Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing (Biomedical and Health Research)

TRADITIONAL RATING OF NOISE VERSUS PHYSIOLOGICAL COSTS OF SOUND EXPOSURES TO THE HEARING

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ISSN 0929-6743

Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing

Edited by

Helmut Strasser Ergonomics Division, University of Siegen, Siegen, Germany

Amsterdam x Berlin x Oxford x Tokyo x Washington, DC

© 2005 The authors. All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without prior written permission from the publisher. ISBN 1-58603-553-3 Library of Congress Control Number: 2005930538

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Preface Current legal occupational health and safety threshold values and suggested curves for the rating of environmental stress are dominated by integral approaches, in particular by the physical principle of “dose.” This is rather understandable given the need to easily characterize work stress and work-environmental influences with simple, straightforward characteristic values in everyday work situations. Such a quest for simplicity often results in the use of single-value or summary measures such as the rating level for noise, the rated vibration intensity associated with vehicles and handheld tools, the effective temperature for climate, and the dose for toxic substances and stress due to radiation (e.g., ultraviolet immissions or radioactive radiation). Conversions that are based on the principle of energy equivalence equate singular high intensities of short duration with an accordingly lower intensity “leveled” over an 8-hour day. Such a rating – which is solely stress-oriented, i.e., based on the combination of intensity and time – does not do the human body’s characteristics justice (e.g., for the evaluation of impulse noise). A single noise exposure with a high level of 160 dB of 1 ms duration or 100 impulses of 140 dB each and a duration of 1 ms are identical to a (still permissible) continuous sound exposure of 85 dB for 8 hours in a physical sense, i.e., in terms of energy. From an ergonomics perspective, however, continuous and peak stress cannot be assumed to have the same effect on the human body. On the other hand, restitution periods after exposure to short-term high noise levels can be filled with additional noise without a numerical change in the rating level as long as the additional noise is only 10 dB below the peak levels. It should be evident that the effect on the human body cannot remain the same. Thus, threshold values or suggested values in ordinances, guidelines, and regulations concerning occupational health and safety – which are based on the dose maxim – can be associated with substantial risks from an ergonomics perspective. Blindly following these laws and rules without knowledge of the underlying compromises can result in substantial misjudgments of the effects on the human body. Inevitably, it becomes increasingly difficult to draw conclusions about strain or acute and potentially long-term effects or damage based on stress data the more integral characteristic values are formed to summarize the dimensions “intensity,” “frequency,” and “exposure time” of physical environmental stress. It may only be seemingly safe when a – mathematically easily accomplished – equilibration of peak levels or mutual compensation of stress level and duration based on an 8-hour workday takes place. This is especially relevant because the compressed acting of a noxa over time, i.e., the growing energy or pollutant concentration, makes it increasingly likely that physiological thresholds are exceeded since the human body does not have sufficient “buffering capabilities.” This book extensively addresses this topic. It is an attempt to increase the transparency in existing rating methods and – in the interest of pertinent disclosure of risks associated with common procedures of occupational health and safety – to work towards the elimination of unacceptable simplifications and faulty ratings. The emphasis is on a discussion of rating methods of acoustic stress since partial loss of hearing due to noise is still the leading occupational hazard in practically all industrialized nations even though herewith only the tip of an iceberg of aural and other extra-aural effects of noise is visible. The introductory Chapter 1 demonstrates the conventional method of measuring, evaluating, and rating of physical environmental stress using the example of noise exposures. For example, the fact that noise exposures are measured, evaluated, and – using complicated for-

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mulas – rated with impressive precision should not suggest that the procedure is sufficiently “tailored” to the human body. The problems begin with the acoustic measuring systems, which exhibit a lack in compatibility with the hearing’s physiological characteristics. Among other things, the obvious differences between the stress specifications during physical work according to the principle of equal work and the various strain reactions of the circulatory system already suggest that the hearing can hardly be capable of handling extremely high, short noise exposures equally well as energy-equivalent lower, but accordingly longer stress. Consequently, there are risks associated with the use of the dose maxim or the energy equivalence in the context of occupational health and safety. Similarly, individual hearing protectors do not always deliver the level of effective protection suggested by common rules and regulations – especially against exposures to impulse noise. Chapter 2 provides an overview of occurrences and characteristics of impulse noise, which, in addition to posing a particular threat to the hearing (e.g., as rebound on the shooter) associated with the use of firearms both in the civil and military sector, also occurs more often than typically assumed during various work processes. Using bolt setting tools as an example, it is demonstrated that it would be unwise to rely “blindly” on a tool’s advertised “acoustic quality” which is based on standardized measuring procedures. Similarly, the level of expected noise emissions and the resulting tolerable work cycles per day should not be taken “at face value.” The use of steel profiles instead of concrete (the standard material), for example, results in noise emissions of a different, substantially more dangerous nature, which means that an employee’s protection cannot be guaranteed under real-life working conditions. Chapter 3 presents field studies on the use of bolt setting tools as advantageous, mobile tools for roofing and paneling of industrial buildings. The studies show that in addition to the noise caused by the tool’s operator, extraneous noise, which occurs with at least equal frequency, must be taken into consideration as well despite its slightly reduced volume. While such extraneous noise and the general noise level at a construction site do not substantially increase the rating level, a substantially increased risk of damage to the hearing results. The bulk of the book consists of more than a dozen chapters, which present comprehensive statistically secured results of studies on audiometrically determined hearing threshold shifts and their restitution behavior after various sound exposures. Chapter 4 describes specifically developed measuring methods and statistical evaluation procedures for the determination of hearing threshold shifts (with precision to 1 dB) at the frequency of maximum threshold shift immediately after the exposure (TTS2), the restitution course, and the time t(0 dB) after which all threshold shifts have subsided. The integral over the restitution function, the so-called Integrated Restitution Temporary Threshold Shifts (IRTTS), is a global characteristic value for the “physiological costs” that must be “paid” by the hearing for the sound exposure. The results presented in Chapter 5 demonstrate via experiments that measurements of threshold shifts at a single frequency capture the majority of metabolic fatigue in the inner ear, thus permitting the use of such a procedure for the remaining studies. The quantitative study in Chapter 6 shows that energetically equivalent stress from continuous and impulse noise with a legally permitted rating level of 85 dB(A)/8 h results in extremely different physiological costs. The already substantial threshold shifts of more than 20 dB after exposure to continuous noise of 94 dB(A)/1 h (equivalent to 85 dB(A)/8 h) only increase by a few dB as a result of exposure to impulse noise (e.g., after 9,000 impulses with a level of 113 dB and a duration of 5 ms, administered at 3-s intervals). However, the restitution times increase from approximately 2 h (after continuous noise) to more than 10 h (after impulse noise) which is associated with a substantially higher risk of permanent hearing threshold shifts.

Preface

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Among other things, the studies concerning threshold shifts in Chapter 7 examine the effect of additional continuous noise (which increased the rating level by only 0.1 dB and was thus of no energetic relevance) after a preceding exposure to continuous noise. The documented results clearly show that there is a “price to be paid” if restitution periods are “filled” with additional noise. The marginal increase in the rating level caused the hearing’s physiological costs to more than double relative to the costs that were associated with the “initial” stress of 94 dB/1 h. Without the determination of the restitution course and the IRTTS-values, it would not have been possible to show such an effect. Chapters 8 and 9 investigate the effects of variations in the number and duration of noise impulses on the threshold shift. Again, the effects on the hearing when duration and number of impulses were “swapped” against each other (while maintaining energy equivalence) showed differences that were of statistical and practical significance. In addition to noise, music can pose a threat to the human hearing. Thus, the following three Chapters 10 through 12 present studies which examine stress from various styles of music (heavy metal, techno, and classical music) by comparing their aural effects to those caused by energy-equivalent industrial noise and “white noise.” The results suggest that heavy metal has effects similar to industrial noise. Furthermore, the human hearing seems best suited to tolerate harmonic and sine-shaped sounds. On the one hand, noise exposures rarely occur in isolation. In the workplace, they are often compounded by physical stress. On the other hand, stress from noise or music often coincides with alcohol or cigarette consumption during leisure activities. Therefore, Chapter 13 analyzes the effects of such combined stress on the hearing and the circulatory system. It was found that such “double stress” is not necessarily negative: For example, restitution processes of the hearing can be accelerated by limited physical work, and “reasonable” amounts of alcohol also exhibit positive effects. Exposure to nicotine and carbon monoxide from cigarette smoke, however, has a negative impact on the restitution processes of the hearing. Chapters 14 through 16 present experimental data on the objective determination of hearing protection devices’ attenuation effectiveness via the artificial head measuring technique versus the subjective hearing threshold method. Additionally, 2 extensive test series establish that short time periods during which no hearing protection is worn does not lead to the drastic negative effects on the protection’s effectiveness that mathematical models – on which national and international standards are based – predict. The experimental results with respect to the physiological costs of various sound exposures, which have been accumulated over more than 10 years refute the concept of energyequivalence along virtually all dimensions. On the one hand, the use of this paradigm – which is solely based on laws of physics – substantially underestimates the risk of impulse noise, and it legalizes the “filling” of resting periods with noise. It is certainly true, however, that such noise results in “physiological costs” as well as mental effects. On the other hand, the concept of energy equivalence ignores that short-term, high-level noise exposures are quite favorable for the human body. If such exposures remain below a threshold of approximately 120 dB, the human body can handle them quite easily, both from a mental and physiological perspective. The concept of energy equivalence even beats its supporters at their own game when – incorrectly – drastic losses in attenuation after short time periods of not wearing personal hearing protection devices are prognosticated, making them sound worse than they are. All results regarding the “physiological costs” to the hearing are based on legally permissible acoustic stress, which is equivalent to a rating level of 85 dB(A)/8 h, since dangerously high levels must not be used in tests involving human test subjects for ethical reasons. According to the consistent experimental findings, but also based on plausible scientificcritical evaluations in Chapter 1 as well as the concluding Chapter 17, the dose principle or the concept of energy equivalence cannot be viewed as ergonomic paradigm for occupational

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health and safety and ergonomics with respect to assessment of environmental stress during an 8-h workday. Supporters of the dose maxim like to cite Paracelsus who – following the spirit of the Renaissance – adopted this name in lieu of his original name (Philippus Aurelius Theophrastus Bombastus von Hohenheim) approximately 450 years ago. He is credited with the phrase “dosis facit venenum.” By no means does that imply, however, that alternatingly high and low immissions should be expressed as a single mean value to describe a workplace’s typical amount of stress (an often-cited justification for the concept of energy equivalence or the dose maxim for the rating of physical environmental stress and toxic substances). A thorough study of Paracelsus’ work reveals that his phrase reflects a medical doctor’s experience and knowledge of toxicology that medication (the extract of a medicinal plant) in several smaller amounts (the “right dosage”) has healing effects while the same amount administered at once (the dose) could be fatal. This does not suggest that stress, which is repeated daily for years (as work dose) cannot correlate with noticeable effects on the human body, which may even include “wear and tear” on an organ. However, the dose maxim may certainly not be used – following Paracelsus – to legitimize the leveling of variable physical and toxic environmental stress during the course of a workday, whose effects are often hastily equated with those of energy-equivalent continuous stress. In order to live up to the claim that work protection is based on the human body, the presented unambiguous, statistically significant experimental results regarding the vastly different “physiological costs” of, e.g., continuous noise and impulse noise must effect changes in the way they are rated. In the area of occupational health and safety, it would be irresponsible to take the convenient position of limiting the assessment of stress to the physical aspects while ignoring the fact that human beings react to exposures according to physiological and psychological characteristics rather than “function” according to the laws of physics as they apply to dead matter. Thus, ergonomics and occupational medicine must insist vehemently on the inclusion of current knowledge regarding short- and long-term effects of stress on the human body in rules and regulations of occupational health and safety. I wish to extend my sincere thanks to Ms. Jenny Deter Gritsch who did a great job in translating large parts of this book from German into English. Prof. Dr.-Ing. habil. Helmut Strasser

Siegen, 2005

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Contents Preface Chapter 1 Problems of Measurement, Evaluation, and Rating of Environmental Exposures in Occupational Health and Safety Associated with the Dose Maxim and Energy Equivalence Principle H. Strasser

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Chapter 2 Impulse Noise Exposures, Present in Civil and Military Sectors J.M. Hesse

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Chapter 3 Noise Immissions from Working with Bolt Setting Tools in the Construction Sector M. Rottschäfer, J.M. Hesse, H. Irle and H. Strasser

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Chapter 4 Methods for Quantifying Hearing Threshold Shifts of Sound Exposures and for Depicting the Parameters TTS2, t(0 dB), and IRTTS Indicating the Physiological Costs to the Hearing H. Irle and H. Strasser Chapter 5 Hearing Threshold Shifts and Restitution Course after Impulse and Continuous Noise at the Frequency of the Maximum Threshold Shift and the Adjacent Lower and Upper Frequencies H. Strasser, H. Irle and S. Linke Chapter 6 Hearing Threshold Shifts and Their Restitution as Physiological Responses to Legally Tolerable Continuous and Impulse Noise Exposures with a Rating Level of 85 dB(A) H. Strasser, J.M. Hesse and H. Irle Chapter 7 Physiological Costs of Energy Equivalent Exposures to Continuous and Additional Energetically Negligible Noise H. Irle, J.M. Hesse and H. Strasser Chapter 8 Influence of the Number of Impulses and the Impulse Duration on Hearing Threshold Shifts H. Irle and H. Strasser

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Chapter 9 Investigations into the Efficiency of the Stapedius Reflex with Impulse Noise Series H. Irle and H. Strasser Chapter 10 Physiological Costs of the Hearing after Exposures to White Noise, Industrial Noise, Heavy Metal, and Classical Music of 94 dB(A) for 1 Hour H. Strasser, H. Irle and R. Scholz Chapter 11 Temporary Hearing Threshold Shifts and Restitution Associated with Exposures to Industrial Noise and Classical Music of 94 dB(A) for 1 Hour and 91 dB(A) for 2 Hours H. Strasser, H. Irle and R. Legler Chapter 12 Comparative Investigations into the Physiological Responses to Heavy Metal, Techno, and Classical Music H. Irle, F. Körner and H. Strasser Chapter 13 Effects of Noise Exposures during Physical Rest, Additional Physical Exercise and Combined Exposures to Alcohol and Cigarette Smoke on Hearing Threshold Shifts and their Restitution H. Strasser and H. Irle Chapter 14 Quantification of the Insertion Loss of Personal Hearing Protection Devices by Means of a Subjective Method and an Artificial Head Measuring System H. Irle, H. Fidan, J.M. Hesse and H. Strasser Chapter 15 Substantial Protection Loss Associated with a Minimally Reduced Wearing Time of Hearing Protectors – Fiction or Reality? H. Irle, Ch. Rosenthal and H. Strasser Chapter 16 Influence of Reduced Wearing Time on the Attenuation of Earplugs – Prognosis by the 3-dB Exchange Rate versus Audiometric Measurements H. Strasser, H. Irle and T. Siebel Chapter 17 Dubious Risk Prevention via Traditional Rating of Whole-Body Vibrations, UV Radiation, and Carbon Monoxide J.M. Hesse and H. Strasser Author Index

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 1

Problems of Measurement, Evaluation, and Rating of Environmental Exposures in Occupational Health and Safety Associated with the Dose Maxim and Energy Equivalence Principle H. Strasser

0 Summary Based on the principle of “equal work,” the conventional, energy equivalent approach of rating physical environmental exposures – which is derived from physics rather than physiology – is presented. Straightforward examples demonstrate the difficulty of summarizing exposure intensity and duration under human-physiological aspects. Results from experiments with continuous noise exposures of different time structures confirm that an energetically equivalent noise exposure does not correspond with equal “costs” for the human hearing. The energy equivalent rating according to national and international standards as well as the Accident Prevention Regulation “Noise” (UVV Lärm) or the cut-off level diagram is questionable at best, but may even be dangerous.

1 Introduction Preventative work safety, via threshold values of permissible stress, attempts to avoid occupational diseases and health problems or at least to limit their prevalence. According to the current law in the European Union, an employee’s safety and health can be demanded in a court of law. This applies to all kinds of stress which result from the type of work, the operation of (ideally ergonomically designed) machines, and the work environment. Humanoriented occupational safety faces the challenge of estimating the stress and its effects from the various potential sources on the human body which typically occur for varying time periods. Yet, it must be the main goal of a human-based occupational health and safety approach to assess the immissions, i.e., the effects of, e.g., environmental exposures on man instead of just simple measuring emissions. Everyday experience suggests that lower-level stress can be tolerated for a longer time period while higher-level stress can only be tolerated for a shorter time period, i.e., can be endured when the level of strain is still acceptable. However, work science and ergonomics must not be satisfied with the principle of equal work, i.e., the simple multiplication of the stress intensity and the exposure time cannot be considered an ergonomic principle.

2 Principle of equal work Physically, 50 Watts (stress intensity H3) for 60 minutes (stress duration T3) represent the same work as 100 Watts (H2) for 30 minutes (T2) or 200 Watts (H1) for 15 minutes (T1) (cp. Figure 1). There is no doubt, however, that despite computational equivalence, there is a limit to this kind of multiplication. For example, 1,000 Watts for 3 minutes or 5,000 Watts for an even shorter time span would not be feasible for biological reasons. As Figure 2 indicates, similar equivalency computations would be highly problematic for climatic exposures as well. For example, 15 % of work time at -10 °C in a cold-storage depot, 75 % at +30 °C outdoors, and the remaining 10 % of work time at -10 °C in a refrigerator truck average to +20 °C which suggests a comfortable environment while the employee in reality feels uncomfortable for the entire time.

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Figure 1:

Multiplication of different stress levels with stress durations according to the principle of equal work (cp. STRASSER 1990)

Figure 2:

Problems associated with averaging over time with alternating cold and hot environments during the same shift (cp. HETTINGER 1984)

Similar is true for a smelter at a blast furnace who experiences heat radiation of 800 W/m2 from the front and simultaneously is exposed to a heat loss of 200 W/m2 (during winter in an open building) on the back (cp. Figure 3).

H. Strasser / Problems of measurement, evaluation, and rating of environmental exposures

Figure 3:

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Questionability of averaging spatial-locational climatic differences with asymmetric impact on a smelter at a blast furnace (cp. HETTINGER 1984)

Such an asymmetric climatic effect can hardly be beneficial even though (800 - 200) W/m2 = 600 W/m2 seem tolerable. The work situation can almost be compared to a shower which dispenses warm and cold water in separate streams rather than in a balanced mixture. Averaging exposures can be problematic with respect to the effects on the human body. In addition to the application to stress data, averaging of values can also be questionable and misleading for data on strain. Caution and awareness of their work-physiological significance are required in analyses of strain data, too. A relatively constant heart rate of approximately 90 beats per minute (bpm) which results from medium physical activity must be viewed differently from a level with the same computational average which results from values between 80 and 180 bpm (cp. Figure 4).

Figure 4:

Problems with averaging strain data such as heart rate (HR) over time

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3 Biological costs in dependence on time structure of the exposure In the context of the effects of an exposure's intensity-duration constellation, the principle of equal work and the respective physiological costs will again be discussed. In addition to the domain of stress (cp. Figure 1), Figure 5 shows the fundamental cardio-vascular reactions to dynamic muscle work in an additional level. If, for example, 50 W (i.e., a physical activity below the Endurance Level (EL)) is required, the work pulses typically enter a “steady state,” even after extended time periods. At 200 W, however, pronounced increases with possibly critical heart frequencies can be expected already after short periods of time (e.g., 15 min).

Figure 5:

Difference in biological cost of physically equal work of different intensity (course of work-related increase in heart rate during and after stress) (cp. STRASSER 1990)

But not only the reactions during the workload are relevant for the human body, but also the restitution processes. It is well known that an increase in stress leads to a superproportional increase in the recovery time course. Allowing recovery time after comprehensive dynamic muscle work solely according to the principle of equal work, i.e., based on stress (e.g., using the BÖHRS-SPITZER method) does not adequately take human characteristics into account, especially in areas far above the endurance level (cp. Figure 6). If the area of required stress height above the endurance level and stress duration is simply transformed into an equivalent area of endurance level and the required restitution time TH, then the recovery times RT in dependence of the stress level are determined in a linear rather then superproportional fashion with this method which is based on measurements of the metabolic rate of energy. It has been well known for some time, however, that a superproportional method is appropriate. The circulatory system’s activations after high short-term stress do not subside linearly, but in a delayed fashion as indicated in an empirical formula which was developed by ROHMERT in the 1960s. The individual exponents for the duration and level of stress have the “flavor” of STEVENS’ power law. The resulting recovery time requirements – which must be seen as minimum requirements – are substantially prolonged (cp. Figure 7).

H. Strasser / Problems of measurement, evaluation, and rating of environmental exposures

Figure 6:

Depiction of recovery time requirements according to BÖHRS-SPITZER as stress-based recovery time method

Figure 7:

Strain-oriented, empirically determined recovery time formula with an example (according to ROHMERT 1981)

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This kind of thinking can be applied to environmental stress to which the human body is exposed in the form of some substance with a concentration C for certain periods of time. In many situations, it can be expected for the dose-effect curves (cp. Figure 8) that it becomes more and more likely that physiological limits are exceeded as energy or a toxin becomes more concentrated with a shortened time period unless the human body is somehow capable of “buffering” over space or time.

Figure 8:

Toxin’s dose or amount D as product of concentration C and exposure time T (domain of stress) with hypothetical dose-effects E for short-term and long-term exposures of equal dose D = C x T (domain of effects) (cp. TIETZE 1981, modified)

While a higher effect E1 can be expected for a short-term exposure D1, a long-term exposure of equal dose D3 (computed as product of concentration C3 and exposure time T3) can be expected to result in a subdued effect E3, i.e., the organism may be able to tolerate higher total amounts of stress. It is oftentimes difficult to simultaneously address the practitioner’s need for easily applicable evaluation and rating methods on the one hand and the human-physiological responsibility, based on a human-related way of thinking, on the other hand. The more integral characteristic values are used in the summary of the dimensions intensity or level of stress and stress duration (and oftentimes the dimension frequency of environmental exposures), the easier it becomes for sometimes problematic resulting compromises to go unnoticed. This is especially true for noise exposures which can be measured, evaluated, and rated on the basis of national and international standards and guidelines as well as several work safety regulations which allow the use of exact mathematical formulas, promising high mathematical precision with respect to quantifying the objectively measured data.

4 Energy equivalence of noise exposures from an ergonomics point of view The intensity of sound events has always been quantified in decibels by the sound pressure level in a logarithmic scale (cp. upper row of Figure 9). Of course, that is a pragmatic scale because a tremendous span of, e.g., 12 decimal powers of sound intensity can be condensed into easily manageable values of only 3 digits (e.g., 0 to 120 dB).

H. Strasser / Problems of measurement, evaluation, and rating of environmental exposures

Figure 9:

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Gradual assessment of the physical dimensions “intensity,” “frequency,” and “exposure duration” of sound events for the development of an integral characteristic value (cp. STRASSER 1993)

However, scientists and practitioners nowadays still have to work with this scale, despite the somewhat paradoxical fact that the psychophysical basic law of Weber-Fechner has meanwhile proven to be incorrect for acoustic stimuli. Although the formula for the sound pressure level is due to Weber-Fechner’s law (cp. upper part of Figure 10), the resulting logarithmic scale is not in accordance with human sensation. For example, 90 dB are not 10 % less than 100 dB but represent just 1/10 of the sound energy which is inherent in 100 dB and, e.g., a sound event with a sound pressure level of 100 dB is not double as loud as an event with 50 dB. It represents a totally changed “acoustic world.”

Figure 10: Incompatible logarithmic scale of sound pressure level in dB and scale of loudness according to Stevens’s power law

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Therefore, instead of the incompatible logarithmic scale, a scale of loudness with linear units in Sone (cp. middle part of Figure 10) due to sensation derived from Stevens’s power law should be used. If we were to make our money transactions utilizing the traditional logarithmic scale, we would, no doubt, handle the decibels a little bit more cautiously than we sometimes do in practice, when we say, e.g., 93 dB seem to be almost the same as 90 dB. Provided that • 0 dB corresponds with 1 €, • 30 dB would be equivalent to 1,000 €, and • 60 dB would mean that we would already be millionaires. • But also trillions or quadrillions in national debt expressed in the small figures 120 or 150 dB would seem to be not that tremendously much more than the money that “have-nots” have in their pockets (cp. Figure 11).

Figure 11: Level in dB and noise energy multiples

With the intention of specifying sound immission with regard to intensity and frequency in one single value, frequency-dependent filters A, B, C, or D should take into account the physiological characteristics of hearing (cp. middle row of Figure 9). The filters A, B, C, and D (cp. Figure 12), however, as a reciprocal approximation of the phon curves in different volume ranges, are based on the subjective comparison of sequentially presented tones and, therefore, cannot lead to an adequate assessment of noise, which normally is a mixture of inharmonious sounds. Furthermore, in most cases today, only the A-weighting network is used for all volume ranges, although doing so conflicts with scientific knowledge. This discrepancy sometimes leads to the fact that, to the disadvantage of man, sound pressure levels of some noise sources do not represent the real sensations of man. Sound pressure levels mentioned in ergonomics and in all legal regulations, standards, and prevention instructions (cp. e.g., Accident Prevention Regulation “Noise”; N.N. 1996; N.N. 1998; ISO DIS 1999) do not refer to a momentary sound event; they normally refer to the rating level Lr calculated via the formula in the lower row of Figure 9, as an average value for the noise exposure associated with an 8-h working day. The energy equivalent calculation of the mean value is, of course, applicable to a great many working situations. However, situations also exist where a purely formal calculation yields peculiar results which lead to a serious misinterpretation.

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Figure 12: Curves of equal subjective sound level intensity (in Phon) and frequency response characteristics of the weighting networks A, B, C, and D

When applying energy equivalence (cp. Figure 13), 85 dB for 8 h are equivalent to 88 dB for 4 h, 91 dB / 2 h, or 94 dB / 1 h. This mutual settlement of noise level and exposure time is correct as far as sound dose and sound energy are concerned. However, with regard to physiological and psychological aspects of work, inevitably some discrepancies result.

Figure 13: Sound pressure levels of different durations leading to an equal rating level (in this case 85 dB(A)) when applying the “3-dB exchange rate”

Ninety-four dB / 1 h (cp. right part of Figure 14) – as previously described are energetically equivalent to 85 dB / 8 h, i.e., they correspond to a Lr of 85 dB. If only the energy, i.e., the sound dose, is considered, what is shown in the left part of Figure 14 also holds true. In this case, 94 dB for 1 h and an additional 75 dB for the remaining 7 h also result in a Lr of only 85 dB. Physically seen, this is correct, but it is comparable to filling up quiet periods with noise, and from a psychological point of view it is likely that nobody would prefer a situation as described in the left part of Figure 14.

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Figure 14: Elucidation of discrepancies in rating noise via the 3-dB exchange rate (cp. STRASSER 1981)

Provided that the noise distribution shown here would stem from 2 machines, strange effects would also result with respect to technical approaches of noise control. If an engineer in this case would decide to completely insulate the machine which emits the lower level, the rating level would not be influenced at all. The application of the measure “rating level” consequently allows these strange ratings, as long as the lower value of noise remains a certain amount below the peak levels. For an equal exposure time, a difference of only 10 dB between the two levels is already enough to neglect the lower level, which absolutely agrees with legal regulations, standards, and national or international guidelines. When continuing to halve the exposure time and when applying the “3-dB exchange rate” as shown in Figure 13 – from a purely arithmetical point of view – even a quarter of an hour at 100 dB would correspond to an 8-h working day at 85 dB, which is still tolerated in the production sector according to almost all international standards (cp. N.N. 1997). Nevertheless, physiologically seen, high sound levels for a short period of time, e.g., 100 dB over 15 min or consequently also 113 dB for 45 s have to be assessed much more advantageously than continuous noise (cp. Figure 15).

Figure 15: Sound pressure levels of different durations leading to an equal rating level when applying the 3-dB exchange rate

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But may continuous noise also be split up into energy equivalent impulse noise? Lead, e.g., 9,000 impulses with a level of 113 dB and a duration of 5 ms, each, to the same effects as continuous noise with the same level and a duration of 45 s? The answer is no! This has to be demonstrated, e.g., by temporary threshold shifts (TTS) resulting from different noise levels with corresponding exposure times in an energy equivalent arrangement (see Chapter 7). Furthermore, may the mutual compensation of level and exposure time be applied without limit? Can 120 dB, 140 dB, or even 160 dB at an adequately reduced exposure time be assessed to be identical to or even more advantageous than, e.g., the above-mentioned 113 dB / 45 s? From a physiological point of view the answer must be “no,” even though TTS may level off completely as physiological responses to an extremely short-lasting peak level. Nevertheless, in the past, the energy equivalent compensation of a halving of the duration with a level increase by 3 dB and vice versa (or the factor 10 in duration versus level) has become the basis for cut-off level diagrams to avoid hearing impairment which are applied in civil as well as in military sectors (cp. N.N. 1987). In the case of impulse noise, exposure times even reach down into the range of ms. When establishing a logarithmic scale for the exposure time in addition to the already existing one for the noise level in dB, the straight line in Figure 16 illustrates the energy equivalence for the rating level of 85 dB, e.g., • 1 x 1-ms impulse of 160 dB, • 10 x 1-ms impulses of 150 dB, • 100 x 1-ms impulses of 140 dB, • 1,000 x 1-ms impulses of 130 dB, • 9,000 x 5-ms impulses of 113 dB, and • 85 dB for 8 hours (28,800 s), respectively.

Figure 16: Conventional noise rating according to the principle of equal energy with a tolerable rating level LArd of 85 dB(A)

Although the unweighted noise level in industry may not exceed 140 dB according to revised noise regulations (e.g., Accident Prevention Regulation “Noise”; N.N. 1997) due to

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the limit line in Figure 16, the varying time structures of the sound exposures are not considered. However, in terms of actual strain, short-term high noise exposures seem to be favorable to lower exposures over longer time periods, as illustrated via studies on hearing threshold shifts (cp. Figure 17).

Figure 17: Temporary threshold shift (TTS) of the hearing in the frequency range 0.5 to 8 kHz after a shortterm high o and long-term low acoustic exposure q as well as TTS restitution p after equal energetic sound exposures of 94 dB(A) / 60 min and 113 dB(A) / 3/4 min n relative to a rating level of 85 dB(A) for 480 min (8 h) (cp. HESSE and STRASSER 1990)

The figure shows the measured hearing threshold shifts of a group of individuals after exposures of 94 dB(A) for 1 h and 113 dB(A) for ¾ min, both of which are energetically equivalent to 85 dB for 8 h. It can be seen that sound exposures of Lr = 85 dB(A) – which are generally considered tolerable – led to substantial “physiological costs,” i.e., threshold shifts which were still measurable more than 100 minutes after the exposure. If the exposure time is shortened in exchange for an increase in the level – e.g., the energy equivalent stress of 113 dB for 3/4 min – there is a substantial reduction in physiological costs. It is tempting to consider the extrapolation to higher levels as advantageous to the human body and, possibly, to make a linear continuation, especially since the psychological “nuisance factor” of high, but short levels is certainly reduced. Furthermore, diagrams which show the results from many studies on the relationship between the Temporary Threshold Shift (TTS) and stress duration as well as levels (cp. Figure 18) indicate that the threshold shift can be expected to decrease almost to “0” if high levels occur for only a short time. The energy equivalent combinations 85 dB for 8 h and 94 dB for 1 h which are indicated with the symbol “}” led to roughly equal TTS2 values of approximately 30 dB, and 113 dB for 45 s resulted in minimal TTS2 values of less than 10 dB which is consistent with the results of the study shown above. However, it must be cautioned against the extrapolation to levels in excess of 120 dB particularly since the “metabolically” determined, audiometrically measured threshold shifts can no longer be considered to be adequate physiological reactions. Instead, the focus with short-term stress in excess of 120 dB shifts to mechanical damage which can hardly be objectified. Thus, the updated version of the Accident Prevention Regulation “Noise” (UVV Lärm) from 1990 (cp. Figure 16) which does not limit peak levels until they reach 140 dB and

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applies the principle of energy equivalence based on LAeq = 85 dB(A) down to the range of seconds must be considered inadequate for certain situations.

Figure 18: Hypothetical TTS2-course associated with duration and level of noise and examples of energy equivalent exposures with a rating level of LArd = 85 dB(A) (cp. MILLER 1974)

The former PFANDER method which was used for bang noise in the military sector (with a higher tolerable rating level of LArd = 90 dB, cp. also Figure 19) equated, for example, • 1 x 165 dB for 1 ms with • 10 x 155 dB or • 100 x 145 dB or • 1000 x 135 dB for 1 ms and also • 90 dB for 28,800 s (8 h) which seems highly problematic at least for superliminal bang noise exposures (i.e., levels in excess of 120 dB) for plausibility reasons. The rather convenient energetic “tradeoff” between exposure level and exposure duration has long been used in the cut-off level diagram which was used to avoid damage to the hearing from bang exposure. In addition to the existing scale which is measured logarithmically (in dB), the exposure time is also plotted logarithmically (cp. Figure 19). As can be seen from the lowest of the three lines, 90 dB for 8 h on the one hand and 165 dB for 1 ms or 175 dB for 0.1 ms on the other hand are energetically equivalent. Assuming a reduction of 20 to 40 dB from individual hearing protection devices, high exposures seem permissible as long as the intersection of peak level (Lpeak) and exposure time (tw) in the level-exposure time diagram stays below the critical lines which have been shifted up vertically by 20 or 40 dB, respectively. Based on this method, exposure times of approximately 1 ms from the use of a standard rifle G3 in the military sector can be plotted twice horizontally at approximately 160 dB without touching the critical line (cp. Figure 20).

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Figure 19: Cut-off level diagrams (with logarithmic scale on both axes) to avoid hearing impairment (with and without hearing protection) in case of bang noise valid for exposures per day with a following recovery of at least 8 hours (for details, see STRASSER 1987)

Figure 20: Exposure for the shooter (measuring point MP 1) and determination of the tolerable number of shots (noise events) with and without ear protectors when using the rifle G3 (extract from ZDV 90/20 sheet II/1.2 and 10.1, 1980)

If practice ammunition with peak levels of 140 dB, i.e., 20 dB less, is used, approximately 100 times more individual exposure times of approximately 1.5 ms can be plotted on the horizontal line at 140 dB so that substantially more (exactly 186) bangs per day are tolerable

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based on this graphical-computational method. For a single bang whose level is 30 dB lower, i.e., approximately 130 dB (e.g., when a silencer is used), 1,250 daily exposures with a duration of approximately 1.5 ms seem harmless. If, additionally, hearing protection with 20 or 40 dB sound attenuation is worn, several thousand exposures with levels around 160 dB would be considered safe. Is it possible, however, to determine such exact numbers in these marginal areas of medical and ergonomic knowledge?

6 Effective attenuation of ear protectors - Remaining doubts from an ergonomics point of view Using the linear cut-off level diagram, the high precision usual in engineering can be achieved in case of bang events according to “law and rule” by means of the aforementioned graphical-mathematical procedure. The question is, however, whether not occasionally a portentous responsibility may result for the person who has to take responsibility for health protection? Can he feel comfortable, if in extreme ranges of the knowledge of medicine and ergonomics he has to argue with a numerical precision in a field, in which exact knowledge and pure supposition are merged, in which both definite and vague facts have to be taken into account? But even provided that the mutual settlement of level and exposure time would at all be applicable for impulse noise, there still remain a number of unsolved problems (cp. Figure 21).

Figure 21: Questions concerning application of the cut-off level diagram in the case of bang events (supposing that in the case of impulse noise the energy equivalent mutual settlement of intensity and duration is allowed)

One question for example comes up, whether exposure time can be constant for all applications, or if there are not essential variances between the bang events in different surroundings. Formerly measured exposure times of 2 ms at the ear of the standing marksman would already involve a halving of the tolerable number of shots. Furthermore, the question should be answered, whether for example the noise attenuation of the ear plug can be generally estimated to be 20 dB and thus a 100 times higher number of shots may be tolerated? Noise attenuation values are always dependent on frequency and they are especially smaller in the case of low frequencies, which often bear the main energy of noise sources. Moreover, must there not arise certain doubts with regard to the “fixed” attenuation value of 40 dB of the ear muff WILLSON, an attenuation, which shall allow for a shifting of the cut-off level diagram to higher values, with the result that even a 10,000 times higher number of bang events becomes permissible? A mean attenuation of 40 dB – as supposed very often

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until now – and this is clearly shown in the frequency range - does indeed exist only at a few measuring points (Figure 22 left).

Figure 22: Frequency-dependent mean values of two usual ear protectors and standard deviation for one type as well as “fixed” attenuation values in comparison with statistical safety according to VDI 2560 (Guideline of the Association of German Engineers)

Furthermore, isn’t there any necessity of guaranteeing that effective noise attenuation values of ear protectors are taken into account? If a procedure in accordance with a German standard is taken as a basis, the mean attenuation values have to be reduced by the standard deviation – and these are indeed not unimportant. Furthermore, in addition to this a humanrelated safety factor has to be subtracted. Strictly speaking the frequency-dependent attenuation effect has even to be adjusted to the sound intensity in the respective frequency ranges. Arithmetical mean values of series of measurements – as shown in Figure 22 – in any case guarantee a suitable protection for only half of the population, and even the attenuation value reduced by the standard deviation can promise safety for only 84 % and not at all for 100 % of a group of persons equipped with the hearing protection. Finally, there is an additional, perhaps decisive question. Does the noise attenuation value having been measured at the hearing threshold still exist with the same effect in the case of extreme high levels, or has not at least an increase of the exposure time to be taken into consideration due to the partial “evaporation” of the sound in the ear protection? From a physical point of view this effect is already obvious. Experimental data about the actual acoustic load behind ear protectors show that the attenuation value, which has been assumed in the past, seems to be most problematical. For example, measurements have been made concerning the acoustic load behind and in front of the personal ear protection. A group of 12 subjects wearing a new active ear muff (Ceotronics, type A GS/SB-VK/K) participated in shooting tests with the rifle AK74 (i.e., the standard gun of the former National People's Army of Germany). During 10 shots for each subject, noise levels outside the ear and behind the ear muff were measured by means of pressure transducers. The differences between the free-field peak levels L1 and the peak levels L2 in the auditory canal, taking into account the respective exposure time, both calculated in a computer-aided evaluation procedure, served to estimate the real protective effect of the ear muff. Peak levels in the range of about 163 to 165 dB as well as exposure times tW1 between 0.5 and 0.8 ms from the free-field signals proved to be at rather constant values irrespective of the numbers of shots and subjects (cp. Table 1).

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Table 1: Peak levels and exposure times in the free-field (L1, tW1) and behind the ear muff Ceotronics (L2, tW2) from shooting tests with the rifle AK 74 (subjects 1 and 12 with 10 bang events, each). (Source: STRASSER and HESSE 1993)

But at least interindividually acoustic load behind the ear muff turned out to vary essentially. For subject 1 differences between L1 and L2 did not exceed approximately 13 dB, and taking into account an increase of about 30 to 40 % in exposure time in the ear muff, at least 1 dB has to be subtracted, so that overall an effective attenuation value of not more than roughly 10 dB can be attributed to the protective device. In contrast to that worst case, peak level differences from subject 12 were almost about twice as high and represent results of the test series at its best. Yet, according to doubling in exposure time real attenuation value of the ear muff must be reduced by 3 dB. Comparing these results from real field conditions with attenuation values measured at the hearing level in third-octave bands between 63 Hz and 8 kHz according to guideline DIN ISO 4869 realism about loss in protection from bang noise has to be recognized. Effective attenuation during laboratory situations between 30 and 35 dB at the most, which can be attained in frequencies above 1 kHz, can in no case protect equally from live-fire exposures. These results also correspond with measures of bang noise from a small bazooka and the conventional ear muff WILLSON SB 258 (cp. Table 2). The bang pressure in the free-field at the ear was about 180 dB and the exposure time was about 0.7 ms. Assuming as a precaution a noise attenuation of only 30 dB of the ear muffs approximately 30 bang events per day seem to be allowable. But the real value measured behind the ear muff, i.e., in the ear, was not reduced by 30 dB. It was after all still 160 dB and only 20 dB less than in the free-field. These results are mostly congruent with those measured by YLIKOSKI et al. (1987). Moreover, the exposure time – which increased by more than the factor 10 to approximately now 12 ms – levels off at least a further decrease of 10 dB of the attenuation, so that – even with the ear muffs – in the case of a numerical value of approximately 0.3 only one single bang event of this kind is permissible within 3 days.

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Table 2: Peak level (Lpeak/dB) and exposure time (te/ms) at the ear and behind ear muffs when shooting with a small bazooka (PzF 44 mm) and the calculated number of tolerable bang events per day Nallowed assuming an attenuation value of 30 dB (right upper part) and under real conditions (right lower part) (data from BRINKMANN 1982)

But, if the bang event is assessed with the B- or even with the A-weighting network, the acoustic situation seems to be a little bit more favourable, at least for those who trust in the cut-off level diagram. Of course, neither A- nor B-weighting are able to lower the peak level “at the ear” or “in the ear” substantially, but with regard to the exposure time there are remarkable reductions “in the ear,” so that again real bang events between approximately 2 and 8 per day seem to be allowable. But that cannot at all be permissible, that would be tolerable in the case of a fixed attenuation value of 30 dB, and above all it is not admissible to assume that the ear muffs have an attenuation effect of 40 dB. Whether, however, the bazooka marksman is equipped with an A-weighting network in his ear, is doubtful when considering the “acoustic trophies,” which experienced members of certain armed forces in former times liked to boast of.

7 Equal energy of environmental exposures or equal work, a principle beyond ergonomics limits Even the fact that it will be impossible to avoid bang load in the future, should not be a reason to call into question from time to time pragmatic compromises, such as the cut-off level diagram, with regard to its origin and deficiencies. The conflict of interest between scientific responsibility on the one hand and the requirements of practice on the other hand surely may not be faded out because of reasons of convenience, and an objective and pertinent manifestation of risks of usual guidelines for occupational health protection is indispensable. This is especially true, if noise is not only to be regarded as a factor which will induce annoyance or handicaps in acoustic communication, but health may be at stake. The traditional cut-off level diagram as well as determining the rating level cannot be anything else but a certain aid to evaluate the sound energy acting on man. But when stress is quantified only in such a physical manner, man and his physiological characteristics are principally not included in the approach of the assessment. The calculation of the total stress by a multiplication of stress height and stress duration is, of course, an often practiced procedure also for other kinds of stress. Always, however, if man is involved and that, of course, is unalterable in ergonomics, even at this datum level, that means in the domain of stress, plausible limiting conditions may not be neglected.

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It is not possible, for example, for the human body as a “chemo-dynamic power plant” to deliver 1,000 Watts for 3 min instead of 50 Watts for 60 min. Can the sense organ ear then be expected to perform so phenomenally – possibly because it is passive – that it can tolerate the sound energy of 90 dB for 8 h (i.e., 28,800 s) equally well as the same dose of 165 dB in mere split seconds? Is such a formal equalization not almost the same as the ironic transformation of a slap in the face into gentle caressing over the course of a day? Can the effects really be the same? One must not forget that bang noise is comparable to a hurricane or tornado inside the ear which cannot be expected to pass over the carpet of hair cells without consequences. This is similar to a gust of wind which sweeps over a field of grain or blows up a sail which is quite different from a continuous, long-term breeze. After all, the mathematical offsetting of stress level and stress duration according to the dose maxim is not applied at will to other sense organs either. • Isn’t it true that increases in temperature – short as they may be – can cause local or global burns? Or can the local temperature increase resulting from touching a hot burner with a finger tip be converted into short-term comfortable warmth of the entire body? • Doesn’t a “laser beam” cause damage when it accidentally hits the retina? • Isn’t a quick prick one and only one discrete event of a mechanical irritation, and doesn’t it cause pain? Is it convertible into tactile caressing of the affected area over long time periods? • Isn’t it true that medicine which is administered in small doses over a long time period to bring healing can be fatal if taken as a one-time high dose? The question whether the concept of “dose” as evaluation basis for the assessment is even justifiable is actually only a rhetorical question. The fact that the principle of dose maxim is used for other sources of environmental stress must not be seen as justification for the application to the field of acoustics. To the contrary, it should be alarming from an ergonomics point of view. Should laws for human beings be solely based on physical principles or is it not rather science’s duty to deal with humans’ physiological and psychological characteristics?

8 Traditional rating of other environmental exposures and risks in occupational safety and health The maximum permissible ultraviolet (UV) radiation according to the American Conference of Governmental Industrial Hygienists (ACGIH) for UV-B and UV-C light does not exactly represent the best available knowledge in the fields of ergonomics or occupational medicine (cp. Figure 23). The conversion of 0.1 PW/cm2 for 8 h into 30,000 PW/cm2 for 0.1 s almost displays ignorance of the underlying physiological or patho-physiological processes. Also, the current way of rating mechanical whole-body vibrations is unsatisfactory from an ergonomics point of view. Especially since changes are still occurring in this field, it would be desirable that the results of the rather comprehensive effects research will be considered to a larger extent in the relevant regulations. The revision of VDI Guideline 2057 in 1987 (Guideline of the Association of German Engineers) was a step in the wrong direction compared to the mathematically easily applicable dose maxim. As a result of the linearization of the single remaining guidance curve for the avoidance of health damage – along with a decrease of tolerable vibration exposure in the middle range of the intensity continuum which must be seen favorably – resulted in a reduction of work safety towards the ends of the range (cp. Figure 24).

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Figure 23: Maximum permissible irradiance Eeff in dependence of exposure time

Figure 24: Exposure limits according to VDI Guideline 2057 (1979 and 1987) for the rating criterion “Health”

Peak stress, represented by a K-value of 112, was permissible for 1 min until 1987. It is now permissible for 10 min. Energetically equivalent evaluated vibration levels of 16.2 are now considered tolerable rather than the previous maximum of 12.5. It is also a concern that 200 ppm (parts per million) for 30 min or, e.g., the equivalent (in terms of dose) 6,000 ppm for 1 min, are considered the upper limit for short-term CO exposure in the military sector (cp. Figure 25). Considering the body’s buffering capabilities and the delayed transition of CO into COHb in the bloodstream, and given the fact that it is typically young, healthy males who are exposed, the dose maxim may not be too harmful. However, there is a number of factors which affect the effect on the organism in a modulating or additive way.

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The dose of the critical values of 60 ppm / 30 min and 14,400 ppm / 7.5 s for short-term CO exposures according to the Ordinance on Hazardous Substances (Gefahrstoffverordnung 1986) are at least lower by more than a factor of 3. These values are also subject to a number of limiting conditions according to the Ordinance on Hazardous Substances so that they appear to offer more protection (cp. Figure 25).

Figure 25: Permissible short-term CO exposures according to BMVG-In San 4 (1972) (top) and according to the Ordinance on Hazardous Substances (Gefahrstoffverordnung 1986) (bottom)

Still, it remains difficult to guarantee a level of safety in real-life situations where peak levels are not always avoidable. This is especially true since clear depictions of the complex relations between exposures, Maximum Workplace-Concentrations (“Maximale Arbeitsplatz Konzentrationen”, MAK values), and the respective Biological Exposure Index (“Biologische Arbeitsstoff-Toleranz-Werte”, BAT values) as an additional, ultimately decisive, protective device are currently more the exception than the rule.

9 Conclusions First of all, it must be mentioned that the surprisingly tight system of regulations and rules (both at the national and the international level) as well as the system of governmental or semi-governmental institutions for the observance of work safety is organized in an exemplary fashion and is unequaled worldwide. However, since so far work safety in a global perspective has typically been fixated on objective critical stress levels, it can happen that the effects of physical environmental stress on the human body are misjudged when the rules and regulations are followed “blindly.” More transparency of existing rules and appropriate supplementation and clarifications are necessary. Critical assessments of existing regulations in the spirit of risk disclosure of commonplace guidelines for health protection must not be avoided out of convenience. Instead, they must be considered a matter of course out of science’s responsibility for the working individual. In that sense, the dose maxim and the equal energy concept do not prove

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to be methods which would be appropriate for regarding the psycho-physiological human characteristics in all relevant ranges of the stress continuum. It cannot be stressed often enough that even data from experimental studies on strain resulting in critical lines can at best be considered data with index character for ranges rather than for specific points. The level of precision which is common in technology will never be achievable in this area. The averaging of stress and the leveling of energy, materials which influence the human body, or materials with which humans work is – and always will be – a problematic task. Even for the human body as “chemo-dynamic power plant” which requires energy in the form of food, it is not possible without consequences to starve for extended periods of time only to make up for it by subsequent “stuffing.” In the context of the energy equivalence principle in rating the physical environment (cp. e.g., MARTIN 1976; STRASSER and IRLE 2001), one must not forget a mechanical analogue where deformations of materials are the intended aim of an energy concentration. Fast, energetic manufacturing operations, such as, e.g., beating, bumping, or punching, are the essential presuppositions for deformations of materials (cp. Figure 26).

Figure 26: Energy concentration by a fast impact of a large mass (1 x 10,000 kg) enabling deformation of materials

Therefore, it is only a stringent consequence that short but intensive events of environmental exposures must involve a greater potentiality of health hazards for man. So, the validity of acceptable equivalences of environmental stress to guarantee health protection must be called into question. There should be no doubts that the effect of a dose which is dispensed within two different time intervals is more striking within the shorter one. Also, unquestionably, in the case of increasing density of energy or concentrations of harmful agents, the exceeding of physiological barriers with simultaneous intensifications of the effects becomes much more probable. This is especially true when the organism does not possess effective potentialities of temporal and/or spatial buffers. Therefore, the well-known endeavor for simplification and standardization which drives attempts to squeeze the rating of complex environmental situations into simple models or integrated measures as is done, e.g., for noise, ultra-violet radiation, mechanical vibrations, and carbon monoxide, cannot be adopted by ergonomics. Via this procedure, multidimensional connections get lost. In this context, a simple but slightly meditative comparison may be convincing for skeptics: The leveling of short lasting high intensity stress, based on physical rudiments, indeed seems to be as trustworthy as the statement that nobody can drown in a river with a statistical average depth of 50 cm (cp. Figure 27).

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Figure 27: Safe crossing of a river with an average depth of 50 cm?

The previous discussion notwithstanding, the “offsetting” of exposure intensity and duration will occasionally be discussed in the following chapters, but it will solely be for pragmatic reasons. It would be foolish to ignore real-life applications in which, e.g., level, exposure time, and number of noise exposures in a certain time period must somehow be evaluated. The dealing with the desired precision – which is in essence already dangerous – will only be used to demonstrate the associated risk, i.e., it is meant to indicate that there is no scientific justification which is appropriate for the issues at hand for such a procedure It should again be mentioned that the application of the equal energy concept, dependent on the range of intensity and time during which the relevant exposure occurs, results in a grave underestimation or – and this should not be ignored – overestimation of potential effects on the human body. To expect that the various effects of noise can be captured with just one method of assessment would be illusory anyway. There is no question that a single short-term high noise exposure without pauses (especially if it is below a level of 120 dB) is much more favorable than an energy equivalent long-term low-level exposure without pauses. This is comparable to a skilled dentist’s onetime determined drilling which is highly uncomfortable, yet, bearable, and thus more favorable for the patient than hesitant and overly-cautious repeated attempts. The principle of energy equivalence cannot be satisfactory if one is concerned about the protection of individuals. Therefore, the goal must be to reduce noise exposure to the absolutely necessary minimum. This is especially true if personal protective devices do sometimes not deliver the level of protection they promise.

9 References BRINKMANN, H. (1982) Die Dämmwirkungen von Gehörschützern gegenüber Waffenknallen. In: NIXDORFF, K. (Hrsg.) Anwendungen der Akustik in der Wehrtechnik, Meppen, 361-337 HESSE, J.M. und STRASSER, H. (1990) Hörschwellenverschiebungen nach verschieden strukturierter energieäquivalenter Schallbelastung. Zeitschrift für Arbeitswissenschaft 44 (16 NF) 3, 169-174 HETTINGER, Th. (1984) Probleme der Übertragbarkeit arbeitswissenschaftlicher Forschungsergebnisse in die Praxis. Leistung und Lohn 150/151, 5-17 MARTIN A.M. (1976) The Equal Energy Concept Applied to Impulse Noise. In: HENDERSON D.; HAMERNIK R.P.; DOSANJH D.S. and MILLS J.H. (Eds.) Effects of Noise on Hearing. Raven Press, New York, 121-153 MILLER, J.D. (1974) Effects of Noise on People. J. Acoustics Soc. America 56 (3) 729-764 N.N. (1987) Effects of Impulse Noise. NATO Document AC/243 (Panel 8/RSG 6) D/9, Final Report of the Research Study Group on the Effects of Impulse Noise

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H. Strasser / Problems of measurement, evaluation, and rating of environmental exposures

N.N. (1996) Preventing Occupational Hearing Loss – A Practical Guide, Revised October 1996, DHHS (NIOSH) Publication No 96-110. U.S. Department of Health and Human Services, Cincinnati, OH N.N. (1997) Technical Assessment of Upper Limits on Noise in the Workplace – Final Report. Approved by the International Institute of Noise Control Engineering. Noise/News International, 203-216 N.N. (1998) Criteria for a Recommended Standard – Occupational Noise Exposure, Revised Criteria 1998. DHHS (NIOSH) Publication No. 98-126. U.S. Department of Health and Human Services, Cincinnati, OH ROHMERT, W. (1981) Physische Beanspruchung durch muskuläre Belastungen. In: SCHMIDTKE, H. (Hrsg.): Lehrbuch der Ergonomie, Carl Hanser Verlag, München/Wien, 115-131 STRASSER, H. (1981) Beurteilung des Lärms aus arbeitswissenschaftlicher Sicht. Leistung und Lohn 105/107, 3-28 STRASSER, H. (1987) Richtlinien des Gesundheitsschutzes bei Schallbelastungen aus arbeitsphysiologischergonomischer Sicht. Zeitschrift für Arbeitswissenschaft 41 (13NF) 37-43 STRASSER, H. (1990) Ergonomische Überlegungen zur Dosismaxime bzw. zur Energieäquivalenz bei Umgebungsbelastungen. Zentralblatt für Arbeitsmedizin, Arbeitsschutz, Prophylaxe und Ergonomie 40 (11) 338-354 STRASSER, H. (1993) Ergonomie – Umgebungseinflüsse. Kap. 2.5.1. Lärm. In: HETTINGER, Th. und WOBBE, G. (Hrsg.): Kompendium der Arbeitswissenschaft. Kiehl-Verlag, Ludwigshafen/Rhein, 243-274 STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Work Load. Archives of Complex Environmental Studies 5 (1-2) 9-25 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: W. KARWOWSKI (Ed.) International Encyclopedia of Ergonomics and Human Factors. Volume I, Part 3, Performance Related Factors, Taylor & Francis, London and New York, 516-523 TIETZE, A. (1981) Energie-Wechselwirkungen in Mensch-Maschine-Umwelt-Systemen, Teil II. Vorlesungsmanuskript, Bergische Universität – GH – Wuppertal YLIKOSKI, J.; PEKKARINEN, J. and STARCK, J. (1987) The Efficiency of Earmuffs against Impulse Noise from Firearms. Scand Audiol 16, 85-88 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln BMVG-In San I 4 (1972) CO-Belastung von Besatzungsmitgliedern im Panzer, Bundesministerium der Verteidigung, 28.3.1972 DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin ISO DIS 1999.2 (1985) Acoustics – Determination of Occupational Noise Exposure and Estimation of NoiseInduced Hearing Impairment Ordinance on Hazardous Substances (1986) Verordnung über gefährliche Stoffe (Gefahrstoffverordnung GefStoffV) BGBl. I, Nr. 47. Bundesanzeiger Verlagsgesellschaft mbH VDI 2057-3 (1979, 1987) Effect of Mechanical Vibrations on Human Beings - Assessment.. February 1979 (Draft) and May 1987. Beuth Verlag, Berlin VDI 2560 (1983) Personal Noise Protection (Guidance Document of the Association of German Engineers). VDI-Verlag, Düsseldorf ZDv 90/20 (1980) Lärmschutzkatalog – Katalog über Lärmminderungsmaßnahmen bei der Verwendung von Wehrmaterial



Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

Chapter 2

Impulse Noise Exposures, Present in Civil and Military Sectors J.M. Hesse

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

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 Figure 1:

Typical shapes of real-life sound pressure time courses

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 Figure 2:

Sound pressure-time course of several powder-actuated tools during driving in of setting bolts into concrete and steel profiles

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

 Figure 3:

Definition of exposure time durations of impulse noise events

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

2 Presentation of impulse and short-time impulse noise exposures 7KHOLWHUDWXUHUHYLHZUHYHDOHGDZLGHYDULDWLRQLQWKHTXDOLW\RIVRXUFHV,QVRPHLQVWDQFHV SUHFLVH LQIRUPDWLRQ UHJDUGLQJ FRQFHSWLRQ DQG FRQGLWLRQV RI PHDVXUHPHQWV DV ZHOO DV WKH PHDVXUHG SDUDPHWHUV ZDV LQFOXGHG 0RUH IUHTXHQWO\ KRZHYHU WKH VRXUFHV ZHUH PLVVLQJ SHUWLQHQWLQIRUPDWLRQZKLFKPDNHVDGLUHFWFRPSDULVRQGLIILFXOWRUHYHQLPSRVVLEOH 7KH IROORZLQJ GDWD FROOHFWLRQ OLVWV WKH DYDLODEOH LPSXOVH SDUDPHWHUV ³SHDN OHYHO´ /SHDN ³PD[LPXP LPSXOVH OHYHO´ /$,PD[ ³ULVH WLPH´ WULVH ³H[SRVXUH WLPH´ 7$ DQG 7% DFFRUGLQJ WR &+$%$ ³H[SRVXUH WLPH´ 7& DFFRUGLQJ WR 3)$1'(5 ³ORFDWLRQ RI WKH PD[LPXP LQ WKH IUHTXHQF\ VSHFWUXP´ I0 DQG WKH UHVSHFWLYH VRXUFH GHSHQGHQW RQ WKH QRLVH VRXUFH DQG WKH ORFDWLRQRIPHDVXUHPHQW ,QGXVWULDOLPSXOVHQRLVHVZKLFKRFFXULQWKHIRUPLQJWHFKQRORJ\ZHUHPHDVXUHGLQIRUJHV DWH[FHQWULFSUHVVHVDQGDWWKHVWUDLJKWHQLQJRIODUJHVL]HVKHHWPHWDODQGIUDPHFRQVWUXFWLRQV FS7DEOH  7KH SHDN OHYHOV ZHUH EHWZHHQ  DQG G% ZLWK H[SRVXUH WLPHV DFFRUGLQJ WR 3)$1'(5 RI  WR PV 7KH YDOXHV IRU WKH %GXUDWLRQ DFFRUGLQJ WR &+$%$ ± ZKLFK ZDV RQO\ GHWHUPLQHG WKUHH WLPHV ± UDQJHG IURP  WR PV 7KH IUHTXHQF\ PD[LPXP ZDV PDLQO\LQWKHUHJLRQDURXQG+] Table 1: Industrial impulse noise parameters at metal forming (source: HESSE 1994)



Noise Source

Location of Measurement

Lpeak [dB]

LAImax [dB]

trise TA TB TC [ms] [ms] [ms] [ms]

fM [Hz]

tup 400 kg

Ear

135

134

—

—

—

—

630

HOHMANN (1984)

tup 750 kg

Ear

144

144

—

—

—

1–2

2050

HOHMANN (1984)

Source

tup 812 kg

Ear

135

118

0.12

—

60

—

—

DIEROFF (1980)

drop-hammer GH 18

Ear

152

—

—

—

—

—

—

DIEROFF (1980)

drop-hammer 2000 t

—

133.4

—

0.7

—

—

—

0–1000

SULKOWSKI and LIPOWCZAN (1982)

Board drop-hammer

Ear

141

123

—

—

—

3–5

1025

HOHMANN (1984)

Forging hammer

—

144

—

—

—

—

—

—

DIN EN 458 (1991)

Eccentric press PEEV 40

Ear

132

114

0.30

—

23

—

—

DIEROFF (1980)

Straightening impact on sheet metal

0.5 m

142

127C

0.08

—

—

4.9

—

SUVA (1976-79)

Straightening impact during box manufacturing

Ear

149

—

—

—

—

2–8

400

HOHMANN (1984)

Straightening impact during frame manufacturing

Ear

138

—

—

—

—

—

1000

HOHMANN (1984)

Straightening impact (max.)

Ear

152

—

—

—

—

3–6

—

HOHMANN (1984)

Straightening impact in fitter’s shop

Ear

143

—

—

—

—

1–3

—

HOHMANN (1984)

Straightening impact 5 kg

0.8 m

152

136

—

—

—

4.5

—

HOHMANN (1984)

Straightening and buckling work

—

—

130

—

—

—

—

—

DÖLLE (1979)

Straightening impact

—

152

—

—

—

—

—

—

DIN EN 458 (1991)

Centre punching of sheet metal

Ear

152

—

—

—

500

—

—

DIEROFF (1975)

Centre punching during boiler construction

Ear

136

121

—

—

—

—

5000

HOHMANN (1984)

,QGXVWULDOLPSXOVHQRLVHGXHWRFXWWLQJDQGDVVHPEOLQJRIPDWHULDOZDVPHDVXUHGRQSXQFK PDFKLQHV ULYHW KDPPHUV DQG SQHXPDWLF FKLVHOV GXULQJ WKH OHYHOLQJ RI VWRQHV RQ FRUH VKRRWLQJPDFKLQHVGXULQJWKHWKURZLQJRISDUWVLQWRDFRQWDLQHUDQGGXULQJRWKHUWDVNVFS 7DEOH  7KH UHJLVWHUHG SHDN OHYHOV UDQJH IURP  WR G% LI D ERWWOH¶V EXUVWLQJ LV VWLOO FRQVLGHUHG DQ LQYROXQWDU\ SURFHVV 7KH H[SRVXUH WLPHV 7& DFFRUGLQJ WR 3)$1'(5 ZHUH LQ





J.M. Hesse / Impulse noise exposures, present in civil and military sectors

WKH UDQJH IURP  WR PV 7KH IUHTXHQF\ PD[LPXP ZDV W\SLFDOO\ KLJKHU UHODWLYH WR WKH LPSXOVHQRLVHHPLVVLRQVGXULQJIRUPLQJSURFHVVHV Table 2: Industrial impulse noise parameters during separating and assembling processes (source: HESSE 1994)



LAImax [dB]

trise TA TB TC [ms] [ms] [ms] [ms]

fM [Hz]

Location of Measurement

Lpeak [dB]

Punching machine 40 t

Ear

121A

98

—

—

—

—

—

BRÜEL (1976)

Guillotine shear (sheet metal manufacturing)

Ear

140

122

—

—

—

0.5–2

6030

HOHMANN (1984)

Pneumatic rivet hammer

0.5 m

135

115

—

—

—

2–8

—

SUVA (1976-79)

Pneumatic rivet hammer

Ear

130–135

—

—

—

—

—

—

HERMANNS und KNOCH (1986)

Pneumatic chisel (foundry cleaning room)

Ear

142

123

—

—

—

—

5000

HOHMANN (1984)

Noise Source

Source

Pneumatic air hammer and chisel

Ear

—

–135

—

—

—

—

—

DÖLLE (1979)

Pneumatic hammer on tar

Ear

130

110

—

—

—

—

12500

HOHMANN (1984)

Stone straightening with hammer and chisel

Ear

137

117

—

—

—

—

2000– 6000

HOHMANN (1984)

Core extrusion machine (without silencer)

2m

—

108–121

—

—

—

—

—

BRULLE (1982)

Core extrusion machine (with silencer)

—

—

92–101

—

—

—

—

—

BRULLE (1982)

Throwing frames into container

—

—

130

—

—

—

—

—

DÖLLE (1979)

Throwing screws and bolts into sheet metal container

1m

—

117

—

—

—

—

—

SCHMIDT (1977)

Blast of an one-litre bottle at the filling machine

Ear

145

133

—

—

—

1–3

800

HOHMANN (1984)

Ramming with tup

Ear

133

116

—

—

—

—

630

HOHMANN (1984)

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors Table 3: Parameters of industrial bang events or short-time impulse noise during assembling processes (source: HESSE 1994) Noise Source

Location of Meaurement

Bolt setting tools

Ear

Lpeak [dB]

LAImax [dB]

139

—

141.8– 118–131 157.8

Bolt setting tools

DIN 45635-34

Bolt setting tools

DIN 45635-34

Bolt setting tools

Ear

Bolt setting tools

Ear

Bolt thrust tools

DIN 45635-34

Bolt thrust tools

DIN 45635-34

133– 157A

—

Bolt driving tools

DIN 45635-34

144– 165A

Pneumatic nail driver

Ear

Pneumatic nail driver

140.8

121.4

trise TA TB TC [ms] [ms] [ms] [ms] — —

—

FM [Hz]

Source

—

1.2

800

SUVA (1976-79)

—

—

0.29– 0.89

—

FRICK und GEINOZ (1980)

—

SCHEFFEL (1989)

—

—

1.07

0–4000

—

— 5–10

—

—

MAUE (1985)

—

—

—

0.1

1600

HOHMANN (1984)

—

—

—

—

—

HESSE et al. (1992)

—

—

—

0.16– 2.47

2000– 3000

VREKE (1982)

—

—

—

— 0.5–1.4

500– 1000

VREKE (1982)

148A

120

—

—

—

—

—

BRÜEL (1976)

Ear

134

108

—

—

—

—

16000

HOHMANN (1984)

Pneumatic nail driver

0.5 m

130–140

—

—

—

—

—

—

MAUE and CHRIST (1980)

Pneumatic nail driver

—

159

—

—

—

—

—

—

DIN EN 458 (1991)

Tack-riveting

—

—

110–115

—

—

—

—

—

SCHMIDT(1980); JEITER (1981)

128–152 110–130 151

—

125–147 103–137

Table 4: Short-time impulse noise parameters from rifles (source: HESSE 1994) Noise Source

Location of Measurement

Lpeak [dB]

LAImax [dB]

Assault rifle (CH)

Left ear

165

—

trise TA TB TC [ms] [ms] [ms] [ms] —

fM [Hz]

Source

—

—

1.4

800

SUVA (1976-79)

0.0001 0.6

—

RATHÉ (1969)

Muzzle bang of standard rifle

7m

154

—

—

(280)

Bullet bang of standard rifle

7m

144

—

—

0.3

—

—

(550)

RATHÉ (1969)

Automatic rifle

1m

171

—

—

0.33 1.0

—

(500)

FORREST (1967)

Automatic rifle

Left ear

162

—

—

0.33 5.0

—

(500)

FORREST (1967)

Rifle G3 (G)

Ear

162.5– 164.0

—

—

—

— 1.6–2.5

—

BRINKMANN (1970)

Rifle K 98

Ear

161.0– 161.5

—

—

—

— 1.9–4.0

—

BRINKMANN (1970)

M-14 Rifle (GB)

Ear

156

—

—

0.25 4.0

—

(650)

FORREST (1967)

.303 Rifle (GB)

Ear

154

—

—

0.23 2.7

—

(720)

FORREST (1967)

Rifle M16 (USA)

1.5 m

160

—

1.0

0.29

—

—

600

PRICE (1974)

Rifle 5.6 mm (former GDR)

Ear

133

—

—

—

—

—

1600

ERTEL (1974)

Rifle AK 74 (former GDR)

Ear

161.6– 165.4

—

—

—

—

0.45– 0.92

—

STRASSER and HESSE (1993)



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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

 Figure 4:

Schematic set-up for the measurement of acoustic and physiological parameters during the firing of a standard weapon

7DEOH VKRZV WKH GHWHUPLQHG SHDN OHYHO YDOXHV /SHDN DORQJ ZLWK WKH VWDQGDUG GHYLDWLRQV DQGWKHH[SRVXUHWLPH7&DFFRUGLQJWR3)$1'(5(DFKILJXUHLVUHSRUWHGIRUERWKLQVLGHDQG RXWVLGHRIWKHHDUPXII Table 5: Acoustic parameters inside and outside of the hearing protection device (HPD) Group 1

Group 2

Group 3

Group 1-3

Lpeak [dB]

164.0

164.0

163.2

163.7

S [dB]

0.5

0.5

0.9

0.8

TC [ms]

0.69

0.69

0.67

0.69

Lpeak [dB]

141.2

140.8

149.5

144.4

S [dB]

2.0

1.6

2.1

4.6

TC [ms]

1.36

1.96

1.32

1.50

'Lpeak = Dreal [dB]

22.8

23.2

13.7

19.3

S [dB]

2.1

1.7

2.1

5.0

Outside of HPD

Inside HPD





J.M. Hesse / Impulse noise exposures, present in civil and military sectors



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able 6: Number N of maximum permissible noise events per day dependent on various attenuation values Group 1

Group 2

Group 3

Group 1-3

Approximately 2,800



N (D = 32.3 dB)

2,834

2,810

3,473

3,050

N (D = 27.7 dB)

983

974

1,204

1,057

N (D = Dreal)

161

123

24

70

N (D = Dreal - S)

99

83

15

22

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

Table 7: Short-time impulse noise from small arms (source: HESSE 1994) Noise Source

Location of Measurement

Lpeak [dB]

LAImax [dB]

trise TA TB TC [ms] [ms] [ms] [ms]

fM [Hz]

Source

Pistol 9 mm

5m

146A

114

—

—

—

—

—

BRÜEL (1976)

Pistol (Practice ammunition)

—

151

—

—

—

—

0.2

1000

BRINKMANN (1975a)

Pistol .22 GB

Right ear

153

—

—

—

1.0

—

—

FORREST (1967)

Pistol .22 lfB

Ear

157

—

—

—

—

—

—

HESSE et al. (1993)

Pistol 9 mm Para

Ear

164

—

—

—

—

—

—

HESSE et al. (1993)

Pistol .45 ACP

Ear

165

—

—

—

—

—

—

HESSE et al. (1993)

Revolver .38 Special

Ear

165

—

—

—

—

—

—

HESSE et al. (1993)

Revolver .357 Magnum

Ear

169

—

—

—

—

—

—

HESSE et al. (1993)

Revolver .44 Magnum

Ear

169

—

—

—

—

—

—

HESSE et al. (1993) HESSE et al. (1993)

Revolver .454 Casull

Ear

168

—

—

—

—

—

—

Signal pistol

1m

169

—

—

—

1.0

0.2

3000

BRÜEL (1976)

Blank gun

0.5 m

151

124C

—

—

1.6

0.5

2000

SUVA (1976-79)

Blank gun

0.5 m

159

127

—

—

—

0.1

—

HOHMANN (1984)

Toy pistol

0.1 m

165

—

—

—

—

0.4

—

BRÜEL (1976)



&RQWUDU\WRWKHEDQJRUVKRUWLPSXOVHQRLVHVIURPZHDSRQVZKLFKDUHQRWRQO\XVHGE\WKH PLOLWDU\EXWDOVRLQQRQPLOLWDU\VLWXDWLRQVHJE\KREE\VKRRWHUVRQILULQJUDQJHVDQGE\ SROLFHRIILFHUV 7DEOHUHSRUWVRQODUJHFDOLEHUZHDSRQVDQGZHDSRQV\VWHPVZKLFKDUHXVHG H[FOXVLYHO\ E\ WKH PLOLWDU\ 3HDN OHYHOV UHDFKHG YDOXHV RI XS WR G% LQ DGGLWLRQ WR DQ H[SRVXUHWLPHZKLFKLQVRPHFDVHVZDVFRQVLGHUDEO\SURORQJHGXSWRPV 

Table 8: Short-time impulse noise parameters from weapon systems (source: HESSE 1994) Noise Source



Location of Measurement

Lpeak [dB]

LAImax [dB]

trise TA TB TC [ms] [ms] [ms] [ms]

fM [Hz]

Source

Bazooka PzF44

1m

181.9

—

—

—

—

0.71

—

BRINKMANN (1982)

Anti-tank gun PzB 84 mm

Left ear

185.9

—

—

—

—

1.5

—

PFANDER (1975)

Cannon 105 mm Tank “Leopard” HEAT Ammunition

Command stand firing tunnel 600 m

147.4

—

—

—

—

105

—

BRINKMANN (1975b)

Cannon 105 mm Tank “Leopard” HESH Ammunition

Tank crew

147.0 through 150.3

—

—

—

—

54–78

—

GÖLNITZ (1971)

Cannon 105 mm Tank “Leopard” APDS Ammunition

Tank crew

156.1 through 166.1

—

—

—

—

30–49

—

GÖLNITZ (1971)

120 mm Mortar Tampella

Gunner’s ear

174

—

—

—

—

10

—

PFANDER (1975)

120 mm Mortar Tampella

Loader’s ear

182

—

—

—

—

5

—

PFANDER (1975)

Tank howitzer PzH 155 mm

20 m

172.5

—

—

—

—

3

30

PFANDER (1975)

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors Table 9: Short-time impulse noise parameters from explosive devices (source: HESSE 1994) Noise Source

5 x 1 kg TNT

10 kg TNT

25 kg TNT

Tunnel blasting

Location of Measurement

Lpeak [dB]

LAImax [dB]

trise TA TB TC [ms] [ms] [ms] [ms]

behind barrier (height: 1,5 m); at 50 m distance

158

—

—

—

—

fM [Hz]

Source

19.6

—

ZDV 90/20 (1980)

at 100 m distance

153

—

—

—

—

23.7

—

ZDV 90/20 (1980)

without shelter wall; at 300 m distance

146

—

—

—

—

15.6

—

ZDV 90/20 (1980)

behind barrier (height: 1,5 m); at 50 m distance

160

—

—

—

—

28.7

—

ZDV 90/20 (1980)

at 100 m distance

158

—

—

—

—

34.6

—

ZDV 90/20 (1980)

without shelter wall; at 300 m distance

151

—

—

—

—

19.4

—

ZDV 90/20 (1980)

behind barrier (height: 1,5 m); at 50 m distance

163

—

—

—

—

39

—

ZDV 90/20 (1980)

at 100 m distance

161

—

—

—

—

39.5

—

ZDV 90/20 (1980)

without shelter wall; at 300 m distance

154

—

—

—

—

25.9

—

ZDV 90/20 (1980)

300 m

168

152C

—

—

—

65

50

SUVA (1976-79)

fM [Hz]

Source HOHMANN (1984)

 Table 10: Short-time impulse noise from firecrackers (source: HESSE 1994)



TC trise TA TB [ms] [ms] [ms] [ms]

Noise Source

Location of Measurement

Lpeak [dB]

LAImax [dB]

“Flashing Thunder”

1m

171

146

—

—

—

0.2

—

Cracker

1m

168

141

—

—

—

0.2

—

HOHMANN (1984)

Balloon bursting

0.4 m

150

128

—

—

—

0.5

—

HOHMANN (1984)

Blank cartridge shooting

—

—

>120

—

—

—

—

—

KLUMPP (1985)

Alarm petard

2m

159

—

—

—

—

—

500

SUVA (1976-79)

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J.M. Hesse / Impulse noise exposures, present in civil and military sectors

 Figure 5:

Construction of bolt setting tools (cartridge-operated fixing tools)

Figure 6:

Noise measurements for powder-actuated tools (bolt thrust tools)



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J.M. Hesse / Impulse noise exposures, present in civil and military sectors



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 Figure 7:

Effect of different influence parameters during bolt setting on the workplace-specific emission value LAImax = 114.6 dB(A) (according to DIN 45635-34) (source: HESSE 1994)

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 Figure 8:

Comparison of different noise rating procedures for hearing damage-risk criteria

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Driving process



Sound pressure level [dB]

Maximum permissible impulse number per work day ________ Rating method

Lpeak

LAImax

LAeq = 85 dB(A)

FRICK / GEINOZ (PFANDER)

LAIm = 85 dB(A)

A

HILTI DX 450, concrete B35, free-field, yellow cartridge, lowest charge

127.4

104.1

7,254

7,943

219

B

HILTI DX 600 N, angle iron on concrete B35, free-field, black cartridge

135.4

114.6

617

707

19

C

HILTI DX 450, angle iron on concrete B35, free-field, black cartridge, highest charge

137.3

119.1

204

251

7

D

HILTI DX 600 N, angle iron on steel, reverberant, black cartridge

142.8

125.5

31

57

1

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

43

Chapter 3

Noise Immissions from Working with Bolt Setting Tools in the Construction Sector M. Rottschäfer, J.M. Hesse, H. Irle and H. Strasser

0 Summary The utilization of bolt setting tools (cartridge-operated fixing tools) which guarantee maximum flexibility and mobility since the worker does not depend on stationary energy supply (via electrical cable or high-pressure air line) has gained substantially in importance for roofing and paneling of industrial buildings with trapeziform profile metal plates. In order to allow the surveillance of work safety requirements and to ensure preventive occupational safety and health, it is essential to find out when, where, and for how long, how often and with which tools, with which bolts and materials, with which power charges, and in which body postures and under which working environment conditions work is carried out. Intensive work analyses showed, e.g., that for the workers’ effective noise exposure, the most relevant tasks are the setting of bolts through metal plates into steel and into scaffolding-like steel constructions with various noise transmission mechanisms. The impulse noise in this context is not only due to a bolt setter’s own work, but also other operators of a team in the immediate surroundings. Because of the large number of bolts which are set in a typical work day (approximately 800) with peak levels between 120 and 150 dB, the additional noise level on a construction site (even at a continuous noise level of approximately 85 dB(A)) is not of much practical relevance for a calculated rating level using the energy equivalence principle. However, the effects of such continuous noise should not be completely disregarded either. The fact that bolt setters oftentimes must lean forward in order to apply the necessary pressure to set a bolt aggravates the problem because of the reduced emission-immission distance which can lead to impulse noise at peak levels which are a danger to the hearing. Additionally, impulse noise can disturb the work course, can be detrimental to work safety, and can be a nuisance. Frequent changes in body posture and varying use of tools almost necessarily leads to varied noise immissions. Thus, there is some debate whether standardized noise emission measuring methods (e.g., according to DIN 45635-34, 1984) are suitable for the reality on a construction site.

1 Introduction In order to determine the noise exposure from working with bolt setting tools in the construction sector, it was examined when and for how long, how and how often, with which tools and which materials, and in which body postures work is carried out. In the past, bolt setting tools were mainly used in the installation trade and in concrete construction. Nowadays, however, powerful hammer drills with appropriate dowel technology have partially replaced bolt setting tools in those fields. However, there are other construction sectors (in particular, profile and trapeziform metal plate assembly) in which bolt setting tools have increased in importance. Profile metal plates are put onto and attached to previously erected steel constructions. There are numerous applications for the galvanized, painted, or plastic-covered trapeziform metal plates. They allow the erection and paneling of manufacturing, office, and administrative buildings. Because such tasks are representative, they were analyzed in a work course study. The “work course study of metal plate assembly” is subdivided into the following parts: • data acquisition for the collection of information, • work characteristics on site, • results of a questionnaire, • rating of noise immissions, • additional workload and its characteristics, • discussion and conclusion (cp. Figure 1).

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M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

Figure 1:

Work course study of metal sheet assembly

With respect to the applications of bolt setting tools, it was found that they are used in all construction trade companies. The information gathering process started with a list of roofing companies in the Yellow Pages category “paneling and flat roof work in industrial applications.” The companies’ answers to the question about the frequency of use of bolt setting guns ranged from “very rarely” over “we have used them before” all the way to “whatever is available.” Oftentimes, the companies made references to larger firms in the category “industrial assembly” and “industrial construction.” Other firms, even though they were listed under “profile sheet assembly,” did not themselves specialize in that line of work. Such firms were in charge of providing estimates, ordering supplies, making calculations, and supervising construction activities. The actual assembly, however, was carried out by subcontractors. Also, large profile sheet manufacturers often have their own assembly departments, i.e., all the work from manufacturing to the finished building is done by just one company. In addition to the practical observation in the construction sector, research, administrative, industrial, and construction institutes were questioned regarding acoustic and work-specific characteristics in the handling of bolt setting tools dependent on work-specific requirements. Publications by the building employers’ liability insurance association refer to potential problems associated with bolt setting tools. Results from real noise level measurements from the use of bolt setting tools on building sites, however, were not available.

2 Interviews The goal of interviewing several workers was to identify certain task characteristics which have an impact on noise exposure in dependence on method-specific parameters during bolt setting. Additionally, physical strain characteristics were considered in the interviews. The resulting questionnaire for the acquisition of acoustic task characteristics during the use of bolt setting tools is shown in Figure 2.

M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

Figure 2:

45

Questionnaire for the recording of acoustic task characteristics during the use of bolt setting tools

The questionnaire was based on well-known methods in the literature (cp. e.g., ROHMERT and LANDAU 1979). Deficits which were discovered via this approach should – from a workphysiological point of view – be considered among design aspects (cp. STRASSER 1982). The tasks are characterized by: • the workplace, • the work course, • the organization, and • the workers.

3 Workplace description The assemblers’ work starts immediately after the pillars and substructures have been completed. On new constructions, the outside walls are typically completed before the roof. That means that it is an outdoor workplace. For the assembly of the wall panels, the workers are on a scaffolding (cp. the left part of Figure 3). The metal plates for roofs are laid out next to each other and one behind the other. Assembly starts at the bottom of the roof and moves from the bottom up. The assemblers either move on the already assembled sheets or the remaining exposed construction of the roof (cp. the right part of Figure 3 and Figure 4). Further profile sheets are transported via the already installed ones and are affixed at the corners. Only then are the sheets finally fixed with setting bolts at pre-marked spots. Dependent on the size of the trapeziform sheet and the area which needs to be covered, there are 2 to 4 workers in an assembly team.

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M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

Figure 3:

Profile sheet metal assembly with bolt setting tools (source: N.N. 1992)

Figure 4:

Sheet metal placement (source: N.N. 1991)

A different assembling company used the following assembly concept for a large-size roof: • After the profile sheets have been put down on the roof construction, a team of four workers distributes the sheets. • After the sheet metal has been aligned and attached, 2 assemblers affix it completely. • Once a number of sheets has been laid out in such a fashion, a second assembly team starts its work. • These 4 assemblers proceed in an identical fashion to that of the first assembly team. With this staggered work method, up to 4 assembly teams are in action dependent on the size of the roof (cp. WEBER no year).

4 Organization course at the construction site The knowledge gained in these studies shows that assembly work is oftentimes not carried out by the design company, but that it is subcontracted. This kind of work division is common in the construction sector. The assembly is carried out by a subcontractor that works independently (and is paid by the design company rather than the customer). The division and number of subcontractors depends on the size of the contract. Due to fluctuations in the number of orders, the dependence on weather, and high turnover, the staffing situation of such subcontractors is often very problematic. A high degree of

M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

47

fluctuation, unreliability, and high absenteeism rates characterize the business. Insufficient instructions and flawed adherence to safety standards during the work with bolt setting tools are often mentioned as causes of accidents on construction sites. Extremely short construction times require the simultaneous use of several firms which can lead to coordination problems, time pressure, and reciprocal endangerment. According to the interviewed employees, bolt setting is considered to be quite practical once appropriate advisement has been received. For both assemblers and management, the independence from an energy source, flexibility, and time reduction are highly important in the use of the setting tools. The time that the completion of a project requires directly influences the workers’ payment as well as the assembly company’s calculations in submitting a proposal. Since the form of the assemblers’ payment often includes piece wages or possibly a bonus, bolt setting is favored over alternative methods such as drilling and affixing by using self-cutting screws. Evaluation of the interview results showed that the “actual noise exposure situation” is characterized by: 1. Number of bolts set A rough estimate indicated by the interviewees for the number of bolts set is approximately 1 bolt per m2 of profile sheet metal. The maximum number of bolts set during an 8-hour work day seems to be up to 150 per hour and assembler. The surveyed companies agreed that approximately 100 bolts per hour and assembler is a good guidance. Thus, the number of noise impulses during an 8-hour work day ranges from 800 to 1200 individual impulses. 2. Additional noise exposure On a construction site, several tasks are carried out simultaneously which leads to additional noise exposure. Other sources of noise are excavators, cement mixers, pumps, pneumatic hammers, compressors, wheel loaders, etc. Additionally, there is frequent traffic to and from the site for the delivery of supplies (cp. Figure 5). The putting down and attaching of metal plates on the steel construction during assembly in particular leads to a noise level increase relative to the existing noise level. If two or more assemblers work with bolt setting tools simultaneously, levels might be increased as well. Furthermore, the noise caused by the bolt setting affects the other workers of the assembly team.

Figure 5:

Noise exposure in the construction sector (source: N.N. 1991)

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M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

5 Noise rating with different methods for the calculation of the maximum permissible number of impulses The tolerable noise exposure can be determined by the maximum number of permissible noise impulses. The rating method presented here refers to an 8-hour work day without taking additional noise into consideration. A sound pressure level Lpeak of 140 dB (which is not unrealistic for the working site) leads to the following permissible numbers: • maximum: 263 impulses according to the energy equivalence principle (LAeq) • minimum: 6 impulses according to the impulse rating method (LAIm) (cp. Table 1). Table 1: Maximum number of permissible impulses per work day based on different rating methods (without additional noise) Sound pressure level [dB]

Maximum permissible number of impulses per work day ___________ Rating method

Lpeak

LAImax

LAeq = 85 dB

PFANDER

FRICK / GEINOZ

LAIm = 85 dB

140

120

263

207

204

6

This may be explained by Table 2. The effect of taking additional noise into consideration is illustrated with an example of one assembler (cp. Figure 6). The noise from a second assembler (at a distance of 2 m) and overall construction site noise from simultaneous work must be added to the noise created by the assembler himself. The number of noise impulses for both assemblers is 800 bolts over an 8-hour work day. For the 3 individual sources of noise, the following rating levels Lr1, Lr2, and Lr3 result: • approximately 96 dB from bolt setting, • approximately 84 dB from the bolt-setting assembler at a distance of 2 m, and • approximately 85 dB from typical construction site noise. The sum of the 3 individual levels (97 dB) shows that even though the additional noises have very high levels, they only add a small amount of just 1 dB to the noise level created by the affected assembler. Working with bolt setting tools does not only lead to aural strain, but also – dependent on the body position relative to the tool – to strain in other regions of the worker’s body. The mainly affected area is the hand-arm system and the shoulder belt and is due to recoil forces of the bolt setting tools as well as pressure forces which workers must apply in order to set bolts. Shocks and vibration caused by the bolt setting on steel constructions also occur. Additionally, the distance between tool and assembler – which is dependent on body position and body posture – has a direct impact on the individual noise exposure (cp. Figure 7).

M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector Table 2: Determination of the maximum number of permissible impulses per work day

49

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M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

Figure 6:

Approximation of the noise immission (Lr) during work with bolt setting tools

Figure 7:

Stress characteristics resulting from the work method

6 Conclusions Due to the varied noise situation on construction sites, an organizational staggering of noise-intensive tasks would be desirable and beneficial to the workers. In real life, however, time pressure often leads to the exact opposite, i.e., additional noise from co-workers. Furthermore, providing the employees with intensive information about noise-preventative behavior would be highly important for the safe long-term use of bolt setting tools. Unfortunately, workers are prejudiced against the use of hearing protection devices. Only continuous and persistent education can change that, but only if the appropriate sensitization and a change in awareness take place first.

M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

51

Organizational measures as well as providing workers with the appropriate information to ensure work safety on a construction site would be desirable, especially in the areas • instruction, • coordination among different companies, • work safety, • sense of duty, • as well as safe behavior despite time pressure. The relevant legal rules are rather comprehensive. For the assembly of profile metal plates, a number of regulations from the German institutions for statutory accident insurance and prevention (the Berufsgenossenschaften - BGs) must be adhered to in addition to the BG regulations governing health and safety at work concerning “noise” (BGV B3) (former Accident Prevention Regulation “Noise” (VBG 121)) (cp. Figure 8).

Figure 8:

Relevant BG regulations governing health and safety at work (BGVs) (in brackets: former Accident Prevention Regulations (VBGs))

Furthermore, in addition to the workers, the surrounding area is negatively affected by noise as well. The issue of noise does often not allow to separate work safety from immission protection (cp. Figure 9). Noise exposures in the construction and assembly sector is covered simultaneously by several entities. Work safety and worker protection is supervised by the German labour inspectorate (Gewerbeaufsicht) as well as German institutions for statutory accident insurance and prevention (Berufsgenossenschaften). The labour inspectorate follows acts which specify various work safety regulations, ordinances, and general administrative regulations. The institutions for statutory accident insurance and prevention are bound by regulations governing health and safety at work and implementation directives (Durchführungsanweisungen). The federal immission control act (Bundesimmissionsschutzgesetz) is relevant for immission and neighborhood protection. In addition to the noise control ordinance (Lärmschutzverordnung), certain tools must comply with various restrictions which govern their import and use. Additionally, there exist numerous administration regulations (Verwaltungsverordnungen). Generally acknowledged rules such as national and international standards, VDI guidelines (guidelines of the German Association of Engineers), and regulations form a unified basis to deal with noise issues. The relevance of acts, regulations, technical rules, etc., must be open to questioning and discussion. If obvious shortcomings are found, improvements should be integrated in order to provide appropriate protection to the affected individuals.

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M. Rottschäfer et al. / Noise immissions from working with bolt setting tools in the construction sector

Figure 9:

Simultaneous responsibilities for noise exposure in the construction and assembly sector

7 References N.N. (1991) Trennwandbau- und Verputzarbeiten. Merkheft der Bau-Berufsgenossenschaften N.N. (1992) Informationsmaterial über Bolzensetzwerkzeuge. HILTI GmbH, München ROHMERT, W. und LANDAU, K. (1979) Das arbeitswissenschaftliche Erhebungsverfahren zur Tätigkeitsanalyse (AET). Handbuch und Merkmalheft, Verlag Huber, Bern STRASSER, H. (1982) Integrative Arbeitswissenschaft – Möglichkeiten und Grenzen arbeitsphysiologisch orientierter Feldforschung. Zeitschrift für Arbeitswissenschaft 36 (4) 201-206 WEBER, H. (no year) Dach und Wand. Hannover Standards, Guidelines, Regulations BGV A1 (VBG 1) Accident Prevention Regulation “General Regulations.” C. Heymanns Verlag, Köln BGV A4 (VBG 100) Accident Prevention Regulation “Preventive Occupational Medical Care.” C. Heymanns Verlag, Köln BGV B3 (VBG 121) Accident Prevention Regulation “Noise.” C. Heymanns Verlag, Köln BGV C22 (VBG 37) Accident Prevention Regulation “Construction Work.” C. Heymanns Verlag, Köln BGV D9 (VBG 45) Accident Prevention Regulation “Work with Shooting Apparatus.” C. Heymanns Verlag, Köln BGV D36 (VBG 74) Accident Prevention Regulation “Ladders and Steps.” C. Heymanns Verlag, Köln DIN 45635-34 (1984) Measurement of Airborne Noise Emitted by Machines; Enveloping Surface Method; Cartridge-Operated Fixing Tools. Beuth Verlag, Berlin



Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

Chapter 4

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 Figure 1:

Energy equivalent level-exposure time constellations with a rating level LArd of 85 dB(A) for 8 hours permitted in the production sector according to national and international standards (cp. Accident Prevention Regulation “Noise” 1990; N.N. 1997)

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 Figure 2:

Schematic test set-up for the sound exposure and the audiometric determination of physiological responses

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 Figure 3:

Representation of a typical individual hearing threshold shift 2 minutes after the exposure (TTS2) at the frequency of maximum threshold shift (4 kHz) as well as at the adjacent lower and upper half-octave frequencies (3 and 6 kHz)

Figure 4:

Typical individual restitution time course TTS(t) after sound exposure with real measured and regression analytical determined characteristic values TTS2 (temporary threshold shift 2 minutes after the exposure) and t(0 dB) (restitution time) in logarithmic time scale







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 Figure 5:

Typical TTS values with respect to restitution time with a linear and a logarithmic time scale, as well as representation of the characteristic values TTS2 and t(0 dB)

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

67

Chapter 5

Hearing Threshold Shifts and Restitution Course after Impulse and Continuous Noise at the Frequency of the Maximum Threshold Shift and the Adjacent Lower and Upper Frequencies H. Strasser, H. Irle and S. Linke

0 Summary Audiometric measurements are typically limited to the determination of the TTS2 value, the Temporary Threshold Shift within 2 min after the exposure at 4 kHz, a frequency which represents the highest sensitivity in most individuals. Measurements during the restitution phase are mostly carried out only in laboratory studies due to the long time that is required. Thus, economic considerations, results from previous studies, and the fact that the C5 dip (characteristic hearing loss at 4 kHz) is considered an undeniable sign of beginning hearing loss due to noise exposure, are all arguments for limiting studies to a single measuring frequency. The objective of this study was, however, to examine whether such threshold shift measurements at a single frequency can reliably capture the main metabolic processes in the inner ear after noise exposure. With respect to practical relevance, this was to be shown for continuous and impulse noise exposures. In a cross-over test design, 10 test subjects (Ss) with normal hearing (ages 28.7 ± 9 years) were exposed to 3 different kinds of sound exposures. In Test Series I (TS I), “White Noise” of 94 dB(A) for 1 h – which is energy equivalent to 85 dB(A) for 8 h – was used. In TS II, an energy equivalent impulse noise exposure with 9,000 impulses (5 ms, each, with a level of 113 dB(A)) in 1-s intervals was used. In TS III, the duration of each impulse was shortened to 2.5 ms, i.e., the applied noise dose was halved. In all 3 test series, the TTS2 values and the hearing threshold shifts’ restitution were determined until the individual resting hearing threshold was once again reached. This was done at the frequency at which the maximum threshold shift occurs as well as at the upper and lower adjacent frequencies. Additionally, using the areas underneath the restitution curves (the IRTTS values) at the 3 frequencies, the total physiological costs which the hearing must “pay” for the preceding exposure were quantified. The results in all Ss and across all 3 experiments were consistent in the sense that the maximum hearing threshold shifts, i.e., the TTS2 values, at the lower and upper adjacent frequencies were substantially lower than at the main frequency. Additionally, the restitution time was shorter. The IRTTS values, i.e., the areas under the restitution curves also displayed substantial differences. On average, the physiological costs of the 10 Ss after exposure to continuous noise at the adjacent frequencies were only approximately 25%, i.e., 1/4, of the physiological costs which were measured at the main frequency. For the energy equivalent exposure to impulse noise, the calculated total threshold shifts were even less than 20%. Exposure to impulse noise which was reduced by half in terms of energy resulted in total hearing threshold shifts at the adjacent frequencies which were even less than 10%. In conclusion, the excitation of the hair cells in the inner ear seems to affect larger areas with continuous broadband noise than with impulse noise. However, measurements of hearing threshold shifts at the frequency of the hearing’s highest sensitivity seem already to capture the main metabolic processes.

1 Introduction Noise exposures with a rating level of 85 dB(A), which do not require the wearing of hearing protectors, cause substantial hearing threshold shifts and it may take hours for them to completely subside. These non-trivial threshold shifts typically occur around 4 kHz as well as at somewhat lower and higher frequencies, which is the hearing’s range of highest sensitivity. According to the halving parameter (exchange rate) q = 3, an increase of 3 dB in exposure level represents the same noise dose if the exposure time is cut in half, that is, 85 dB(A) for 8 h are equivalent to 88 dB(A) for 4 h or 91 dB(A) for 2 h, as well as 94 dB for 1 h. Concerning this physically correct equivalence and conventional rating of sound exposures which, however, has to be regarded sceptically from an ergonomics point of view (cp.

H. Strasser et al. / Hearing threshold shifts and restitution course

68

STRASSER and IRLE 2001, 2003), it is expected that higher levels and correspondingly shorter exposure times do not lead to the same physiological responses. On the one hand, lower physiological and psychological adverse effects can be expected for noise exposures when the exposure duration is reduced according to the 3-dB exchange rate. On the other hand, an increase of the human’s responses might be associated with impulse noise (IRLE et al. 2001), when, e.g., the duration of an energy equivalent continuous noise exposure with a high level is split up into thousands of impulses with a duration of some milliseconds, each. According to prior studies (HESSE and STRASSER 1990) a continuous noise exposure of 113 dB(A) for 45 s, for example, was associated with substantially lower threshold shifts. But an exposure to 94 dB(A) / 1 h led to similar threshold shifts as 85 dB(A) / 8 h. Therefore, noise exposure experiments in the laboratory which have to be relevant for practical conditions can also be carried out with an exposure duration of 1 h instead of 8 hours. In prior studies (cited above) the Temporary Threshold Shifts (TTS2) were measured within 2 min after the exposure – starting from 4 kHz – whereby the temporary hearing threshold shifts were determined at various frequencies. The restitution course was then determined at only one frequency, however: the frequency of the maximum threshold shift. Hearing threshold shifts’ behavior at adjacent frequencies during restitution, however, could not be addressed so far. Thus, in a special test series, an attempt was made to objectify the magnitude of threshold shifts at the main frequency as well as at the adjacent lower and upper frequencies for both continuous and impulse noise exposures; the restitution courses were also examined and compared to those after threshold shifts at the main frequency.

2 Methods Ten Ss with normal hearing participated on 3 days in laboratory noise exposure tests which have been arranged in a cross-over test design. According to the left part of Figure 1, the exposures were administered with high precision by using headphones.

Figure 1:

Schematic test set-up

H. Strasser et al. / Hearing threshold shifts and restitution course

69

The hearing threshold shifts were measured audiometrically in a sound proof cabin (cp. right part of Figure 1). As can be seen in Figure 2, 94 dB(A) for an exposure time tExp of 1 h were administered in Test Series I (TS I) which is equivalent to a LArd value of 85 dB(A). In Test Series II (TS II), the sound exposure consisted of impulse noise at levels of 113 dB(A) which was administered at 1-s intervals over 150 min. The 9,000 impulses of 5 ms, each, correspond with an effective exposure time of 9,000 x 0.005 s = 45 s. Thus, they were energy equivalent to 94 dB(A) for 1 h in TS I or 85 dB(A) for 8 h. In Test Series III (TS III), the 9,000 impulses had a duration of only 2.5 ms, each. With the exchange rate q = 3, it can be calculated that these impulses were energy equivalent to continuous noise of LArd = 82 dB(A). Thus, the noise dose in TS III was only half of the doses in TS I and TS II.

Figure 2:

Schematic representation of the exposures

According to Figure 3, all Ss’ hearing had to conform to the “quality criteria” specified in DIN ISO 4869.

Figure 3:

Criteria for the selection of the test subjects according to DIN ISO 4869

70

H. Strasser et al. / Hearing threshold shifts and restitution course

Figure 4 shows an example of a threshold shift above the individual hearing threshold, with a maximum value of TTS2 at 4 kHz. However, an individual hearing threshold does not necessarily correspond with the mean value of thousands of Ss (that is, the normal hearing threshold).

Figure 4:

Selection of the frequency with maximum threshold shift during the first 2 minutes after the exposure

As can be seen in Figure 5, the hearing threshold shift after the sound exposure was determined not only at the frequency of the maximum threshold shift fm (at 4 kHz), but also at the adjacent lower and upper frequencies fl and fu (3 kHz and 6 kHz).

Figure 5:

Representation of a typical individual hearing threshold shift 2 minutes after the exposure (TTS2) at the frequency of maximum threshold shift (4 kHz) as well as at the next lower and upper frequency (3 and 6 kHz)

Figure 6 shows how at all measuring frequencies, the restitution was measured at predetermined time intervals until the time t(0 dB), that is, until a hearing threshold shift above the resting hearing threshold could no longer be detected. If a linear time axis is chosen for the restitution course, the measured values approximate an exponential function. Using a logarithmic time axis for the restitution course instead, results in a straight line.

H. Strasser et al. / Hearing threshold shifts and restitution course

Figure 6:

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Typical individual restitution time course TTS(t) after a sound exposure with characteristic values TTS2 (temporary threshold shift 2 minutes after the exposure) and t(0 dB) (restitution time) in logarithmic and linear time scale

3 Results Figure 7A shows for TS I the (expectedly high) threshold shifts at the main frequency and their restitution over time for all 10 Ss. With a range of individual measured values from 13 to 30 dB immediately (that is, 2 min) after the sound exposure, and measured restitution times of up to 90 min, regression-analytical characteristic values of 21.7 dB for the mean TTS2 and 85 min for the time t(0 dB) were determined. Figure 7B, in the same set-up, shows measured values for the energy equivalent exposure to 9,000 impulses of 113 dB for 5 ms at 1-s time intervals which corresponds with loud hammering. It can easily be seen that the threshold shifts were less severe and, on average, subsided more quickly. Similar threshold shifts resulted – as shown in Figure 7C – for impulses of half the duration, but the restitution was quicker. The following series of figures from TS I, that is, the continuous noise exposure to 94 dB for 1 h (Figure 8A - 8E) is meant to illustrate that, on the one hand, the Ss exhibit substantial differences between the main and the adjacent upper and lower frequencies; on the other hand, the frequencies at which the maximum threshold shifts occur vary between individuals. While test subject 2 (Figure 8A) had initially exhibited threshold shifts in excess of 20 dB at the main frequency of 4 kHz, which took 60 min to subside, threshold shifts at the adjacent lower frequency of 3 kHz were barely measurable. At the adjacent upper frequency, however, substantial threshold shifts – at least at the beginning of the restitution phase – occurred. Test subject 3 (Figure 8B) only exhibited threshold shifts worth mentioning (of approximately 15 dB) at 8 kHz, which did not completely subside until approximately 1 h after the exposure. In test subject 5 (Figure 8C), TTS values at the adjacent upper frequency of 8 kHz were only marginal. Contrary to that, test subject 7 (Figure 8D) exhibited virtually no hearing threshold shifts at the adjacent lower frequency. Finally, in test subject 10 (Figure 8E), the TTS values of the adjacent upper frequency were rather low relative to those at the main frequency. Just like a regression graph was added in this and the previous figures for the main frequency, these curves also were determined for the adjacent frequencies. Figure 9 shows the 3 courses as well as the calculated characteristic values of the variables TTS2 and t(0 dB), that is, the maximum threshold shifts and restitution times, both as the actually measured values and the regression-analytically determined values.

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Figure 7:

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Individual and mean restitution time course TTS(t) for all 10 subjects after the exposure to “94 dB(A) / 1 h White Noise" A, “113 dB(A) / 9,000 x 5-ms Impulses” B, and “113 dB(A) / 9,000 x 2.5-ms Impulses” C. Measurements taken at the frequency with maximum threshold shift.

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Figure 8:

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Restitution time course TTS(t) with characteristics TTS2 and restitution time t(0 dB) at the frequency with maximum threshold shift as well as measured values of next lower and upper frequency of the Subjects 2 (A), 3 (B), 5 (C), 7 (D), and 10 (E) after the exposure to “94 dB(A) / 1 h White Noise”

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Figure 9:

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Measured values and regression functions TTS(t) from the time 2 min after the exposure until the time t(0 dB)real at the frequency of maximum threshold shift and the next lower and upper frequency

In order to quantify the total “physiological costs,” which the hearing must “pay” in the form of threshold shifts for the preceding exposure, it is straightforward to determine the area below the restitution curves. Figure 10 shows such a determination as the integral over the restitution curve (TTS(t)); that is, all hearing thresholds from 2 min after exposure to t(0 dB) are added up. The numeric value of this (green) area with the unit of measurement dB x min is a global characteristic value for the hearing’s physiological reactions.

Figure 10: Exemplary representation of the IRTTS (Integrated Restitution Temporary Threshold Shift)

Since the regression-analytically determined times t(0 dB)reg. do not coincide exactly with the actually measured values t(0 dB)real – as shown in Figure 11, it is necessary to adhere to the following methods in order to accurately determine the physiological costs in the integration process. In case the actually measured t(0 dB) values are less than the regressionanalytically determined values, the evaluation only extends to the time t(0 dB)real. If the calculated regression curve reaches the horizontal time axis before the actually measured t(0 dB) values, a negative area would result.

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Figure 11: Determination of the “Physiological Cost” IRTTS dependent on the position of the times t(0 dB)real and t(0 dB)reg.

In order to avoid that threshold shifts – which did persist until after the regressionanalytically determined time t(0 dB)reg. – are evaluated as a negative number, the evaluation only extends to t(0 dB)reg. As can be seen in Figure 12, the resulting areas underneath the restitution curves measured 889 dBmin at the main frequency of 6 kHz, whereas the respective value for the adjacent upper frequency of 8 kHz was only 100 dBmin and 303 dBmin for the adjacent lower frequency. This means that the threshold shifts at the adjacent frequencies are only 34.0 % or even only 11.3 %, respectively, of that at the main frequency.

Figure12: “Physiological Cost” IRTTS as the integral of the regression function TTS(t) from the time 2 min after the exposure until the time t(0 dB)real at the frequency of maximum threshold shift and the next lower and upper frequency

Figure 13A shows the IRTTS values for all Ss in a vertical bar chart at the adjacent lower and upper frequencies as a percentage of the IRTTS values at the frequency at which the maximum threshold shift occurs. The 10 Ss’ mean, shown in the right part of the figure, shows that the adjacent frequencies only lead to approximately 25 %, that is, approximately 1 /4 of the physiological costs of the main frequency.

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Figure 13: Integrated Restitution Temporary Threshold Shift (IRTTS values) at the next lower and upper frequency in % of the IRTTS values at the frequency of maximum threshold shift of TS I (A), TS II (B), and TS III (C)

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Test Series I showed for continuous noise that the threshold shifts at the adjacent frequencies are substantially lower than at the frequency of the hearing’s highest sensitivity; this result was confirmed with impulse noise. As can be seen in Figure 13B, the “physiological costs” of energy equivalent impulse noise at the adjacent frequencies are no more than approximately 20 % of those at the main frequency. With the short impulses in TS III, where the noise dose was only half of that in TS I and TS II, the sum of the hearing threshold shifts at the two adjacent frequencies were even less than 10 % – as can be seen in Figure 13C. In some Ss (Ss 2, 3, and 9), hearing threshold shifts could only be detected at one frequency. Figure 14 summarizes the results of the 3 test series; the hearing’s reactions at the main frequency are much more pronounced than at the adjacent frequencies.

Figure 14: Integrated Restitution Temporary Threshold Shift (IRTTS values) at the next lower and upper frequency in % of the IRTTS values at the frequency of maximum threshold shift

4 Discussion Audiometric measurements of TTS2 values and especially the following restitution course of the hearing threshold require quick action in order to address the conflicting goals of precision and time. While it is possible to increase the precision of audiometrically determined values through repeated measurements, TTS values can change very rapidly. This is one of the reasons why many studies only determine the frequency at which the maximum threshold shift occurs via several measurements; subsequently, the restitution is examined at this frequency exclusively (cp. DIEROFF 1975). The results of this test series are a justification of this procedure and stress that the main effect of a threshold shift can be captured that way. Beyond this justification, it is of interest why the adjacent frequencies – apparently in dependence of the type of sound exposures – play a rather minor role in the metabolic processes in the inner ear. The very small (in percentage terms) IRTTS values of the adjacent frequencies may be due to the generally rather low threshold shifts of impulse noise with very short impulses. The somewhat longer impulses during the other exposure to impulse noise lead to somewhat larger and more persistent threshold shifts at the main frequency with larger effects in the adjacent frequencies.

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With the generally largest and most persistent threshold shifts after continuous noise, the reactions in the adjacent frequencies were also more pronounced. In addition to this assumed dependence on the overall magnitude of the TTS-reaction, impulse noise generally seems to lead to narrowly defined (in terms of frequency) effects on hair cells in the inner ear. In addition the impulse duration certainly plays a substantial role as well. Short “clicks” are apparently focussed on the immediate neighborhood of the basilar membrane. A comparison of the results of this study with FUDER and KRACHT’S results from several decades ago (shown in Figure 15) clearly shows that adjacent upper and lower frequencies exist (both in the rise and the descent of temporary hearing threshold shifts) which are distinctly different from the main frequency – which was 4 kHz in this study.

Figure 15: Growth and restitution of Temporary Threshold Shifts (TTS) after exposure to broadband continuous noise with a level of 100 dB(A) (mean values of 25 subjects) (according to FUDER and KRACHT 1972)

The results in this figure stem from a substantially higher broadband noise exposure of 100 dB(A) with rating levels of LArd = 88 dB(A) (top), 94 dB(A) (middle), and even 97 dB(A) (bottom); threshold shifts at the adjacent upper frequency (6 kHz in this case) were almost as high as at the main frequency. Thus, it seems obvious that the documented excitation of the basilar membrane would occur in larger areas with increasing exposure levels. Overall, it seems safe to assume that the main effects of threshold shifts due to sound exposure can be appropriately captured via audiometric measurements at one frequency, at least as long as no overcharge of the hearing due to extremely high levels (approximately 100 dB and more) occurs. It is not possible, however, to further explore this matter since ethical considerations prohibit experiments with sound exposures of such high levels.

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5 References DIEROFF, H.G. (1975) Lärmschwerhörigkeit. Urban und Schwarzenberg, München/Berlin/Wien FUDER, G. und KRACHT, L. (1972) Zur gehörschädigenden Wirkung quasistationären Lärms auf den Menschen. Dissertation, Technische Universität Dresden HESSE, J.M. und STRASSER, H. (1990) Hörschwellenverschiebungen nach verschieden strukturierter energieäquivalenter Schallbelastung. Z.Arb.wiss. 44 (16 NF) 3, 169-174 IRLE, H.; HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposure: Temporary Threshold Shifts. In: KARWOWSKI, W. (Ed.): Int. Encyclopedia of Ergonomics and Human Factors. Volume II, Taylor & Francis, London and New York, 1050-1056 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: KARWOWSKI, W. (Ed.): Int. Encyclopedia of Ergonomics and Human Factors. Volume I, Taylor & Francis, London and New York, 516-523 STRASSER, H. and IRLE, H. (2003) Conventional Measurement, Assessment, and Rating of Sound Exposures – A Critical Review from an Ergonomics Point of View. Journal of Ergonomic Study 5 (1) 49-58 Standards, Guidelines, Regulations DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 6

Hearing Threshold Shifts and their Restitution as Physiological Responses to Legally Tolerable Continuous and Impulse Noise Exposures with a Rating Level of 85 dB(A) H. Strasser, J.M. Hesse and H. Irle

0 Summary In traditional standards, rules, and safety regulations environmental stress is assessed by connecting intensity and duration by means of a multiplication, i.e., a mutual settlement of high workload and short duration and a lower intensity exposed for a correspondingly longer duration. This principle is based on the hypothesis that equal energy or dose – also known for dynamic muscle work as the principle of equal work – involves equal human related effects. But such guidelines are more closely related to physics than to physiology. Yet, ergonomics and occupational medicine have to concern themselves not only with physics but principally with physiological cost as well as short-term and long-term effects on humans in the domain of strain. Experimental data from laboratory studies in noise elucidate that physiological cost is not congruent with the principle of equal energy. 10 subjects between the ages of 23 and 43 participated in a cross-over trial laboratory study. They were exposed to 94 dB(A) for 1 h, 113 dB(A) for 45 s, and impulsive noise with a mean level of 113 dB(A) for 250, 100, 25, and 5 ms. The 6 noise situations were all energy equivalent to 85 dB(A) / 8 h. Temporary hearing threshold shift and its recovery was measured at 4 and 6 kHz, respectively. A shorter exposure duration and a corresponding increase in noise level lead to a significant decrease in the TTS2 as well as in the restitution time. Yet, the fractionation of continuous noise at 113 dB(A) for 45 s into an energy equivalent number of impulses – which lasted only 5 ms in the end – was associated with a considerable and highly significant increase in the TTS2 and in the required restitution time up to 10 hours. Finally, the integrated restitution temporary threshold shift (IRTTS), representing overall physiological cost, revealed a risk for 5-ms impulses that is 2.5 times higher than that of 94 dB(A) / 1 h or 85 dB(A) / 8 h.

1 Introduction Both ergonomists and practitioners responsible for occupational health and safety in a company normally use and appreciate indices of workload and environmental exposures presented in the simplest possible figures and numbers. Therefore, in traditional standards, rules, and safety regulations, the physical environment is normally rated in 8-hour based mean values via connecting intensity and duration of stress by means of a multiplication, i.e., a mutual settlement of high load within a short exposure time and a low stress height within a longer lasting exposure. This principle is well-based on the experience that a low workload can be tolerated for a longer duration than a high workload. But does this confirm the hypothesis that equal energy or dose, e.g., represented in the noise rating level, also involves equal short or long-term human responses? Standards and conventional guidelines for occupational health and safety are more closely related to physics than to physiology (N.N. 1986, N.N. 1990, N.N. 1998). Yet, in order to really protect man at work, ergonomics must be much more concerned with physiological costs of work and environmental stress than with physical principles of equal energetic dose (cp. STRASSER and HESSE 1993).

2 Temporary and permanent threshold shifts High noise levels, at least, lead to Temporary Threshold Shifts (TTS) which are dependent on the preceding exposure and last for varying time periods. If such TTS are not subsided completely at the beginning of a subsequent noise exposure, the hearing threshold shifts can

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become permanent. Moreover, high noise exposures beyond a level of 120 dB inhere a high risk for acute damage in the inner ear. Permanent Threshold Shifts (PTS) as the long-term aural effects normally do not occur after just days, weeks, or months of risky noise exposures but are the then irreversible consequences of noise exposures over years. PTS can be regarded as the accumulated sum of the temporary threshold shifts which still remain 16 h (i.e., 1000 min) after the end of an 8-h working day. Therefore, they may be represented in the equation PTS = 6 TTS1000. As TTS can be regarded as a predecessor of PTS, this measure has some prognostic value and is used in ergonomics to evaluate sound exposures with respect to “physiological costs” the ear has to pay for noise with varying levels, exposure times, frequencies, and time structure (e.g., continuous noise, impulse noise).

3 Audiometric measurements Figure 1 shows what can be measured audiometrically when subjects have been exposed to noise. The lower curve represents the hearing threshold of otologically normal young subjects. It means, e.g., that a minimum signal amplitude of about 4 dB is necessary to hear a 1 kHz tone. Test tones with a lower frequency have to be increased in level according to the hearing threshold to, e.g., about 10 dB at 250 Hz or even more than 60 dB at 16 Hz, and lowered to negative values (below 0 dB) in the area of about 4 kHz which represents the most susceptible frequency range of the human ear.

Figure 1:

Hearing threshold of otologically normal young subjects (lower line), example of a resting hearing threshold (middle line) and threshold shift of an individual after a noise exposure (upper curve) with a maximum in the area of 4 kHz, measured within 2 min after the end of the sound exposure

As the normal hearing threshold is the result of averaging over hearing thresholds of a representative large group of subjects, the individual hearing threshold of a subject must not be congruent with this curve but can take its course below or (as shown in Figure 1) some decibels above the normal threshold. When the physiological responses of noise exposures have to be quantified it is always necessary to determine the individual resting hearing threshold as a reference base. After an acoustic exposure, threshold shifts can be measured in a standardized procedure as represented in the upper curve of Figure 1. Normally within 2 minutes after the end of an exposure the maximum threshold shift has to be quantified in dB audiometrically. It is advisable to concentrate on the maximum relative hearing threshold shift

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above the individual resting hearing threshold (in this case at 4 kHz). This parameter, the socalled TTS2, is a classic characteristic value of audiometric examinations (cp. IRLE et al. 2001). Furthermore, it is advisable with this frequency of the maximum threshold shift to monitor the restitution of the hearing threshold shift (back to the resting threshold) at exactly predetermined points in time (cp. Figure 2). This point in time t(0 dB) is also an important characteristic value of the acoustic strain analysis. When a linear time scale is used, the shape of the restitution of a temporary hearing threshold shift resembles an exponential function. If, however, it is plotted against a logarithmic time scale (cp. the block in the upper right corner of Figure 2), the regression function TTS(t) is a straight line.

Figure 2:

Example for registered TTS values for the recovery period with a linear and a logarithmic time scale as well as representation of the regression function TTS(t)

In order to allow an overall assessment and a comprehensive statistical analysis, the area under the regression line has to be determined, as can be seen in Figure 3.

Figure 3:

Exemplary representation of the IRTTS (Integrated Restitution Temporary Threshold Shift)

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This Integrated Restitution Temporary Threshold Shift (IRTTS) is computed as the integral of the regression function TTS(t) from 2 min after the exposure to the point t(0 dB). The IRTTS is a numeric value for the total threshold shift (in dB x min) which has to be “paid” by the hearing in physiological costs for the exposure. This global characteristic value is similar to the sum of work-related increases of heart rate which can be measured after finishing dynamic muscle work until total recuperation from cardiovascular stimulation. In the following, results of comprehensive investigations will show that essentially varying physiological responses to equally rated noise exposures do exist.

4 Physiological costs associated with continuous and impulse noise with the same amount of energy During revisions of Occupational Health and Safety Guidelines in Europe and the US (N.N. 1986, N.N. 1990, N.N. 1998), noise exposures exceeding a rating level of 85 dB(A), and at least linearly evaluated noise levels of more than 140 dB (or levels LAI > 130 dB(A)) have been declared as harmful to hearing (cp. Figure 4).

Figure 4:

Conventional noise rating according to the principle of equal energy with a tolerable rating level LArd of 85 dB(A)

Yet, impulse noise with levels below that is not necessarily considered critical, as long as the daily noise dosage, resulting from the number of single impulses and their respective exposure times, does not exceed an energy equivalent rating level of 85 dB(A). This applies for, e.g., 100 x 1-ms impulses of 140 dB, 1,000 x 1-ms impulses of 130 dB, 10,000 x 1-ms impulses or 2,000 x 5-ms impulses of 120 dB or also 9,000 x 5-ms impulses of 113 dB. For gun fire the same energy of a rating level of 85 dB(A) for 8 h is inherent in 1 impulse of 160 dB with a duration of 1 ms (cp. upper left part of Figure 4) (see also N.N. 1987). However, from an ergonomics point of view, the equating of impulse and continuous noise (on the basis of the 3-dB exchange rate) must be questioned (see, e.g., STRASSER and IRLE 2001). The energy equivalent measuring and rating procedure assumes that noise with the same energy inheres the same long-term damage.

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According to Figure 5 this should be valid for 85 dB(A) / 8 h and, e.g., 94 dB(A) / 1 h, as well as 113 dB(A) / 45 s and for the splitting-up of this short-time continuous exposure into energy equivalent series of impulses with the same level of 113 dB(A). Eighty-five dB(A) for 8 hours or energy equivalent exposures are still tolerable for the production sector according to almost all international standards (cp. N.N. 1997).

Figure 5:

Sound pressure levels of different durations leading to an equal rating level (in this case 85 dB(A)) when applying the “3-dB exchange rate”

For safety purposes and with respect to ethical and moral aspects, and to absolutely insure that no harm is done to test subjects, acutely “dangerous” exposures of impulse noise with levels over 120 dB(A) never should be provided in laboratory studies, but 113 dB(A) is still considered acceptable. According to MILLER (1974), similar threshold shifts can be expected from 94 dB(A) / 1 h and 85 dB(A) / 8 h. Therefore, for economical reasons, studies with 94 dB(A) over 1 h can be carried out instead of providing exposures over 8 h. In cross-over trial laboratory studies 10 subjects (Ss) were exposed to 94 dB(A) for 1 h, 113 dB(A) for 45 s, and impulse noise with a level of 113 dB(A), namely as shown in Figure 6, • 180 impulses for 250 ms, each, • 450 impulses for 100 ms, each, • 1,800 impulses for 25 ms, each, and, finally, • 9,000 impulses for 5 ms, each. Five-ms impulses with noise levels between 110 and 120 dB are common in practice, e.g., while hammering metal plates. All impulses were separated by 3-s noise intervals so that test duration lasted up to 7 h 30 min for the 5-ms impulse exposure. Simultaneous to the acoustic exposure of the Ss via headphones, the noise exposure was also checked with an identical pair of headphones with an artificial head measuring system (cp. Figure 7). For the audiometric measurements which have been described before, the Ss were placed in a sound proof cabin in order to reduce disturbing outside noise and guarantee the replication of environmental conditions. For details see HESSE et al. (1994) and IRLE et al. (2001).

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Figure 6:

Exposures TS I through TS VI with a rating level LArd of 85 dB(A)

Figure 7:

Schematic test set-up for noise exposure and audiometric measurements in a sound proof cabin

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The upper part of Figure 8 shows the physiological responses to the exposure of 94 dB / 1 h: the measured values of all subjects, the arithmetically averaged values, as well as the regression line. Also, the statistically calculated data are summarized next to the real measured values in the upper right corner. For continuous noise of 94 dB / 1 h, e.g., the regression function TTS(t) is 28.5 - 12.45 x log t, the TTS2 = 24.8 dB which was calculated via this regression function, and the length of time t(0 dB) it took to reach the resting hearing threshold is 195 minutes. The middle part of Figure 8 shows the results of the short-time continuous noise exposure of 113 dB for 45 seconds. It can be seen that an increase of the level of continuous noise together with a corresponding shortening of exposure time leads to a substantial weakening of physiological responses. The TTS2 values are essentially lower and the restitution times are substantially shorter. The lower part of Figure 8 represents the restitution after the exposure to 180 impulses of 250 ms, each, and shows that the physiological responses already have the tendency to be higher than those resulting from continuous noise at the same level. Both TTS2 – with values around 15 dB – and the restitution time – with values up to 100 min – are increased. This tendency becomes even more pronounced with 100-ms impulses (cp. upper part of Figure 9), whereby the threshold shifts reach values of more than 18 dB and last on average up to more than 300 min. When the length of the impulses is reduced to 25 ms (cp. middle part of Figure 9), the physiological cost again grows substantially. Finally, 9,000 5-ms impulses result in an essential increase of physiological costs. As can be seen in the lower part of Figure 9, the TTS2 exceeds 20 dB and the restitution time is up to 600 min, i.e., 10 h. Figure 10 shows all smoothed restitution time courses TTS(t) arranged together which reveal a substantial increase of physiological costs associated with the splitting-up of continuous noise into shorter and shorter impulses. If the damage-risk based on the energy equivalence principle – with its roughly comparable consequences of strain – is accepted, then the position of the regression lines for all test conditions would have to be at least similar. This, however, not at all can be confirmed by the results of the investigations. The overall physiological costs in the form of IRTTS also differ greatly with varying exposures. The ratio of IRTTS resulting from the two extremes 5ms impulses (2,473 dBmin in TS VI) and the energy equivalent exposure to “113 dB(A) for 45 s” (147 dBmin in TS II) amounts to 16:1. According to the results of significance tests, the consistent increases of physiological costs are in a strong causal-deterministic relation with the shortening of the length of the impulses. In order to quantify these results with regard to the damage-risk based on the energy equivalence principle, the IRTTS values of all five test series with 113 dB were standardized to the total physiological costs resulting from the exposure to 94 dB for 1 hour (TS I) (see Figure 11). Values of this ratio as a “risk factor” > 1 thus stand for an increased danger whereas values < 1 mean a lower risk for hearing. Compared to continuous noise based on IRTTS of 94 dB / 1 h, e.g., the risk with 5-ms impulses is 2.5 times higher. Compared to continuous noise of 113 dB / 45 s, the risk with 5-ms impulses is even 16 times more dangerous. The results of prior studies (cp. HESSE and STRASSER 1990), which were carried out with other subjects and with continuous noise of 100 dB / 15 min, 110 dB / 1.5 min, and 113 dB / 45 s, also energy equivalent to 94 dB / 1 h or 85 dB / 8 h, confirm the high reliability of the results of the studies; e.g., the IRTTS values for 94 dB(A) / 1 h are almost identical.

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Figure 8:

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Individual and mean restitution time courses for all 10 Ss after the exposures to 94 dB(A) / 1 h (n), 113 dB(A) / 45 s (o), and 113 dB(A) / 180 x 250 ms (p)

H. Strasser et al. / Hearing threshold shifts and their restitution as physiological responses

Figure 9:

Individual and mean restitution time courses for all 10 Ss after the exposures to 113 dB(A) / 450 x 100 ms (q), 113 dB(A) / 1800 x 25 ms (r), and 113 dB(A) / 9000 x 5 ms(s)

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Figure 10: Restitution time courses TTS(t) with characteristics TTS2 reg., t(0 dB)reg., and physiological cost IRTTS as well as symbolic labeling of significance levels for the differences between the responses to the exposures with equal exposure level Lm = 113 dB(A) and exposure time tExp (n x tImp) (according to the one-tailed WILCOXON-test) (source: STRASSER et al. 1995)

Figure 11: Hearing damage-risk criterion of energy equivalent continuous and impulse noise exposures determined via the comparison of IRTTS values with IRTTS (94 dB(A) / 1 h) = 1.00 (source: HESSE et al. 1994)

5 Conclusions The results presented have shown that energy equivalent impulse noise exposures that differ in their time structure lead to quite different physiological costs. Whereas the difference in aural strain from impulse noise compared to continuous noise does show fewer substantial effects in the temporary threshold shift two minutes after the end of the exposure (TTS2), the effects are distinctly pronounced in the different restitution times, i.e., the strain compensation.

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Taking this into consideration, the IRTTS as an integral parameter proved to be an indicator for potentially long-term hearing-physiological damage more valid and reliable than commonly used TTS2. Because reversible auditory fatigue is always of certain prognostic value for irreversible hearing impairments, the results disprove the validity of the damage-risk based on the energy equivalence hypothesis. Especially in the areas close to the lower and upper end of the exposure continuum which is still legally allowed (cp. Figure 4), it must be explicitly stated again that hearing cannot be energy equivalent. The results reported show that impulse noise can lead to considerably prolonged restitution times. Therefore, the purely energy equivalent determination of the rating level can lead to an underestimation of latent problems so that over time a reversible TTS can evolve into a permanent threshold shift. This is also of importance for the acoustic design of break rooms for noise-exposed workers. Conditions which allow an undisturbed restitution of hearing should always be present. In summary, it has been shown that the levelling of impulse noise to the usually lower level of continuous noise in the new version of the Accident Prevention Regulation “Noise” (1990) which follows the European directive on the protection of workers from the risk related to noise at work (1986) leads to a distinct deterioration of occupational safety and health. From an ergonomics point of view it is simply unacceptable that humans probably have to suffer damage first before the consequences of such an improper adjustment for the European common market are considered. These considerations will most likely come too late for those who are directly affected. Hearing impairment is irreversible; it should be possible, however, to revise political and economic decisions for the sake of a humane environment.

6 References HESSE, J.M. und STRASSER, H. (1990) Hörschwellenverschiebungen nach verschieden strukturierter energieäquivalenter Schallbelastung. Z.Arb.wiss. 44 (16NF) 3, 169-174 HESSE, J.M., IRLE, H. und STRASSER, H. (1994) Laborexperimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Z.Arb.wiss. 48 (20 NF) 4, 237-244 IRLE, H., HESSE, J.M. and STRASSER, H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A): Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. Int. Journal of Industrial Ergonomics 21, 451-463 IRLE, H., HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposure: Temporary Threshold Shifts. In: KARWOWSKI, W. (Ed.) International Encyclopedia of Ergonomics and Human Factors. Volume II, Part 7, Environment, Taylor & Francis, London, New York, 1050-1056 MILLER, J.D. (1974) Effects of Noise on People. J. Acoust. Soc. Am. 56 (3) 729-764 N.N. (1987) Effects of Impulse Noise. NATO Document AC/243 (Panel 8/RSG 6) D/9, Final Report of the Research Study Group on the Effects of Impulse Noise N.N. (1997) Technical Assessment of Upper Limits on Noise in the Workplace – Final Report. Approved by the International Institute of Noise Control Engineering. Noise/News International, 203-216 N.N. (1998) Criteria for a Recommended Standard – Occupational Noise Exposure, Revised Criteria 1998. DHHS (NIOSH) Publication No. 98-126. U.S. Department of Health and Human Services, Cincinnati, OH STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Workload. Archives of Complex Environmental Studies 5 (1/2) 9-25 STRASSER, H., HESSE, J.M. and IRLE, H. (1995) Hearing Threshold Shift after Energy Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and P.C. CHAMPNEY, (Eds.) Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, London/New York/Philadelphia, 241-248 STRASSER, H., IRLE, H. and SCHOLZ, R. (1999a) Physiological Cost of Energy-Equivalent Exposures to White Noise, Industrial Noise, Heavy Metal Music, and Classical Music. Noise Control Engineering Journal 47 (5) 187-192

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STRASSER, H., IRLE, H. and SCHOLZ, R. (1999b) Hearing Risks Associated with Heavy Metal-Music, Classical Music, and Industrial Noise. In: G.C. LEE (Ed.) Advances in Occupational Ergonomics and Safety III. IOS Press Ohmsha, Amsterdam/Berlin/Oxford/Tokyo/Washington DC, 342-352 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: KARWOWSKI, W. (Ed.). International Encyclopedia of Ergonomics and Human Factors, Volume I, Part 3, Performance Related Factors, Taylor & Francis, London, New York, 516-523 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln Directive on the Protection of Workers from the Risk Related to Noise at Work (86/188/EEC) (1986) Richtlinie des Rates vom 12. Mai 1986 über den Schutz der Arbeitnehmer gegen Gefährdung durch Lärm am Arbeitsplatz (86/188/EWG). Amtsblatt der Europäischen Gemeinschaft Nr. L, 28-34

Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 7

Physiological Costs of Energy Equivalent Exposures to Continuous and Additional Energetically Negligible Noise H. Irle, J.M. Hesse and H. Strasser

0 Summary In industry continuous or impulse noise does not occur exclusively; rather it is a combination of both. If low-level continuous noise or impulse noise (below 120 dB) is added to an already existing high-level continuous noise this often numerically causes no essential increase in the rating level. Yet, it cannot be expected that also aural strain of these exposures is always negligible. Therefore, in a cross-over test series, ten male subjects (Ss) were exposed to white noise of 94 dB(A) for 1 hour (TS I), energy equivalent to an 8-h rating level LArd of 85 dB(A). In a second test series (TS II) the same exposure was combined with 900 energetically negligible 5-ms impulses with a noise level of 113 dB(A) which increased the rating level by only 0.4 dB. The noise exposure of TS I and TS II was followed by an idealized resting phase in a soundproof cabin. In a third test series (TS III) the continuous noise of 94 dB(A) / 1 hour was followed by 3 hours of white noise at 70 dB(A). Such an additional load increases the LArd by merely 0.1 dB to 85.1 dB(A). In all three test series, the noiseinduced temporary threshold shift (TTS2) and its restitution were measured. The continuous noise exposure of 94 dB(A) for 1 hour was associated with a TTS2 of around 20 dB which disappeared completely after about two hours. The additional impulse noise caused a small increase in the TTS2 and a prolongation of the restitution time. The maximum mean temporary threshold shift for the group increased only slightly (from 22.5 to 25.9 dB, which nevertheless can be statistically proven at a significance level of p t 0.99). Yet, more importantly, the restitution time increased from 126 min to 175 min, i.e., 3 h, which can be statistically proven at a significance level of p t 0.95. The TTS2 values of TS III did not differ significantly from those resulting from TS I. That was expected as the conditions up to that point in time were identical. But due to the additional subsequent exposure, the mean restitution time increased considerably from 126 min up to 240 min (4 h). The mean total physiological cost represented by the Integrated Restitution Temporary Threshold Shift (IRTTS) increased in TS II by approximately 40 % and in TS III even by 140 %.

1 Introduction A noise stress of 85 dB(A) for 8 hours and linearly evaluated noise levels up to 140 dB are still a tolerable environmental load in the production sector according to almost all national Working Places Regulations and Accident Prevention Regulation “Noise” (1990). But it is scientifically proven that noise exposures – even if they do not exceed the above-mentioned rating level – are often annoying, disturbing, and performance-reducing, and, furthermore, they cause considerable hearing threshold shifts. As a result of a continuous noise of 94 dB(A) for 1 hour – energy equivalent to 85 dB(A) / 8 h – hearing threshold shifts (TTS2) of approximately 20 to 25 dB must be expected immediately after the exposure. Usually, the restitution time for these threshold shifts is at least 2 hours. The sum of the temporary threshold shifts which can be audiometrically monitored represents a measure of the “physiological cost” of noise, for which the organism – in addition to the psychological effects – must “pay.” Such hearing threshold shifts, if and when recovery is completed before another acoustic exposure occurs, still may represent a “normal” fatigue of hearing. Yet, a considerable increase in physiological cost was shown in various studies when, e.g., continuous noise of high intensity and short exposure time was split up into impulses with a duration in the range of milliseconds. After an exposure to 5-ms impulses of 113 dB(A) – energy equivalent to 85 dB(A) / 8 h – the restitution time was greatly prolonged (up to ca. 10 h) (cp. HESSE 1994; STRASSER et al. 1995), so that a daily exposure to such noise can eventually lead to a Noise Induced Permanent Threshold Shift (NIPTS). The same may occur

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if recovery from a temporary threshold shift as a response to continuous noise cannot take place in quiet surroundings. That happens in situations where, e.g., a continuous noise exposure with a limited duration is followed by a lower noise of longer duration. Such an additional load does not contribute essentially to the rating level which is calculated only on the base of the energy principle. Often, impulse or continuous noise does not occur exclusively; rather, it is a combination of both. There are relatively few studies, however, that deal with the effects of such noise conditions. COHEN et al. (1966) and WALKER (1972) found that the combination of continuous noise and impulse noise may even considerably reduce the TTS. This effect is most probably a result of the repeated stimulation of the stapedius reflex by the superimposed impulses. In case of a shorter (10 min) duration, LUTMAN (1972) found no apparent reduction of the TTS values. KUNDI et al. (1982) measured a TTS-reduction due to the superimposed impulses only at the repetition rate of 4 impulses per second (ips), whereas no such effect was found at a rate of 0.25 ips. The combined noise exposure showed lower TTS values than the respective continuous noise when presented alone. However, statistically significant differences were only found at a 30 min exposure (KUNDI et al. 1984). After a 30 min exposure, the TTS values from the combined conditions steadily approached the TTS values which were produced by their respective continuous component. The authors concluded that, seemingly, the stapedius reflex adapts after 30 min combined exposure. From studies on combined exposures in chinchillas, HAMERNIK et al. (1974; 1981) found that the combined exposure may produce a greater hearing loss than each component alone, but obviously only at extremely high peak levels of the impulses. Therefore, on the one hand, it was the objective of this study to examine the above described sometimes contradicting results more closely, using noise exposures as they are typical in reality. On the other hand, it was the aim of this study to determine whether a legally permissible situation (also with a LArd of 85 dB(A)) where an essential noise exposure is followed by longer lasting low-level noise of negligible energy involves risks for the hearing. To prove this, the influence of an energetically insignificant additional load (70 dB(A) / 3 h) on the restitution of a threshold shift resulting from a preceding continuous noise (94 dB(A) / 1 h) was analyzed. According to a number of studies (e.g., SLEPECKY et al. 1982; HAMERNIK et al. 1982; SHADDOCK et al. 1985; SAUNDERS et al. 1987; ALLEN 1988; HAMERNIK et al. 1989; LIBERMAN 1990) which dealt with the evaluation of noise-induced cochlear damage (i.e., irreversible alterations of the cochlea in the inner ear), impairment of the stereocilia of the sensory hair cells – which are probably the most sensitive part of the hearing system – is the reason for the first signs of hearing loss. The transition from the splaying apart and fusion of the stereocilia is a mostly smooth one (cp. MULROY and WHALEY 1984). Whereas the splaying apart and fusion are still considered potentially reversible, the formation of giant stereocilia is accompanied by permanent hearing loss (Permanent Threshold Shift, PTS). Considering this, the Temporary Threshold Shift (TTS) is also of particular importance. It represents the portion of hearing impairments that are reversible under certain conditions, such as stopping the noise exposure or applying an inner ear therapy. Due to the smooth transition to pathological reactions, however, the TTS must be considered an early stage of potentially irreversible damage. Therefore, studying temporary, i.e., reversible, threshold shifts allows the possibility of judging the damage potency of noise exposures.

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2 Methods and materials 2.1 Exposures and test hypotheses In a first Test Series (TS I) the subjects were exposed to continuous white noise of 94 dB(A) for 1 hour (cp. TS I in the lower part of Figure 1, domain of stress). This acoustic load is equivalent to the rating level LArd of 85 dB(A) for 8 hours permissible in almost all countries. Due to the usually more complex stress situations in daily professional life, the same subjects participated in another Test Series (TS II) with the same exposure combined with energetically negligible impulse noise. The impulse noise exposure consisted of 900 impulses with a duration of 5 ms, each, a noise level of 113 dB(A), and a peak level of 125 dB at regular intervals of 4 s. These impulses correspond with an energy equivalent continuous noise level of approximately only 75 dB(A) / 8 h. The energetic addition of both exposures presented in Test Series II (cp. TS II in Figure 1 leads to an increase in the rating level LArd of 85.4 dB(A) only at the first digit after the decimal point.

Figure 1:

Hypothetical restitution time course of the Temporary Threshold Shift (TTS) associated with three noise exposures with nearly equal rating level LArd per day

Such a noise situation is comparable with the noise stress that is caused by air-powered gun-nailers. In practice, however, very often some 5,000 to 15,000 nails are shot with such an apparatus on an average working day, and the extraneous continuous noise exposure is approximately 85 dB(A). Although this additional load consequently increased the rating level Lr only by 0.4 dB (for the calculation of Lr cp. Figure 2), the working hypothesis was that the strain on the cochlea structure would be higher than it was in Test Series I. This increase in TS II should lead to higher TTS2 values and a longer restitution time (cp. upper part of Figure 1, domain of strain). In the case of Test Series III the exposure of TS I was immediately followed by 3 hours of white noise of 70 dB(A) (cp. TS III in Figure 1). Such an additional environmental load – which increases the 8-h rating level by merely 0.1 dB to 85.1 dB(A) – would be tolerated for simple office activities for as long as 8 hours.

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Figure 2:

H. Irle et al. / Physiological costs of energy equivalent exposures

Converting of impulse noise in TS II into an energy equivalent rating level LArd 1 and its numeric contribution to an already existing sound exposure LArd 2 of 85 dB(A) according to the formula for adding noise events

In order to form a hypothesis, it was assumed that repair mechanisms are time-dependent and it is hardly possible to evaluate environmental conditions by means of a simple onedimensional, energy equivalent assessment. Therefore, contrary to the idealized restitution conditions in Test Series I, the hypothesis was that in Test Series III an energetically insignificant additional load causes a delay in the restitution process, thus leading to an increase in physiological cost (cp. upper part of Figure 1). 2.2 Selection of the test subjects A group of 10 male subjects (Ss) between the ages of 22 and 40 years (28.0 ± 5.1 years) with normal physiological and aural constitution was selected according to DIN ISO 4869 (1991). Thus, the sound audiometrically determined resting hearing threshold of a potential test subject should not exceed the hearing threshold of reference subjects with no hearing impairment by more than 15 dB for frequencies through 2 kHz, and 25 dB for frequencies above 2 kHz. 2.3 Test set-up and audiometric measurements Figure 3 shows the schematic test set-up with the noise generation, the exposure of a subject via headphones, and the adjustment and control with an artificial head measuring system. The hearing thresholds were always measured in a soundproof cabin. The exposures of continuous white noise were provided by a signal generator and applied via headphones. Each subject’s resting hearing threshold was determined before each exposure at frequencies of 3, 4, 6, and 8 kHz as a reference base for each individual test series. The audiometer’s technical features made it possible to carry out the hearing threshold measurements in 1-dB steps (cp. also STRASSER et al. 1995; HESSE et al. 1996; IRLE et al. 1996).

H. Irle et al. / Physiological costs of energy equivalent exposures

Figure 3:

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Schematic test set-up

During the first two minutes after the end of the acoustic exposure, the frequency with the maximum threshold shift with respect to the resting hearing threshold was determined (cp. Figure 4) and thereafter its restitution time course was recorded. That frequency was 4 kHz for 2 subjects and 6 kHz for 8 subjects.

Figure 4:

Selection of the frequency with the maximum hearing threshold shift during the first 2 minutes after the exposure

Due to the expected exponential TTS restitution, the shortest possible time intervals were chosen for the measurements at the beginning of the restitution time. Instead of the mostly utilized logarithmic time scale for presenting the results of restitution in a linear graph (cp. upper right part of Figure 5) in this case a linear time scale was used (cp. main part of Figure 5), thus showing the restitution processes more distinctly.

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Figure 5:

H. Irle et al. / Physiological costs of energy equivalent exposures

Example for registered TTS values for the recovery period with a logarithmic and a linear time scale as well as the representation of the regression function TTS(t)

3 Results and discussion 3.1 Temporary threshold shift (TTS) and restitution time (t(0 dB)) Figure 6 displays the measured TTS values of all 10 subjects and the arithmetically averaged measured values for Test Series I with the continuous noise exposure of 94 dB(A) for 1 hour. The statistically calculated data are summarized next to the real measured values in the upper right corner: the regression function TTS(t), in this case 27.6 - 13.14 x log t, the TTS2 (23.6 dB) which was calculated via the regression function, and the length of time t(0 dB) it took to reach the resting hearing threshold (126 min). The physiological responses which vary from subject to subject can be seen in the real measured values of the TTS2 – with an interindividual range from 15 to 28 dB – and in the time required to reach the resting hearing threshold t(0 dB) – with a range from 40 to 165 min. If the arithmetically averaged values and their course along the regression function are considered, then the approximation is excellent, which can also be seen in the statistical correlation value (r2) of 0.98. Figure 7 shows the results of Test Series II after the combined exposure of 94 dB(A) for 1 hour and a simultaneous, energetically irrelevant exposure of 900 5-ms impulses. The additional exposure caused an increase in the TTS2 and a prolongation of the restitution time. The maximum mean temporary threshold shift for the group increased only slightly from 23.6 to 24.6 dB. Yet, more importantly, the restitution time increased from 126 min to 175 min, i.e., about 3 h. Figure 8 represents the results of Test Series III after the exposure to continuous noise of 94 dB(A) for 1 hour and a second energetically irrelevant exposure of 70 dB(A) for 3 hours during the restitution. The averaged TTS2 in this Test Series III was 23.6 dB, exactly the same as in Test Series I. According to the identical exposure up to this time this was to be expected. Yet, due to the succeeding additional exposure, the mean restitution time increased considerably. Whereas the restitution under resting conditions (in TS I) lasted up to 126 min, threshold shifts now existed for up to 234 min (ca. 4 h) when restitution did not take place in quiet surroundings.

H. Irle et al. / Physiological costs of energy equivalent exposures

Figure 6:

Individual and mean restitution time course for all 10 subjects after the exposure to 94 dB(A) / 1 h (Test Series I)

Figure 7:

Individual and mean restitution time course for all 10 subjects after the exposure to 94 dB(A) / 1 h + 113 dB(A) / 900 x 5 ms (Test Series II)

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Figure 8:

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Individual and mean restitution time course for all 10 subjects after the exposure to 94 dB(A) / 1 h + 70 dB(A) / 3 h (Test Series III)

The restitution process can roughly be split up into sections from 2 min to 50 min (Section A-B), from 50 min to approximately 180 min (Section B-C), and from 180 min to t(0 dB) (Section C-D). Depending on the measured values, the least squares method leads to three different regression functions with points of intersection at tB = 53 min and tC = 175 min. For the section up to tB, the exponential function TTS(t) = 24.5 • 0.98t is the best approximation for the averages. For the subsequent section from tB to tC, the best approximation is the equation for the straight line 9.6 - 0.03 • t. From tC on, the restitution “undisturbedly” follows the equation TTS(t) = 82.1 - 34.64 • log t, which is in accordance with the usually described restitution tendency with the logarithm of the restitution time (cp. KRYTER 1970; BUBB et al. 1978; SCHMIDTKE 1989). There are also quite different individual responses, especially in TS III. Seven of the 10 Ss had an evident delay with a plateau in the TTS recovery caused by the energetically negligible noise of 70 dB(A). The restitution time for the other 3 Ss, who by nature had low TTS2 values around 16 dB, was not influenced by this additional stress. 3.2 Total physiological cost indicated by the Integrated Restitution Temporary Threshold Shift Due to the “two-dimensional” strain process, it seems sensible to consider the restitution process not only with respect to the height of the physiological responses (i.e., the TTS2), but also with respect to their duration (i.e., the restitution time t(0 dB)). Analogous to the sum of heart rate increases above the resting level during the recovery period as a characteristic of the physiological cost of dynamic muscle work, the integral of the temporary threshold shift over the restitution time can be calculated (cp. HESSE et al. 1994; STRASSER et al. 1995). This parameter, named Integrated Restitution Temporary Threshold Shift (IRTTS) is the area that is bordered by the time axis, the TTS axis at 2 minutes, and the regression line TTS(t). It represents the total physiological cost for the respective hearing frequency where these data have been measured. For reasons of comparison, the time courses of the threshold shifts of the test series TS I, TS II, and TS III together with the calculated IRTTS values are arranged in Figure 9. It can be

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seen that a relatively low number of 900 noise impulses increases metabolic strain which is indicated by the somewhat higher TTS2 values and by distinctly prolonged restitution times. Furthermore, it must be stressed that impulse noise with levels of more than 120 dB can also lead to a more direct, mechanical impairment of the cochlea structure which cannot be shown in TTS values. The energetically unimportant successive load (in TS III) causes a stagnation of the restitution process during the entire duration of the energetically insignificant acoustic load. This restitution process is in accordance with the working hypothesis. Drawing inferences from the results, it must be stated that the energetically insignificant additional acoustic load significantly increases the physiological cost.

Figure 9:

Regression lines TTS(t) and IRTTS (in dBmin) as physiological cost associated with a preceding exposure to 94 dB(A) / 1 h (TS I), to 94 dB(A) / 1 h + 113 dB(A) / 900 x 5 ms (TS II), and to 94 dB(A) / 1 h + succeeding 70 dB(A) / 3 h (TS III)

Clearly, the overall physiological cost in the form of IRTTS (cp. upper part of Figure 9) differs with varying exposures. In the case of TS I, the IRTTS value is 660 dBmin; in the case of TS II, it is 902 dBmin; and for TS III, it is 1613 dBmin, respectively. 3.3 Statistical verification of the results (measured values and regression-analytically determined parameters) The results of a study only become relevant when differences have been proven for significance, i.e., that the increase of physiological cost is in a causal-deterministic relation with the additional exposure. In order to ensure that, all parameters, i.e., the values of TTS2, t(0 dB), and IRTTS, were checked for their significance via the WILCOXON-test. According to the upper part of Table 1, the values for TS I and TS II differ in most cases at least on a 95 % significance level (D d 0.05). The differences in the real TTS2 values have a significance level of even 99 % (D d 0.01). With this in consideration, the real values seem to be more powerful. The regressionanalytically determined TTS2 values do not show significant differences, i.e., the difference in characteristic values may be coincidental. Both the real t(0 dB) values and the regressionanalytically determined t(0 dB) values differ on the same significance level. The comparison of the IRTTS values yields also significant differences. Thus, the statements of the working hypothesis in this case are statistically proven. The comparison of TS I and TS III (cp. lower part of Table 1) shows, that differences between the TTS2 values were non-significant. This was to be expected as the conditions up to that point in time were identical. But due to the additional exposure thereafter, the mean restitution time increased considerably with significant differences allowing the conclusion

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that varying physiological responses to energy equivalent stress exist. It can be assumed that the noise-determined decrease in the metabolism of the inner ear is responsible for that. Also, for the integral parameter IRTTS, significant effects were found. Table 1: TTS2, recovery time t(0 dB), and physiological cost IRTTS with significance levels for the differences between Test Series I and Test Series II (top) and Test Series I and Test Series III (bottom) according to the one-tailed WILCOXON-test (data from measurement and regression analysis) Exposure TS I 94 dB(A) / 1 h vs. TS II 94 dB(A) / 1 h + 113 dB(A) / 9000 x 5 ms

Exposure TS I 94 dB(A) / 1 h vs. TS II 94 dB(A) / 1 h + 70 dB(A) / 3 h

TTS2 real [dB]

t(0 dB)real [min]

TTS2 reg. [dB]

t(0 dB)reg. [min]

IRTTS [dBmin]

22.5

91

23.6

126

660

‘‘

‘

…

‘

‘

25.9

122

24.6

175

902

Values determined via measurement

Values determined via regression analysis

TTS2 real [dB]

t(0 dB)real [min]

TTS2 reg. [dB]

t(0 dB)reg. [min]

22.5

91

23.6

126

660

…

‘‘

…

‘

‘‘

24.6

171

23.6

234

1613

Values determined via measurement ‘‘ - D d 0.01

IRTTS [dBmin]

Values determined via regression analysis

Significance level ‘ - D d 0.05

… - D > 0.05

3.4 Hearing risk of the different exposures In order to assess these results with regard to a damage-risk evaluation, the IRTTS values from TS II and TS III were related to the total physiological cost resulting from the legally permissible exposure of TS I (cp. Figure 10). According to MILLER (1974), it is assumed that the aural effects of this reference quantity (94 dB(A) / 1 h) are nearly identical to the consequences of a permissible exposure of 85 dB(A) / 8 h.

Figure 10: Hearing Risk Factor depending on the different test conditions

If the calculated so-called “Hearing Risk Factor” (HRF) is greater than 1, the risk of hearing damage is greater; values lower than 1 reflect a lower risk of hearing. Compared to TS I, the addition of impulse noise that increases the noise level by only 0.4 dB(A) leads with

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a Hearing Risk Factor of 1.37 to an increase in the danger to hearing of approximately 40 %. An additional energetically negligible noise exposure is according to the Hearing Risk Factor of 2.44 extremely more hazardous.

4 Conclusions The test series have shown that the energy equivalent leveling of noise exposures with different time structures is not suitable for drawing inferences about the physiological issues on human beings. Different noise exposures – even if they do not exceed the legal rating level of 85 dB(A) – are often annoying, disturbing and performance-reducing, and they still cause essential hearing threshold shifts of about 20 to 25 dB as could be shown here. Furthermore, noise which has no essential influence on the calculated energy equivalent rating level is often of great importance, as it can lead to considerably prolonged restitution times. Therefore, the purely energy equivalent determination of the rating level of both impulse noise and low sound levels can lead to an underestimation of latent problems so that over time a reversible TTS can evolve into a permanent threshold shift. Contrary to various damage-risk criteria like the ones from the CHABA-method (COLES et al. 1968; WARD et al. 1968) or the method according to SMOORENBURG (1982), the study has shown that aural demand may be less obvious in the height of the temporary threshold shift (TTS2) but becomes more evident in the duration of the load compensation. The Integrated Restitution Temporary Threshold Shift (IRTTS) which combines the two parameters seems to be the most reliable variable for the early indication of potential, long-term hearing damage. The results of this study are also of importance for the acoustic design of break rooms for workers who were exposed to noise. There should be conditions that allow an undisturbed restitution of the hearing. It must be expected that values below 85 dB(A) – which is generally considered the damage limit – are still too high for hearing restitution. Even levels of only 70 dB(A) significantly delay hearing restitution.

5 References ALLEN, J. B. (1988) Cochlear Signal Processing. In: JAHN, A.F. and SANTOS-SACCHI, J. (Eds.): Physiology of the Ear. Raven Press, New York, 243-270 BUBB, H.; MÖSCH, S. und SCHMIDTKE, H. (1978) Einfluß von Lärmintensität und -einwirkdauer auf die Vertäubung des Ohres. Z. Arb. wiss. 32 (4NF) 4, 245-253 COHEN A.; KYLIN, B. and LA BENZ, J.P. (1966) Temporary Threshold Shifts in Hearing from Exposure to Combined Impact/Steady-State Noise Conditions. J. Acoust. Soc. Am. 40, 1371-1380 COLES, R.R.A.; GARINTHER, G.R.; HODGE, D.C. and RICE, C.G. (1968) Hazardous Exposure to Impulse Noise. J. Acoust. Soc. Am. 43, 336-343 HAMERNIK, R.P.; HENDERSON, D.; CROSSLEY, J.J. and SALVI, R. (1974) Interaction of Continuous and Impulse Noise: Audiometric and Histological Effects. J. Acoust. Soc. Am. 55, 117-121 HAMERNIK, R.P.; HENDERSON, D. and SALVI, R. (1981) Potential for Interaction of Low-Level Impulse and Continuous Noise. Dept of Mech & Aerosp Engin, Syracuse, NY HAMERNIK, R.P.; HENDERSON, D. and SALVI, R. J. (Eds.) (1982) New Perspectives on Noise Induced Hearing Loss. Raven Press, New York HAMERNIK, R.P.; PATTERSON, J.H.; TURRENTINE, G.A. and AHROON, W.A. (1989) The Quantitative Relation between Sensory Cell Loss and Hearing Thresholds. Hearing Research (38) 199-222 HESSE, J.M. (1994) Theoretische und experimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Dissertation, Universität Siegen HESSE, J.M.; IRLE, H. und STRASSER, H. (1994) Laborexperimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Z. Arb. Wiss. 48 (20NF) 237-244

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HESSE, J. M.; VOGT, E.; IRLE, H. and STRASSER, H. (1996) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – B) Restitution of a Continuous Noise-Induced Temporary Threshold Shift under Resting Conditions and under the Influence of an Energetically Negligible Continuous Noise Exposure of 70 dB(A). In: MITAL, A. et al. (Eds.): Advances in Occupational Ergonomics and Safety I. International Society for Occupational Ergonomics and Safety, Cincinnati, Ohio, 639-644 IRLE, H.; VOGT, E.; HESSE, J. M., and STRASSER, H. (1996) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – A) Hearing Threshold Shifts after Continuous Noise of 94 dB(A) / 1 Hour with and without Additional Impulse Noise Exposure. In: MITAL, A. et al. (Eds.): Advances in Occupational Ergonomics and Safety I. International Society for Occupational Ergonomics and Safety, Cincinnati, Ohio, 633-638 KRYTER, K.D. (1970) The Effects of Noise on Man. Academic Press, New York/San Francisco/London KUNDI, M.; KUNDI, R. und STIDL, H.G. (1982) TTS nach kombinierter Impuls-Dauerlärmbelastung. XII AICBKongreß, 69-75 KUNDI, M.; WENINGER, U.; STIDL, H.-G. and HAIDER, M. (1984) Effects of Combined Exposures to SteadyState and Impulse Noise on Inner Ear Functions. In: MANNINEN, O. (Ed.), Combined Effects of Environmental Factors, 169-178 LIBERMAN, M.C. (1990) Structural Basis of Noise Induced Threshold Shift. Noise as a Public Health Problem. New Advances in Noise Research, Part 1. Swedish Council for Building Research, Stockholm LUTMAN, M.E. (1972) The Effect of Steady-State and Impulse Noise Combinations on Temporary Threshold Shift. M. Sc. Dissertation, University of Southhampton MILLER, J.D. (1974) Effects of Noise on People. J. Acoust. Soc. Am. 56, 729-764 MULROY, M.J. and Whaley, E.A. (1984) Structural Changes in Auditory Hairs during Temporary Deafness. Scanning Electron Microscopy, II, 831-840 SAUNDERS, J.C.; CANLON, B. and FLOCK, Å. (1987) Mechanical Changes in Stereocilia Following Overstimulation: Observations and Possible Mechanisms. In: SALVI, R.S.; HENDERSON, D.; HAMERNIK, R.P. and COLLETTI, V. (Eds.): Basic and Applied Aspects of Noise-Induced Hearing Loss. Plenum, New York, 11-29 SCHMIDTKE, H. (1989) Vertäubung. In: Handbuch der Ergonomie. Kap. 9.3.4, 1-10 SHADDOCK, L.C.; HAMERNIK, R.P., and AXELSSON, A. (1985) Effect on High Intensity Impulse Noise on the Vascular System of the Chinchilla Cochlea. Ann. Otol. Rhinol. Laryngol. 94, 87-92 SLEPECKY, N.; HAMERNIK, R.P.; HENDERSON, D. and COLLING D. (1982) Correlation of Audiometric Data with Changes in Cochlear Hair Cell Stereocilia Resulting from Impulse Noise Trauma. Acta Otolaryngol., 93, 329-340 SMOORENBURG, G.F. (1982) Damage Risk Criteria for Impulse Noise. In: HAMERNIK, R.P.; HENDERSON, D. and SALVI, R. (Eds.): New Perspectives on Noise-Induced Hearing Loss. Raven Press, New York, 471-490 STRASSER, H.; HESSE, J.M. and IRLE, H. (1995) Hearing Threshold Shift after Energy Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and CHAMPNEY, P.C. (Eds.): Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, London/New York/Philadelphia, 241-248 WALKER, J.G. (1972) Temporary Threshold Shift Caused by Combined Steady-State and Impulse Noises. J. Sound & Vibration 24, 493 WARD, D.W. et al. (1968) Proposed Damage-Risk Criterion for Impulse-Noise (Gunfire). Report of Working Group 57, NAS-NRC Committee on Hearing, Bioacoustics and Biomechanics, Office of Naval Research Contract No. NONR 2300 (05) Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin

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Chapter 8

Influence of the Number of Impulses and the Impulse Duration on Hearing Threshold Shifts H. Irle and H. Strasser

0 Summary It has been shown in previous experimental studies on energy equivalent exposure to impulse noise that a shortening of the impulse duration and a corresponding increase in the number of impulses leads to a significant increase in the following variables: the temporary threshold shift TTS2 immediately after the exposure, the restitution time to the complete abatement of the threshold shift t(0 dB), and the integral of the hearing threshold shifts (Integrated Restitution Temporary Threshold Shift or IRTTS). However, it has been unclear so far whether this increase in physiological cost is due to the shortened duration or the increased number of impulses. In order to study this question, 5 test series (TS I to TS V) were carried out with 10 (8 male and 2 female) test subjects from 22 to 41 years of age (27.1 ± 5.8 years) in each test series in order to analyze the effect of variations in the number of impulses with constant impulse duration as well as the effects of the impulse duration with constant number of impulses. The goal was to obtain information about the significance of the parameters “impulse number” and “impulse duration” with respect to the danger to the hearing from the stress of impulse noise by comparing the results of these tests. Consistent with earlier work, it was found that energy equivalent fractioning of continuous noise into impulse noise of shorter and shorter duration leads to an increase of the reversible effects of the noise exposure. If the differences (which are due to the test design) in the rating level (± 3 dB) are taken into account, the tests showed that an increase in the number of impulses leads to a high, i.e., over-energetic increase in the physiological cost (in the form of the IRTTS values) which is consistent with the pre-set hypothesis. With respect to impulse duration, there also exists a critical value; if the actual values are lower than this critical value, the effects of the noise exposure increase over-proportionally as well.

1 Introduction The energy equivalent conversion of noise stress situations of varying time structures has long been met with skepticism by scientists in the field of work physiology. In earlier experiments carried out (cp. STRASSER et al. 1995), for example, the influence of the splitting up of a continuous noise exposure of 113 dB(A) for 45 s into energy equivalent impulse noise events was studied (cp. Figure 1). Experiments with • 180 impulses for 250 milliseconds, each, • 450 impulses for 100 milliseconds, each, • 1,800 impulses for 25 milliseconds, each, and • 9,000 impulses for 5 milliseconds, each resulted in a substantial and highly significant increase of the audiometrically measurable hearing threshold shifts when the number of impulses increases with a corresponding decrease in impulse duration. This can be seen in Figure 2. The increasing physiological responses on the hearing resulting from noise consisting of, for example, 5-ms impulses can be seen in the form of threshold shifts with TTS2 values of 24 dB after noise exposure; it takes approximately 600 minutes – 10 full hours – for them to completely dissipate. Finally, these two characteristic values also determine the sum of the physiological cost of the noise exposure in the form of the integral of the TTS over the restitution time, whose value greatly increases from 147 to 2473 dBmin.

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Figure 1:

Noise exposures with 113 dB(A) and corresponding exposure times for impulse noise which is energy equivalent to 85 dB(A) for 8 h

Figure 2:

Mean restitution time courses TTS(t) and integral ³TTS dt as a function of equal impulse exposures (exposure level Lexp = 113 dB(A)) but a varying number (n) and duration (tImp) (rating level LArd = 85 dB(A) / 8 h) (mean values of 10 subjects)

Although these experiments proved that the “physiological cost” increases when the number of individual impulses is increased and the duration of those impulses is decreased, it remained unclear whether this mainly results from the shorter duration of the impulses or their greater number.

2 Methods In the hypotheses for the influence of the number of impulses on the one hand and the impulse duration on the other hand on the threshold shifts, it was assumed that (according to the left part of Figure 3) an increase in the number of impulses at a constant impulse duration

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and (according to the right part of Figure 3) an increase in the impulse duration of a constant impulse number result in different physiological costs. Corresponding with the steeper course, a stronger influence on the physiological cost could be expected from the increased number of impulses. In order to examine this, 5 different test series were chosen; TS I (shown in the middle of Figure 3) served as a reference measurement for both parameters.

Figure 3:

Hypothetical course of the physiological cost as an integral of the temporary threshold shifts (³TTS dt) after exposures to various impulse numbers (n) and impulse durations (tImp) (TS I, TS II, TS III - left; TS I, TS IV, TS V - right)

The parameter “number of impulses” was studied in the TS III, TS I, and TS II. The influence of the “impulse duration” was examined in TS V, TS I, and TS IV. As in similar preceding experiments, the impulses had a level of 113 dB(A). In order to establish a mathematical relationship between all the noise exposures, the exposures were based on the exchange rate of 3 dB. In other words, in comparison with a level of 82 dB(A) for 8 hours in TS I, the noise level in TS III and TS V was 3 dB lower and the level in TS II and TS IV was 3 dB higher. This resulted in a halving and a doubling, respectively, of the number of impulses on the one hand and of the impulse duration on the other hand in the test design. Since, according to MILLER (1974) similar threshold shifts can be expected from the exposure to a continuous noise of 94 dB for 1 hour and an energy equivalent exposure of 85 dB over 8 hours, an experimentally feasible exposure was defined with this noise leveltime constellation in order to standardize the results of the tests (which were carried out in a cross-over design). Therefore, in a further test series (TS VI), the 10 test subjects were exposed to 94 dB(A) for 1 hour. By using an audiometer during the selection process, it was ensured that only test subjects with normal hearing were chosen for the study. According to DIN ISO 4869, the subjects chosen for the study could have individual hearing threshold shifts of no more than 15 dB over the normal hearing threshold for frequencies through 2 kHz and no more than 25 dB over the normal hearing threshold for frequencies over 2 kHz. Since it is the basis for further measurements and evaluations, each subject’s individual hearing threshold was determined before each test series.

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Figure 4 shows the schematic test set-up. “White noise” was used as the acoustic load in all the test series. A nominal value setting via an artificial head measuring system ensured identical noise exposure situations for all subjects. The actual noise exposure was carried out via headphones in a sound-insulated test booth (sound proof cabin), in which the measurement of the physiological responses to the noise exposure was also carried out via pure tone audiometry.

Figure 4:

Schematic test set-up

While taking into account the individual resting hearing threshold, the frequency of the maximum threshold shift, i.e., the increase in the hearing threshold, was determined within the first 2 minutes after completion of the noise exposures. The value of the hearing threshold shift 2 minutes after the end of the noise exposure, the TTS2, is an important characteristic value for audiometric studies in noise experiments. The hearing threshold shift at the frequency of the maximum threshold shift, which was usually 4 or 6 kHz, was measured at exactly defined times until the resting threshold was again reached. This point in time, the restitution time, is also an important characteristic value for the acoustic strain analysis and is called t(0 dB). Using a linear time scale, the restitution time course of a temporary threshold shift (TTS) generally has the approximated course of an e-function. However, if a logarithmic time scale is utilized, then the course of the regression function TTS(t) follows a straight line.

3 Results As an example for all the test series, Figure 5 shows the results of TS II, in which the subjects were exposed to 1,800 impulses for 25 ms, each. The individually measured values and the mean restitution time course for all 10 test subjects are plotted against the time axis. The results of the empirical and regression-analytical data analysis are summarized on the upper right.

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Figure 5:

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Individual and mean restitution time course TTS(t) for all 10 subjects after the exposure “113 dB(A) / 1,800 x 25-ms impulses“ (TS II)

On average, the threshold shifts TTS2 were a bit less than 20 dB and the restitution time t(0 dB) was approximately 120 minutes. Thus, this noise stress situation of 85 dB(A), which is permissible according to §15 of the “German Workplace Regulations (Arbeitsstättenverordnung)” and the “Accident Prevention Regulation “Noise” (UVV Lärm)” in the production sector, leads to a hearing threshold shift which is worth mentioning and should not be underestimated. In addition to the characteristic values TTS2 and t(0 dB), the physiological cost of noise exposures can be shown with a third parameter as an integral value of the hearing threshold shifts and the restitution time. This “Integrated Restitution Temporary Threshold Shift” or IRTTS is the area which is enclosed by • the time axis • the TTS axis at 2 minutes and • the regression function TTS(t). The unit of this characteristic value is dB x min. If the results of the three test series with equal impulse duration in Figure 6 (TS III, TS I, and TS II) are examined, it becomes clear that an increased, that is, a doubled, number of impulses at an equal impulse duration leads to a highly significant increase in all the characteristic values: • the TTS2 values • the t(0 dB) values and • the IRTTS values. A doubling of the number of impulses leads to more than twice the strain, i.e., an overproportional strain, in the form of the following IRTTS values: • 73 • 172 and • 457 dBmin (cp. upper right part of Figure 6).

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Figure 6:

H. Irle and H. Strasser / Influence of the number of impulses and the impulse duration

Restitution time course TTS(t) with characteristics TTS2 reg., t(0 dB)reg., and physiological cost IRTTS as well as symbolic labeling of significance levels for the differences between the responses to the exposures with equal impulse duration tImp = 25 ms (according to the one-tailed WILCOXON test)

In contrast to the aforementioned series and especially with respect to restitution time, the test series with equal impulse number (TS V, TS I, and TS IV) exhibit smaller differences between the tests (cp. Figure 7). This also leads to a slightly lower significance level between the results of these tests. Finally, the difference between the restitution time of TS V and that of TS I is not significant.

Figure 7:

Restitution time course TTS(t) with characteristics TTS2 reg., t(0 dB)reg., and physiological cost IRTTS as well as symbolic labeling of significance levels for the differences between the responses to the exposures with equal impulse number n = 900 (according to the one-tailed WILCOXON test)

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If the IRTTS values of all the impulse noise exposures are related to the value of the reference exposure of “94 dB(A) for 1 hour,” then a value greater than “1” corresponds with a higher, i.e., greater risk, and a value less than “1” corresponds with a lower or smaller risk for the human hearing than an energy equivalent stress situation of 85 dB(A) for 8 hours. The results in Figure 8 clearly show the influence of the varying number of impulses and the impulse durations. It can be seen that after exposure to impulse noise with a rating level of 85 dB(A), the physiological cost is higher than that resulting from exposure to the energy equivalent continuous noise of the reference measurement by a factor of • 1.08 in TS IV and even • 1.38 in TS II.

Figure 8:

“Normalized physiological cost” of all impulse noise exposures (exposure level = 113 dB(A)) corresponding to an exposure of 94 dB(A) / 1 h (TS VI) (IRTTSTS i / IRTTSTS VI)

Thus, people so exposed are no longer protected by the designated noise exposure limits. Clearly lower values resulted in • TS I with 0.52 • TS V with 0.36, and • TS III with 0.22. However, this should not be unexpected with the 3 and 6 dB lower rating levels of these exposures. The different rating levels LArd which are due to the test design complicate the interpretation of these standardized values. The dose of energy of the test series which varied ±3 dB represents an interference variable whose influence was eliminated as follows: With regard to the influence of the number of impulses (n) on the physiological cost in the form of IRTTS values, a great increase in physiological responses results when the number of impulses is doubled (from 450 to 900 to 1,800 impulses), as seen in the upper left part of Figure 9. Each doubling of the impulse duration (tImp) – according to the upper right part of Figure 9 – leads to a smaller increase in strain than the variation of the number of impulses. The IRTTS values were converted to a reference level (LArd also 85 dB(A)) in order to compare the test series to one another. The IRTTS values that corresponded with a rating level of 82 dB(A) were multiplied by a factor of 2 and those which corresponded with 79 dB(A) were multiplied by 4, since only half and a quarter, respectively, of the energy affected the ear in these cases.

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Figure 9:

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Influence of the impulse number and impulse duration on the physiological cost

At this point it should be mentioned that this is a mathematical procedure which merely allows a comparison of threshold shifts after exposure to noise with varying rating levels. If the energy equivalence principle for aural strain were valid, this principle would have to lead to almost identical IRTTS values since the rating levels are now the same. An increase in the number of impulses still leads to an increase in physiological cost, with the rating level now equal, as can be seen in the lower left part of Figure 9. The influence of the impulse duration on the physiological cost (again with equal rating level) can be seen in the lower right part of Figure 9. At first, the physiological cost is almost constant, i.e., energy equivalent, with decreasing impulse duration; for impulse durations below 25 ms, however (i.e., the 12.5 ms given here), the physiological cost increases greatly. These results show that the energy equivalence principle is not suitable for the evaluation of noise exposures. The influence of the number of impulses in particular, but also the influence of the impulse duration and of the rating level, do not follow this principle by any means.

4 Conclusions and Outlook The evaluation of the experiments in this study has shown that an increase in the number of impulses (with constant impulse duration) leads to a substantially larger increase in the physiological cost – expressed as IRTTS values – than was expected based on the utilized dose of energy. The test series with varying impulse durations also showed that the energy equivalence principle is not relevant in practice for aural stress. The physiological cost does increase with longer impulse durations, but this was to be expected equivalently to the energy. Short impulse durations lead to an over-energetic increase in physiological responses of the hearing. The results of this study clearly show that the number of impulses is more important than the impulse duration when stress from impulse noise is evaluated, at least under the conditions in this study. From an energy equivalent point of view, however, the influences of the impulse duration below a critical impulse duration cannot be underestimated. Since the

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number of persons subjected to impulse noise is even greater than the number of those subjected to continuous noise, these aspects of the research of noise effects should be paid more attention. The rigorous application of the energy equivalence principle in the incorporation of the “Directive on the Protection of Workers from the Risks Related to Exposure to Noise at Work (EG-Lärmschutzrichtlinie)” (1986) into the “Accident Prevention Regulation “Noise” (UVV Lärm)” (1990) will not be able to contribute to a decline in the occupational hazard NIPTS, i.e., “noise-induced hearing loss.”

5 References STRASSER, H.; HESSE, J.M. and IRLE, H. (1995) Hearing Threshold Shift after Energy Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and CHAMPNEY, P.C. (Eds.) Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, London/ New York/Philadelphia, 241-248 MILLER, J.D. (1974) Effects of Noise on People. J. Acoust. Soc. Am. 56 (3) 729-764 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors; Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin Directive on the Protection of Workers from the Risk Related to Noise at Work (86/188/EEC) (1986) Richtlinie des Rates vom 12. Mai 1986 über den Schutz der Arbeitnehmer gegen Gefährdung durch Lärm am Arbeitsplatz (86/188/EWG). Amtsblatt der Europäischen Gemeinschaft Nr. L, 28-34

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Chapter 9

Investigations into the Efficiency of the Stapedius Reflex with Impulse Noise Series H. Irle and H. Strasser

0 Summary Earlier experimental studies showed that during exposure to energy equivalent continuous noise, a reduction in exposure time along with the associated increase in the noise level leads to a significant reduction in “physiological costs” in terms of TTS2, t(0 dB), and IRTTS values. For exposure to impulse noise, however, a significant increase of physiological responses occurred when the impulse duration was shortened and the number of impulses was increased accordingly. However, the kind of consequences of a reduction of the time interval between noise impulses to approximately 1 s (so that impulse salvos would result) are not clear since an effect of the stapedius reflex can be expected. Another unanswered question concerns the particular impact of a reduction in the impulse duration on the experienced strain when the number of impulses remains constant. In order to examine these issues, 10 test subjects (Ss) participated in 3 Test Series (TS) which were carried out in a “cross-over” test design. The 8 male and 2 female Ss ranged in age from 20 to 48 years (28.7 ± 9.5 years). The hearing of all 10 individuals satisfied quality criteria according to DIN ISO 4869 at all frequencies. In TS I, the Ss were exposed to the reference exposure of 94 dB(A) for 1 h which is equivalent to a rating level LArd of 85 dB(A) for 8 h. In TS II, the exposure consisted of energy equivalent 9,000 short-term impulses with a duration of 5 ms and an exposure level of 113 dB(A). In contrast to an earlier study, the time interval between the impulses was reduced from 3 s to 1 s. In TS III, the Ss were exposed to 9,000 impulses with a duration of only 2.5 ms. Using the same exposure level and time interval between impulses as in TS II, the noise dose is reduced by half in terms of energy (LArd value of 82 dB(A)). The results of this study show that the reduction in the time interval between the impulses from 3 s to 1 s leads to significantly reduced threshold shifts. These reductions cannot be explained by interindividual differences since different Ss were used relative to previous studies. Thus, these differences must be caused by the stapedius reflex. While no impact of the stapedius reflex on the threshold shift could be shown with 3-s time intervals between noise impulses, some of the positive, level-reducing effect of this reflex seems to be present with 1-s time intervals between the impulses. While the halving of the impulse duration led to reduced “physiological costs,” the reduction was not statistically significant. This suggests that the parameter “number of impulses” has a stronger impact on the hearing’s physiological reactions than the impulse duration.

1 Introduction In a conventional rating of noise exposures – as can be seen in Figure 1 – 85 dB for 8 h are equivalent to 88 dB for 4 h, 91 dB for 2 h, or 94 dB for 1 h, but this exposure which is tolerable in the production area is also equivalent to 113 dB for only 45 s. These conversions between various constellations of levels and exposure times with the exchange rate 3 dB also allows a splitting up of continuous noise into impulse noise as long as the principle of energy equivalence is not violated. It would be absurd, however, to assume that the effects on the hearing are the same. Impulse noise, without a doubt, is substantially more dangerous, even though that it is not addressed in the "Accident Prevention Regulation Noise" or national and international standards. Under certain conditions, however, a physiological mechanism in the middle ear can possibly modulate the noise transmission. Without discussing the details of Figure 2, the triggering of the stapedius reflex can lead to a “capping” of peak levels, which may very well be in the range of 10 to 20 dB at immission levels between 100 and 120 dB.

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Figure 1:

Sound pressure levels of different durations leading to an equal rating level when applying the 3-dB exchange rate

Figure 2:

Stapedius reflex – Protective mechanism of the middle ear

This should lead to some level of protection for the inner ear since only dampened noise signals can pass the oval window as “gateway” to the inner ear. This dampening effect is mainly limited to low frequencies (below 1 kHz) and exhibits substantial differences from one individual to the next. Furthermore, it can only be expected for those noise signals, which reach the ear after the latency period of the stapedius reflex, i.e., for levels of impulse salvos, which occur in rapid succession after the reflex has been triggered. The reason is that for impulse sequences at 1-s time intervals, the reflex can be expected to still be effective since it has not already subsided.

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Already time intervals between the individual impulses of 3 s, however, require another triggering. Moreover, the triggering will be ineffective whenever the individual noise is shorter than the latency period of the reflex, which, on average, is approximately 70 ms. This is certainly the case with individual impulses of 5 ms duration. It has been shown in an earlier experimental study (HESSE et al. 1994) that impulse noise, which does not appear dangerous when judged by its energy and peak level, leads to substantially higher hearing threshold shifts than continuous noise if the time interval between the individual impulses is 3 s or more. In a cross-over test design, 10 Ss had been exposed to 6 different sound constellations on different days. As can be seen in Figure 3, in Test Series I (TS I), the Ss were exposed to continuous noise for 1 h at a level of 94 dB(A).

Figure 3:

Schematic representation of the exposures

In TS II, the energy equivalent exposure of 113 dB(A) lasted for only 45 s. In TS III through TS VI, the exposure of 113 dB(A) – starting with 180 impulses with a duration of 250 ms each – was distributed over a correspondingly larger number of shorter impulses, which was ultimately expanded to 9,000 impulses of 5 ms duration, which corresponds with a total exposure time of 7 h 30 min, i.e., almost a full workday. Figure 4 shows that the maximum threshold shifts TTS2 immediately after the exposure exhibit a significant increase with the number of impulses from TS II to TS VI. Additionally, the restitution times t(0 dB) also increased significantly when the impulse duration was shortened in exchange for an increase in the number of impulses. The exposure in TS VI, i.e., 9,000 impulses of 5 ms duration each required a restitution time of even more than 600 min, i.e., more than 10 h. The area underneath the restitution curves as an integral value for the physiological costs, which the hearing had to “pay” for the preceding exposures – quantified as the Integrated Restitution Temporary Threshold Shifts – even increased from an IRTTS value of 147 dBmin to more than 2,000 dBmin.

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Figure 4:

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Restitution time courses TTS(t) with characteristics TTS2 reg., t(0 dB)reg., and physiological cost IRTTS as well as symbolic labeling of significance levels for the differences between the responses to the exposures with equal exposure level Lm = 113 dB(A) and exposure time tExp (n x tImp) (according to the one-tailed WILCOXON-test)

The ratio of these IRTTS values, which is shown once more in Figure 5, was even 1 to 16.

Figure 5:

Restitution time courses TTS(t) of all exposures with equal exposure level Lm = 113 dB(A) and exposure time tExp (n x tImp) with exemplary representation of the Integrated Restitution Temporary Threshold Shift IRTTS of the exposures 113 dB(A) / 45 s (TS II) and 113 dB(A) / 9000 x 5 ms (TS VI)

Expressing the results from TS II to TS VI relative to the IRTTS values from TS I, i.e., continuous noise of 94 dB(A) for 1 h (cp. Figure 6), shows two things: On the one hand, an energy equivalent increase in the exposure level to 113 dB along with a corresponding shortening of the exposure time in TS II to 45 s leads to a substantial reduction in the physiological costs. On the other hand, splitting up such high continuous noise into 9,000 impulses in TS VI leads to a substantial increase in the physiological costs, which are 21/2 times as high as after exposure to continuous noise.

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As can be seen in Figure 7, an additional test series examined the effects of impulse noise in TS II and TS III relative to the exposure to continuous noise of 94 dB(A) for 1 h in TS I with an energy equivalent rating level of LArd = 85 dB(A).

Figure 6:

“Standardized Physiological Cost” of the energy equivalent exposures (IRTTSTS i / IRTTSTS I)

The exposure in TS II, which was energy equivalent to the exposure in TS I, and the exposure in TS III, which was reduced by a factor of 2 in terms of the noise dose due to the shortened impulse duration from 5 ms to 2.5 ms, both consisted of 9,000 individual impulses each. Since the impulses were administered at 1-s time intervals, it could hypothetically be expected that the stapedius reflex may have a positive effect on the hearing threshold shifts. However, since the 1-s time intervals required the reduction of the total exposure time to 150 min, the effect of the generally shortened impulse noise exposure with a possibly additional intervening variable could not be excluded.

Figure 7:

Schematic representation of the exposures

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2 Methods According to Figure 8, the Ss were exposed to noise in the laboratory. Headphones were used in order to be able to exactly control the administering of the exposures. The exposures were continuously monitored via an artificial head measuring system. The resting hearing threshold and the restitution time course of the hearing threshold shifts were measured audiometrically in a sound proof cabin.

Figure 8:

Schematic test set-up

The hearing of all Ss had to satisfy certain quality criteria. In accordance with DIN ISO 4869, individuals were only suitable as Ss if their individual hearing threshold shifts were limited to a certain value above the normal hearing threshold. The measurements were not limited to the maximum threshold shift TTS2 (measured within 2 min after exposure). Instead, the hearing threshold shift was measured (at predetermined time intervals) until the time t(0 dB), i.e., until a hearing threshold shift above the resting hearing threshold was no longer detectable. If a linear time axis is chosen for the restitution course, the measured values typically follow a declining exponential function. Plotting the restitution course against a logarithmic time axis, however, results in a straight line.

3 Results Figure 9 (top) shows all measured values of the 10 Ss from TS I with maximum threshold shifts of approximately 20 dB and their restitution over time. Based on the range of individual values from 13 to 30 dB measured immediately, i.e., 2 min, after the acoustic exposure and the measured restitution times of up to 90 min, the regression-analytical characteristic values of 21.7 dB for the mean TTS2 and 85 min for the time t(0 dB) were determined.

H. Irle and H. Strasser / Investigations into the efficiency of the stapedius reflex

Figure 9:

Individual and mean restitution time courses TTS(t) for all 10 subjects after the exposures “94 dB(A) / 1 h White Noise” (top), “113 dB(A) / 9000 x 5 ms Impulses” (middle), and “113 dB(A) / 9000 x 2.5 ms Impulses” (bottom)

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Figure 9 (middle) shows the corresponding values from TS II, i.e., after the energy equivalent noise exposure with 9,000 impulses at a level of 113 dB and a duration of 5 ms each, which were administered at 1-s intervals. It can already be seen at first glance that the threshold shifts in this case were not higher than after the exposure to continuous noise in TS I. Furthermore, on average, they even subsided more quickly. The regression-analytically determined maximum threshold shift was only approximately 14 dB, on average, and the regression-analytically determined restitution time t(0 dB) was only approximately 1 h (63 min). The results in TS III which are shown in Figure 9 (bottom) are evidence that the maximum threshold shifts with a TTS2 value of 13.5 dB after the exposure to the 2.5-ms impulses are comparable to those after the 5-ms impulses. However, they seem to subside somewhat more quickly. The regression-analytically determined restitution time t(0 dB) was only 46 min. In order to allow a better comparison between the results, an integral characteristic value for the threshold shifts was determined. The area underneath the restitution curve TTS(t), i.e., the integral of the temporary hearing threshold shifts from 2 min after exposure to t(0 dB), represents a global characteristic of the Integrated Restitution Temporary Threshold Shifts, the IRTTS value. It quantifies the total physiological costs, which the hearing had to pay for the preceding noise exposure, i.e., it measures the entirety of threshold shifts in the unit dB x min. The compilation in Figure 10 shows the restitution courses of TS I, TS II, and TS III along with the resulting IRTTS values, as well as the results of significance calculations according to the WILCOXON-test for the differences in the characteristic values TTS2, t(0 dB), and IRTTS.

Figure 10: Restitution time courses TTS(t) of all exposures with characteristics TTS2 reg., t(0 dB)reg., and physiological cost IRTTS as well as symbolic labeling of the significance level (according to the one-tailed WILCOXON-test)

The maximum threshold shifts of 14.1 dB in TS II differ significantly from the value of 21.7 dB in TS I. The difference in the t(0 dB) values (63 min vs. 85 min) was not statistically significant, however. Nonetheless, the entire physiological costs of exposure to impulse noise in TS II is significantly lower than those due to exposure to continuous noise in TS I (IRTTS

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value of 222 dBmin vs. 435 dBmin). Also, all 10 Ss’ responses to the impulse noise exposure in TS III were lower in all 3 characteristic values than their responses to continuous noise in TS I; these differences were highly statistically significant. Contrary to that, there was no significant difference to TS II in any of the parameters. With a difference in TTS2 values for the two impulse noise exposures of 14.1 vs. 13.5 dB, which is only marginal and by no means substantial, the somewhat larger differences in restitution time of 63 min versus 46 min still did not result in significantly different IRTTS values as characteristic values for the total “physiological costs.” These almost equal reactions are surprising given that, in TS III, the Ss were exposed to the same number of impulses as in TS II, but that the impulse duration has only half as long (2.5 ms), i.e., the noise exposure has only half as high in terms of energy.

4 Discussion In the test series which was mentioned in the introduction in which a continuous noise exposure was split up into impulse noise, the highly significant increase in physiological costs was thought to be caused by the increase in the number as well as the corresponding shortening in the duration of impulses. After all, the 9,000 impulses of only 5 ms duration each had caused the overall highest threshold shifts, which exceeded the physiological correlate of the continuous noise exposure by a multiple. Due to the energy equivalence of all 6 exposures in the test design, however, it was not possible to determine whether this “more” was only due to the number of impulses or whether the shortening of the impulse duration played a role as well. Specific studies on the influence of the number of impulses and impulse duration on hearing threshold shifts and their restitution (cp. IRLE et al. 1999) already showed several years ago that – still utilizing energy equivalent noise exposures – the number of impulses had stronger effects on the threshold shifts than the impulse duration. Also using levels of 113 dB(A), 1,800 impulses of 25 ms each led to significantly higher physiological costs than half as many, i.e., 900 impulses, which, with an impulse duration of 50 ms, were twice as long. Such exposures were energy equivalent to a LArd value of 85 dB(A). However, compared to 450 impulses of 25 ms duration, exposures with 900 impulses of 12.5 ms each at a level of 113 dB(A) caused significantly higher TTS2 values, i.e., higher maximum threshold shifts, longer restitution times, and higher IRTTS values. The two latter impulse noise exposures contained – according to a LArd value of 79 dB(A) – a noise dose which was lower by a factor of 4. The apparent dominance of the effect of the number of impulses – regardless of a noise exposure’s noise dose – is consistent with the results from TS II and TS III. The effects in TS III were comparable to those in TS II, even though the former, despite the same number of impulses, contained only half of the physical noise dose. While the number of impulses was constant in the two Test Series TS II and TS III, the impulse duration changed by a factor of 2. However, this did not lead to a significant reduction in the aural reactions. Thus, it is fair to assume that the reduction in the physiological costs which could have been expected due to the halving of the noise dose in TS III was almost completely compensated by the constant number of impulses. However, compared to the continuous noise exposure, both impulse noise exposures with a time interval between impulses of only 1 s did not exhibit a “more,” but a significant “less” in physiological costs. It seems obvious to attribute this result to the protective mechanism of the stapedius reflex. If the IRTTS values from TS II and TS III are expressed relative to the value from TS I (cp. Figure 11), such protection with a reduction of 0.51 in TS II apparently leads to a halving of the total physiological costs. For the quotient of 0.38 for the ratio of TS III to TS I, it must be kept in mind that the hearing was only exposed to half of the original noise energy.

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Figure 11: “Normalized Physiological Cost” of the exposures (IRTTSTS i / IRTTSTS I)

If the differences in the energetic exposures are taken into consideration – which is shown in Figure 12 – then weakened protection for the hearing would have to be assumed because of the now-doubled value of 0.76 (compared to 0.51) for extremely short impulse noise exposures with 2.5-ms impulses.

Figure 12: “Physiological Cost” (characteristic value “IRTTS”) of the impulse noise exposures compensated via the 3-dB exchange rate for a rating level LArd = 85 dB(A)

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5 References HESSE, J.M.; IRLE, H. und STRASSER, H. (1994) Laborexperimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Z.Arb.wiss. 48 (20 NF) 4, 237-244 IRLE, H.; HINZMANN, G. und STRASSER, H. (1999) Hörschwellenverschiebungen nach Impulsschall – Einfluß der Impulsanzahl und Impulsdauer. Z.Arb.wiss. 53 (25 NF) 2, 83-94 STRASSER, H. and IRLE, H. (2003) Conventional Measurement, Assessment, and Rating of Sound Exposures – A Critical Review from an Ergonomics Point of View. Ergonomic Study 5 (1) 49-58 IRLE, H.; HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposure: Temporary Threshold Shifts. In: KARWOWSKI, W. (Ed.) Int. Encyclopedia of Ergonomics and Human Factors. Volume II, Part 7, Environment, Taylor & Francis, London and New York, 1050-1056 IRLE, H. and STRASSER, H. (1998) Influence of the Number of Impulses and the Impulse Duration on Hearing Threshold Shifts. In: CARTER, N. and JOB, R.F. (Eds.) Noise Effects '98. Proceedings of the 7th International Congress on Noise as a Public Health Problem, Organized by the International Commission on the Biological Effects of Noise (ICBEN), Sydney/Australia, 236-239

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Chapter 10

Physiological Costs of the Hearing after Exposures to White Noise, Industrial Noise, Heavy Metal, and Classical Music of 94 dB(A) for 1 Hour H. Strasser, H. Irle and R. Scholz

0 Summary In order to disclose the actual physiological responses to Industrial Noise, a medley of Heavy Metal Music and Classical Music, 10 hearing physiologically normal subjects (Ss) participated in test series with 4 sound exposures which were characterized by the same level of 94 dB(A) over 1 h, each. In a first test series the Ss were exposed to White Noise as a reference sound exposure. In a second test series a prototype of Industrial Noise was applied. In a third test series typical Heavy Metal Music was utilized and in the fourth test series Classical Music was provided. The physiological responses to the 4 exposures were recorded audiometrically via the temporary threshold shift TTS2, the restitution time t(0 dB), and the IRTTS value which represents the total physiological cost the hearing must “pay” for the sound exposure. The results show again, in accordance with prior investigations, that the energy equivalent approach of rating sound exposures leads to gravely misconceiving assessments of their actual physiological cost. Industrial Noise with an IRTTS value of 631 dBmin in relation to 424 dBmin quantified as responses to White Noise brought about an increase of approximately 50 % in the total physiological cost. Heavy Metal Music was also associated with tremendous physiological cost (637 dBmin). Classical Music was accompanied by the slightest temporary threshold shifts which also disappeared very quickly. The temporary threshold shifts resulting from this type of music added up to an IRTTS value of only 160 dBmin. Related to the physiological responses to Industrial Noise or Heavy Metal Music, Classical Music caused only one quarter of the physiological cost. Because Heavy Metal Music like Industrial Noise causes multiple temporary threshold shifts compared to that of Classical Music, it can be concluded that those who listen to that modern type of music take a high risk of permanent hearing threshold shifts in the long run.

1 Introduction and objectives The occupational disease Noise-Induced Permanent Threshold Shift (NIPTS) is usually considered the result of long-term, detrimental exposure to noise in industrial workplaces. However, not only Industrial Noise in the form of continuous or impulse noise is problematic; music can also be harmful to the hearing. Previous studies have shown that approximately 1/4 of all orchestra musicians have some kind of hearing problems, and NIPTS – especially for brass players – is the number one occupational disease for these musicians (cp. BLUM 1995). According to new random measurements carried out by FUNK et al. (1997), brass instrument players – when in their usual seating positions in the orchestra pit – were exposed to an energy equivalent continuous noise level (depending on the piece of music) between 90 and 95 dB for the duration of an opera performance, i.e., usually for more than 1 hour. During solo rehearsals, trumpets and bass drums reached levels even up to 130 dB(A) and almost 140 dB(A), respectively, with a distance of approximately 40 cm between player and instrument. A topical and comprehensive survey of the sound exposure of orchestra musicians based on literature evaluation provide MARQUARDT and SCHÄCKE (1998). They conclude that orchestra musicians are regularly exposed to sound levels that are likely to cause hearing loss despite the fact that results of audiometric studies carried out in musicians, with special regard to occupation-related hearing impairments have been contradictionary. Sound levels on Walkmans can be similarly high, often for even longer periods of time; such portable devices whose volume control can be individually adjusted directly affect the hearing of usually young music fans via stereo headphones. According to American studies (cp. HAYNE and SCHULZE 1997), even college students (and not only teenagers) chose volume

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levels up to 120 dB(A). An overview of the dangers to the hearing of youths resulting from exposure to overly loud music can be found in a study carried out by MERCIER et al. (1998). In general, users of personal stereo equipment (headsets or cassette players) are not aware of the danger to their hearing which such equipment poses (cp. for example, HAYNE et al. 1997). However, tens of thousands of “music addicts” (not only single individuals) voluntarily endure acoustic loads (e.g., from the “Pink-Sound” at open-air concerts) which are comparable to those from a Jumbo Jet at take-off in the immediate vicinity. Hard Rock, Heavy Metal, and Techno Music often consists of less well-tempered harmonic music (with sinusoidal waves) than more abrupt impulse noise. Nevertheless, young people voluntarily subject themselves to such noise exposures in their leisure time to satisfy – mainly during group-dynamic emotional highs – their “lust for noise.” Since also music can be responsible for long-term aural consequences – as mentioned above – the reversible short-term effects of different exposures to music and noise on the hearing were studied.

2 Methods 2.1 Test design and working hypotheses The traditional measurement, evaluation, and assessment of noise exposures does not differentiate between various sources of noise. Moreover, in the course of preventative occupational health protection, only whether national limits (which are based on an 8-hour day of energy equivalent continuous noise) are satisfied is evaluated according to existing standards (cp. N.N. 1997). Except for the selection of a time constant (slow, fast, impulse), an exposure time-weighted average is calculated from the individual noise levels according to the 3-dB rule, without any special consideration for the time structure of the noise events. In order to evaluate the acute hearing-physiological effects of energetically equivalent exposures but different sound exposures with respect to time structure and semantic quality, 4 different exposures with a mean level Lm of 94 dB(A), each, over an exposure period of 1 hour were created for threshold shift experiments, as is shown in the front part of Figure 1. According to the 3-dB exchange rate, the exposures were equivalent to a rating level LArd of 85 dB(A) / 8 h and were therefore definitely ethically acceptable for the test subjects (Ss). The first test series (TS I) dealt with White Noise, which is often used as a neutral reference noise. Industrial Noise was used as the acoustic load in a second test series (TS II). A 25-s cut of a demonstration-noise CD was repeatedly recorded and seamlessly strung together in order to produce a 1 hour lasting, continuous sample of noise. This tape included numerous impulses in addition to the broad-band background noise of the machinery of a metalworking factory which was featured on the CD. These noise impulses resulted from hammer strikes, falling metal sheets and pipes, as well as the noise from work such as forging and stamping. A medley of typical Heavy Metal Music was played in TS III. A 10-min block of music which consisted of 3 pieces by Guns n’ Roses and 1 piece by AC/DC (cp. right part of Figure 2) with relatively constant sound levels but penetrating drum sections was seamlessly strung together resulting in 1 h of a continuous exposure. In TS IV, the test subjects were exposed to Classical Music. Solemn passages (“Largo” out of Handel’s “Xerxes”) were included as well as pieces which have frequent changes between slow, mellow parts and fast, loud parts (one part from Vivaldi’s “The Four Seasons” and one from Smetana’s “Moldau”) (cp. lower left part of Figure 2).

H. Strasser et al. / Physiological costs of the hearing after different exposures

Figure 1:

Schematic representation of the energy equivalent exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shifts TTS

Figure 2:

Presentation of the 4 different used sound and music exposures

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As can be seen in the rear part of Figure 1, the hypothesis to be tested was that different threshold shifts would be observed during the test series. These threshold shifts should be measurable in TTS2 values immediately after the sound exposure. Also, the restitution time t(0 dB), i.e., the amount of time necessary for a complete recovery of the threshold shifts, was expected to be a function of the previous acoustic load of test series I through IV.

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The 3 realistic exposures to Classical Music, Heavy Metal Music, and Industrial Noise are comparable with respect to their frequencies. There were no systematic differences in the levels of the 8 octaves in the mid-range from 63 to 8000 Hz. All 4 exposures were transmitted from a DAT-recorder via an amplifier to 2 loudspeakers in a sound-insulated test booth. The test subject was inside the booth in a standardized seating position. 2.2 Test subjects and audiometric selection procedures Each S was subjected to all 4 acoustic exposures in a random sequence on different days, thus acting as his or her own control measure. According to experience, this minimizes the inter-individual variations. Only persons with normal hearing were used as test subjects. According to the standards in ISO 4869-1, the test subjects’ threshold shift in the range up to 2 kHz could not be more than 15 dB above the normal threshold (of healthy men and women between 18 and 30 years of age). In the frequency range above 2 kHz, the maximum threshold shift was 25 dB. Ten Ss (4 women and 6 men) were selected based on these criteria. Their age was 28 ± 7.3 years and their weight was 76.2 ± 16.3 kg. Before each test, their individual resting hearing threshold was determined. The resting hearing threshold was the basis for subsequent measurements and analyses. After the acoustic exposure, the frequency of a test Ss maximum hearing threshold shift TTS2 had to be determined within the first 2 minutes via several measurements. This maximum relative hearing threshold shift above the individual resting hearing threshold 2 min after the end of the exposure (the so-called TTS2) is a classic characteristic value of audiometric examinations in hearing threshold shift experiments. With this frequency of the maximum threshold shift (which usually was 4 or 6 kHz), the restitution of the hearing threshold shift back to the resting hearing threshold was measured at exactly predetermined points in time. This point in time when the hearing threshold measured prior to the exposure was reached again, the restitution time, is also an important characteristic value of the acoustic strain analysis and is called t(0 dB). When a linear time scale is used, the shape of the restitution of a temporary hearing threshold shift resembles an exponential function. If, however, it is plotted against a logarithmic time scale, the regression function TTS(t) is a straight line.

3 Results Figure 3 shows the results of the measurements taken after the exposures to White Noise (Figure 3A), Industrial Noise (Figure 3B), Heavy Metal Music (Figure 3C), and Classical Music at a level of 94 dB(A) for 1 h (Figure 3D). Since the audiometric results from all test series are represented in the same manner, the selected layout of the Figure 3A containing the results of TS I will be explained in further detail. First, all the hearing threshold shifts which were determined for the Ss after their exposure were graphed in a TTS-time-coordinate system. These threshold shifts are the differences between the measured TTS values and the respective individual resting hearing threshold before the sound exposure. Furthermore, the arithmetic mean values averaged over the 10 Ss and the regression curve for the test series at every measuring point are shown in this diagram. The results of the mathematical-numerical evaluation are also shown in the upper box. It shows the regression function itself, from which the audiometrically significant values TTS2 and t(0 dB) were determined. In addition to the regression analytically determined maximum hearing threshold 2 min after the noise exposure (TTS2 reg.) and the time t(0 dB)reg., after which the threshold shifts completely dissipated, the range of the real measured values and, finally, the average values, i.e., the mean measured values TTS2 real and t(0 dB)real averaged over the 10 Ss, are shown.

H. Strasser et al. / Physiological costs of the hearing after different exposures

Figure 3:

Individual and mean restitution time courses TTS(t) for all 10 Subjects after all exposures (TS I through TS IV)

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As can be seen in Figure 3A, systematic threshold shifts with individual variations occurred immediately after the exposure to White Noise; the mean of the threshold shifts was roughly 20 dB (e.g., TTS2 reg. = 18.8 dB). The threshold shifts subsided over time following the course of a decreasing exponential function and completely dissipated after at most approximately 100 min (e.g., t(0 dB)reg. = 97 min). Figure 3B seems to show that the situation which occurs after an exposure to Industrial Noise of (also) 94 dB(A) for 1 h (i.e., an energy equivalent, realistic acoustic load) is similar to that which occurs after exposure to White Noise. However, the TTS2 values are somewhat higher (e.g., TTS2 reg. = 22 dB) and the restitution times are longer (e.g., t(0 dB)reg. = 130 min). The inter-individual variations are also greater, both in terms of the absolute magnitude of the hearing threshold shift and the time until total subsidence. As can be seen in Figure 3C, exposure to Heavy Metal Music of 94 dB(A) for 1 h also leads to maximum temporary threshold shifts of about 20 dB which take approximately 2 hours, i.e., 120 min, to subside. The mean values (e.g., TTS2 reg. = 22.7 dB and t(0 dB)reg. = 127 min) were comparable to the ones from the earlier test series, but the inter-individual variation was somewhat less, i.e., the Ss reacted more homogeneously to this (energy equivalent) acoustic load than to Industrial Noise. Finally, Figure 3D shows the effects of an energy equivalent exposure to Classical Music on the hearing. Already at first glance it can be seen that loud Classical Music at a level of 94 dB(A) for 1 hour leads to considerably smaller threshold shifts. The characteristic values of the threshold shifts with respect to magnitude and duration are substantially smaller. For example, the maximum threshold shifts (TTS2 reg. value = 11.4 dB) are only approximately half as big as after the other exposures, and instead of 2 hours, it only took approximately 1 hour (t(0 dB)reg. = 55 min) for these threshold shifts to completely subside. In order to allow an overall assessment and a comprehensive statistical analysis of the measured differences, the area under the regression line as a global characteristic value for the aural effects of sound exposures was calculated. This Integrated Restitution Temporary Threshold Shift (IRTTS) (cp. IRLE et al. 1998; IRLE and STRASSER 1998) is computed as the regression function’s (TTS(t)) integral from 2 min after the exposure to t(0 dB). The IRTTS is a numeric value for the total threshold shift (in dB x min) which has to be “paid” by the hearing in physiological cost for the exposure. Figure 4 shows a comparison of the results in summarized form between the 4 energy equivalent test series. The regression analytical characteristic values TTS2 reg. and t(0 dB)reg. are presented again for the test series I-IV at the beginning and the end of the smoothed courses of restitution. Additionally, the upper part of the figure contains the IRTTS values. Industrial Noise and Heavy Metal Music (exposures in TS II and TS III) cause almost identical global physiological cost (631 or 637 dBmin, respectively), which could be expected from the TTS2 values and the t(0 dB) values. Yet, the characteristic values of these two test series are substantially larger than the IRTTS value of TS I (424 dBmin). The IRTTS value of Classical Music (160 dBmin), however, is only a fraction of the values from the other 3 test series. Most of the differences between the physiological responses to the 4 exposures are statistically significant according to the two-tailed WILCOXON-test. Compared to White Noise, Heavy Metal Music, and Industrial Noise, Classical Music leads to significantly or even highly significantly smaller aural consequences with respect to the regressionanalytically determined TTS2, t(0 dB), and IRTTS values. Heavy Metal Music leads to significantly graver consequences than White Noise, but it is not significantly different from Industrial Noise, the latter not being significantly different from White Noise.

H. Strasser et al. / Physiological costs of the hearing after different exposures

Figure 4:

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Restitution time courses TTS(t) with values of TTS2 reg., t(0 dB)reg., and physiological cost IRTTS of all energy equivalent exposures

Calculation of the ratio of the IRTTS values of Industrial Noise in TS II, of Heavy Metal Music in TS III, and of Classical Music in TS IV related to the IRTTS value of White Noise in TS I yielded values of 1.49, 1.50, and 0.38. Thus, Industrial Noise and Heavy Metal Music cause threshold shifts which are approximately 50 % higher than those from White Noise, and Classical Music leads to substantially lower physiological cost (roughly 38 % of those resulting from White Noise) (cp. rear part of Figure 5). Moreover, the threshold shifts from Classical Music are only about one quarter the magnitude of the threshold shifts which are due to Industrial Noise and Heavy Metal Music (cp. front part of Figure 5).

Figure 5:

“Relative physiological costs” of the energy equivalent exposures (TS I through TS IV), quantified by using the IRTTS values of TS I (rear) or TS II (front) as a reference

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4 Discussion While the permissible noise exposure in the industrial sector is subject to a number of regulations (N.N. 1976; N.N. 1990; N.N. 1997) and is controlled – at least via random searches – by government and semi-government organizations (e.g., the officers of the factory inspection and the mutual accident insurance associations), there still exists a great deal of freedom in the recreational sector. Often, especially youths seem to equate “noise” with vitality and the joy of life. Measures to avoid dangers to audiences’ hearings from high sound levels from loudspeakers are included in DIN standards. However, the concepts of the energy equivalence and the dose maxim are used to calculate maximum theoretically permissible levels (e.g., DIN 15905-5). Until now it is difficult to fight acoustic excesses in the recreational sector with regulations of any kind, but people should know what amount of risk they take exposing themselves to different sound exposures with the same rating level. In accordance with earlier work done by our group, the presented results again show that an energy equivalent evaluation of acoustic stress leads to grave mis-evaluations of the true physiological cost. STRASSER (1995; 1996) and STRASSER and HESSE (1993), among others, have shown that the dose principle for the evaluation of environmental stress is not compatible with the human characteristics. In a study HESSE et al. (1994) showed that the splitting up of a continuous noise exposure of 85 dB(A) for 8 hours into energy equivalent impulse noise leads to a drastic increase of the threshold shift (see also STRASSER at al. 1995). Impulse noise of 5 ms over 8 hours which resembles actual work patterns led to increased TTS2 values which – although statistically highly significant – would not even be very problematic. Yet, while the restitution time after continuous noise is usually approximately 2 hours, this value was drastically increased to approximately 600 min, i.e., 10 hours after impulse noise. Further, IRLE et al. (1998) empirically showed that energetically irrelevant impulse noise which is added to a base continuous noise of 94 dB(A) / 1 h was not consequence-free for the hearing, although the rating level was only increased by 0.4 dB(A) to 85.4 dB(A) by the addition of the impulse noise. The results presented here show once more that the calculation of the rating level is a highly compromising or even dangerous method for the preventative protection of humans. One must wonder whether it can be justified to exclusively use the measure “rating level” to evaluate sound exposures when this measure leads to equal results, but the physiological responses differ by a factor of 4 (e.g., in the case of Classical Music and Industrial Noise or Heavy Metal Music). It can be mentioned in this context that peak levels do not seem to be the relevant factor in the metabolic fatigue of hearing. After all, the tape with Classical Music had higher peak levels (123 dB) than both the Heavy Metal Music (119 dB) and Industrial Noise (117 dB). If work safety (cp. N.N. 1997; DIN 15905-5) is to be guided by the human body, i.e., if it is to be more than just counting peas or applying schematic management systems which do not self-critically ask whether the utilized principles are correct and suited to solve the problem, then the individuals in charge have to start rethinking their approach, even if this is uncomfortable. If this does not happen, one cannot be surprised when the number of individuals with damage to the hearing is on the rise again despite tightened work safety laws. With respect to music listening as a very popular and enjoying leisure activity with recreational sound exposure, the results of the study should have made clear that Heavy Metal Music has to be regarded as a kind of “relative” of impulsive Industrial Noise. On the other hand – although only two sample exposures out of the broad spectrum of different music styles were tested – the consumer of Classical Music possibly takes a comparatively low hearing risk, even under high volume exposures.

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5 References BLUM, J. (Hrsg.) (1995) Medizinische Probleme bei Musikern. Georg-Thieme-Verlag, Stuttgart/New York FUNK, D.; KESSLER, H. und KURZ, W. (1997) Orchestermusik = Lärm? Sicherheitsbeauftragter 8, 14-18, 9, 1418 HAYNE, M. and SCHULZE, L.J.H. (1997) Personal and Car Stereo Volume Levels: Hazards of Leisure Listening Activities. In: DAS, B. and KARWOWSKI, W. (Eds.) Advances in Occupational Ergonomics and Safety II. IOS Press, Amsterdam, 581-584 HAYNE, M.; SCHULZE, L.J.H. and CHEN, J. (1997) Sound Pressure Levels, Usage, Listening Preference, Situation of Use, and Hazard of Exposure Knowledge of College Students Using Personal Stereo Equipment. In: B. DAS and W. KARWOWSKI (Eds.): Advances in Occupational Ergonomics and Safety II. IOS Press, Amsterdam, 517-520 HESSE, J.M.; IRLE, H. und STRASSER, H. (1994) Laborexperimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Z.Arb.wiss. 48 (20 NF) 4, 237-244 IRLE, H. and STRASSER, H. (1998) Influence of the Number of Impulses and the Impulse Duration of Hearing Threshold Shifts. Proceedings of the 7th International Congress on Noise as a Public Health Problem, organized by the International Commission on the Biological Effects of Noise (ICBEN). In: CARTER, N. and JOB, R.F. (Eds.) Noise Effects ’98 (Sydney/Australia), 236-239 IRLE, H.; HESSE, J.M. and STRASSER H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. International Journal of Industrial Ergonomics 21, 451-463 MARQUARDT, U. und SCHÄCKE, G. (1998) Arbeitsschutz und Ergonomie, “Hearing Impairment in Orchestra Musicians” (in German). Zentralblatt für Arbeitsmedizin 48, 188-204 MERCIER, V.; WÜRSCH, P. und HOHMANN, B. (1998) Gehörgefährdung Jugendlicher durch überlauten Musikkonsum. Zeitschrift für Lärmbekämpfung 45 (1) 17-21 N.N. (1997) Technical Assessment of Upper Limits on Noise in the Workplace. Final Report, Approved by the International Institute of Noise Control Engineering. Noise/News International, 203-216 STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Workload. Archives of Complex Environmental Studies, 5 (1-2) 9-25 STRASSER, H. (1995) Dosismaxime und Energie-Äquivalenz – Ein Kernproblem des präventiven Arbeitsschutzes bei der ergonomischen Beurteilung von Umgebungsbelastungen. In: STRASSER, H. (Hrsg.) Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen – Anspruch und Wirklichkeit des präventiven Arbeitsschutzes. Ecomed Verlag, Landsberg/ Lech, 9-31 STRASSER, H.; HESSE, J.M. and IRLE H. (1995) Hearing Threshold Shift after Energy Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and CHAMPNEY P.C. (Eds.) Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, London/New York/Philadelphia, 241-248 STRASSER, H. (1996) Curiosities of Conventional Noise Rating Procedures. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, 619-626 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN 15905-5 (1989) Acoustics in Theatres and Multipurpose Halls; Measures to Avoid Impairing the Audience’s Hearing by High Sound-Pressure Levels from Loudspeaker Reproduction. Beuth Verlag, Berlin German Working Places Regulations - § 15 Noise Abatement (1988) Arbeitsstätten – Vorschriften und Richtlinien, § 15 Schutz gegen Lärm ISO 4869-1 (1990) Acoustics; Hearing Protectors; Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin

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Chapter 11

Temporary Hearing Threshold Shifts and Restitution Associated with Exposures to Industrial Noise and Classical Music of 94 dB(A) for 1 Hour and 91 dB(A) for 2 Hours H. Strasser, H. Irle and R. Legler

0 Summary In order to investigate whether the energy-equivalence principle is at least acceptable for exposures with a duration in the range of hours and in order to disclose the actual physiological responses to exposures which varied with respect to the time structure and the semantic quality of sounds, a series of tests was carried out where physiological costs associated with varying exposures were measured audiometrically. In a cross-over test design, 10 Subjects (Ss) participated in a test series with 3 energetically equal sound exposures on different days. The exposures corresponded with a tolerable rating level of 85 dB / 8 h. In a first test series (TS I), the Ss were exposed to a prototype of industrial noise with a sound pressure level of 94 dB(A) / 1 h. In a second test series (TS II), the same type of noise was applied, but the exposure time of a reduced level of 91 dB(A) was increased to 2 hours. In a third test series (TS III), classical music was provided also for 2 h at a mean level of 91 dB(A). The physiological responses to the 3 exposures were recorded audiometrically via the temporary threshold shift TTS2, the restitution time t(0 dB), and the IRTTS-value. IRTTS is the integrated restitution temporary threshold shift which is calculated by the sum of all threshold shifts. It represents the total physiological costs the hearing must “pay” for the sound exposure. Physiological responses of the hearing to the industrial noise exposures in TS I and TS II, all in all, were identical in the 3 parameters. Maximum threshold shifts of approximately 25 dB occurred which did not dissipate completely until 21/2 h after the end of the exposure and IRTTS-values of about 800 dBmin were calculated. Therefore, at least for exposure times in the range of hours, the equilibration of intensity and duration of sound exposures according to the energy equivalence principle seems to have no influence on the hearing. Classical music was associated with the least severe TTS of less than 10 dB which disappeared much more quickly. IRTTS added up to just about 100 dBmin and, in comparison with 800 dBmin as specific responses to industrial noise, amounted to only about 12 %. The substantially lower physiological costs of classical music apparently indicate a decisive influence of the type of sound exposures. Making inferences from the results of the study, the conventional approach of rating sound exposures exclusively by the principle of energy equivalence can lead to gravely misleading assessments of their actual physiological costs.

1 Introduction and objectives Previous empirical studies have shown that different time structures of energy equivalent noise exposures can lead to greatly varying “physiological costs.” Such physiological costs can be measured precisely in the form of threshold shifts (cp. IRLE and STRASSER 2001; STRASSER et al. 1999). However, the pattern of restitution after a noise exposure is a better measure than the maximum temporary threshold shifts, the so-called TTS2-values alone, which can be measured audiometrically immediately (typically within 2 min) after the exposure. Dependent on the noise exposure, the restitution time (i.e., the time required for the hearing to return to the resting hearing threshold) varies substantially more than the height of the threshold shifts after the exposure. An important reason for that is that TTS2-values are measured in “dB,” i.e., a logarithmic measure which may lead to drastic underestimations. As is well-known, an increase of only 3 dB corresponds with a doubling of the associated energy. The same is true for TTS-values expressed in a logarithmic scale. Just 3 dB more correspond with doubling of the height of the threshold shift. When, finally, the entire area under the restitution function is analyzed, that is, when the integral over the temporary threshold shifts is calculated from the time of the first measurement after the noise exposure to the time at

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which the resting hearing threshold is reached again, a sensitive integral characteristic value can be obtained. With this procedure, it is possible to establish the causality between the total of the threshold shifts and the preceding stress. It was found in the already mentioned study by STRASSER et al. (1999) that the physiological costs in Ss who were exposed to industrial noise with a mean level of 94 dB(A) for 1 hour were approximately 50% higher than the physiological costs in individuals who were exposed to equally loud white noise. A medley of heavy metal-music led to threshold shifts which were very similar to those caused by industrial noise. Classical music, on the other hand, caused threshold shifts which were only approximately one quarter the magnitude of those after exposure to industrial noise or heavy metal-music. It has, of course, to be admitted that sound exposures for one hour are not necessarily representative for the workplace with a typical work duration of 8 hours. Similarly, the exposure time to music, e.g., during a classical concert can easily be as long as 2 hours. According to the exchange rate of 3 dB, as visualized in Figure 1, an exposure to 94 dB for 1 h, indeed, is equivalent to 91 dB for 2 h, 88 dB for 4 h, and 85 dB for 8 h, and all these exposures are legally tolerable for the production sector. Yet, even if the same energy, the same noise dose is inherent in these exposures the same effects on man cannot always be expected. Especially, noise exposures varying in frequency and time structure or via the energy equivalence principle equally rated continuous and impulse noise have to be evaluated with distinct precaution.

Figure 1:

Sound pressure levels of different durations leading to an equal rating level when applying the exchange rate of 3 dB

Based on some measurements in the 1960s, MILLER (1974) developed a diagram (cp. Figure 2) which makes it possible to hypothetically estimate the TTS2-values from levels (between 70 and 120 dB) after exposures of different duration. According to such calculations, the threshold shifts TTS2 measured at the testing frequency of 4 kHz, indeed, seem to be similar (~30-35 dB) for the above-mentioned situations of 85 dB / 8 h, 91 dB / 2 h, or 94 dB / 1 h.

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Figure 2:

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Hypothetical TTS2-course associated with duration and level of noise and examples of energy equivalent exposures with a rating level of LArd = 85 dB(A) (according to MILLER 1974)

For even higher levels (with reduced exposure times in accordance with energy equivalence), decreasing TTS2-values can be expected. Already some time ago, HESSE and STRASSER (1990) were also able to show that empirically. While threshold shifts of approximately 35 dB were measured after an exposure to 94 dB / 1 h, the respective shifts after an intense energy equivalent exposure to 113 dB / 45 s amounted to only a few dB. As mentioned earlier, however, TTS2-values are just one aspect of this issue, and it must be stressed that the principle of energy equivalence should not be applied to levels in excess of 120 dB. Such extremely loud noises are more likely to lead to mechanical damage in the sound transmission apparatus and in the sensitive structures of the inner ear than to metabolic disturbances in the cochlea which can be immediately documented audiometrically. The equilibration of legally permissible 85 dB(A) /8 h with 120 dB / 10 s or even 140 dB / 100 ms, which is in accordance with national and international standards (cp. Accident Prevention Regulation “Noise” (1990) or N.N. 1998), is not justifiable from a workphysiological point of view. Such equilibration becomes absolutely irresponsible when impulse noise and blast noise are “converted down” to tolerable levels of continuous noise. An example for that would be the working with compressed-air nailers when 2,000 impulses of 5 ms each (i.e., a total duration of 2,000 x 0.005 s = 10 s) at a level of 120 dB are experienced. Another example is 100 individual impulses of 1 ms each with a level of 140 dB – such as during bolt setting – which are also energy equivalent to continuous noise of 85 dB / 8 h. Extensive research, in fact, has been carried out by the in-house working group about the effects of noise exposures in the continuum between ethically still acceptable 113 dB / 45 s and 94 dB / 1 h. The effects of energy equivalent noise with longer exposure times, in essence for an 8-hour day at 85 dB (i.e., with a rating level Lr = 85 dB), have not been examined much from a work-physiological perspective, however. Therefore, noise exposures whose physiological responses are known already (industrial noise and classical music (cp. STRASSER et al. 1999) were re-analyzed with respect to their audiometric effects at varying exposure times and levels.

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2 Methods 2.1 Test design and test set-up Figure 3 shows the schematic test design. The exposures were transmitted as prepared sound sources from a DAT-Recorder via an amplifier to 2 loudspeakers in a sound proof cabin. The test subject was inside the booth in a standardized seating position. A nominal value adjustment of the sound exposure was carried out with an integrating sound level meter. Before and after the exposure, the Ss were audiometrically evaluated.

Figure 3:

Schematic test set-up

As shown in Figure 4, exposures to 91 dB for 2 h have been chosen in TS II and TS III which were reduced in level by 3 dB but prolonged to 2 h, that is, which were twice as long as in reference test series TS I. For the exposure of TS I, a 25 s cut of a demonstration-noise CD was repeatedly recorded and seamlessly strung together in order to produce a 1-hour-long, continuous sample of noise. For TS II, this sound medley was reduced in level by 3 dB but exposed twice as long, that is, two hours. In addition to the broad-band background noise of the machinery of a metal working factory, noise impulses were also featured on the CD. These impulses resulted from hammer strikes, falling metal sheets and pipes, as well as from work such as forging and stamping. In TS III, the Ss were exposed to classical music. Solemn passages (“Largo” out of Händel’s “Xerxes”) were included as well as pieces which have frequent changes between slow, mellow parts and fast, loud parts (one part from Vivaldi’s “The Four Seasons” and one from Smetana’s “Moldau”). The exposures to industrial noise and classical music are comparable with respect to their frequencies. The medley of music and industrial noise intentionally had been composed in such a way that there were no systematic differences in the levels of the 8 octaves in the mid-range from 63 to 8,000 Hz. As can be seen in the back part of Figure 4, the hypothesis to be tested was that different threshold shifts would be observed during the test series. These threshold shifts should be measurable in TTS2-values immediately after the sound exposure. Also, the restitution time t(0 dB), that is, the amount of time necessary for the threshold shifts to completely dissipate, was expected to be a function of the previous acoustic load of tests I through III.

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Figure 4:

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Schematic representation of the 3 energy equivalent exposures (Domain of Stress) and hypothetical physiological responses, that is, growth and restitution of the Temporary Threshold Shift (Domain of Strain)

Since inter-individual differences in the way human beings react to exposures usually cause substantial variation thus leading to a more or less pronounced interference of the specific effects of the test variables, a cross-over test design was chosen. Each test subject was subjected to all 3 sound exposures in a random sequence on different days, thus acting as his or her own control measure. According to experience, this minimizes the inter-individual variations. 2.2 Test subjects and audiometric selection procedures Only persons with normal hearing were used as Ss. According to the standards in DIN ISO 4869-1, the Ss’ threshold shift in the range up to 2 kHz could not be more than 15 dB above the normal hearing threshold (of healthy men and women between 18 and 30 years of age). In the frequency range above 2 kHz, the tolerable maximum threshold shift was 25 dB. Ten Ss (3 women and 7 men) were selected based on these criteria. Their age was 26 ± 3.4 years, their weight was 72.1 ± 7.6 kg, and their height was 179.8 ± 7.2 cm. Before each test, their individual resting hearing threshold was determined which was the basis for subsequent measurements and analyses. After the sound exposure, the frequency of a S’s maximum threshold shift TTS2 had to be determined via several measurements within the first 2 minutes (cp. left part of Figure 5). This maximum of the “individual hearing threshold shift” above the “individual hearing threshold” is a classic characteristic value of audiometric examinations in hearing threshold shift experiments. With this frequency of the maximum threshold shift (which usually was 4 or 6 kHz), the restitution of the hearing threshold shift (back to the resting threshold) was measured at exactly predetermined points in time (cp. right part of Figure 5). This point in time, the restitution time t(0 dB), is also an important characteristic value of the acoustic strain analysis. When a linear time scale is used, the shape of the restitution of a temporary hearing threshold shift resembles an exponential function. If, however, it is plotted against a logarithmic time scale (cp. the upper block of the right part of Figure 5), the regression function TTS(t) is a straight line.

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Figure 5:

Selection of the frequency with maximum threshold shift during the first 2 minutes after the exposure (left part) and typical individual restitution time course TTS(t) after the exposure with characteristic values TTS2 (Temporary Threshold Shift 2 minutes after the exposure) and t(0 dB) (restitution time) in linear and logarithmic time scale

3 Results The results of TS I, that is, after industrial noise at a level of 94 dB(A) for 1 h which was used as a reference, are shown in Figure 6A. Since the audiometric results from the two other test series are also represented in the same manner, the selected layout of the figure will be explained in further detail. First, all the hearing threshold shifts which were determined for the Ss after their exposure were graphed in a TTS-time-coordinate system. These threshold shifts are the differences between the measured TTS-values and the respective individual resting hearing threshold before the sound exposure, as shown previously in Figure 5. Furthermore, the arithmetic mean values averaged over the 10 Ss and the regression graph for the test series at every measuring point are shown in this diagram. The results of the mathematical-numerical evaluation are also shown in the upper right box of the figure. This box shows the regression function itself, from which the audiometrically significant values TTS2 and t(0 dB) were determined. In addition to the regression-analytically determined maximum hearing threshold 2 min after the noise exposure (TTS2 reg.) and the time t(0 dB)reg., after which the threshold shifts completely dissipated, the range of the real measured values and, finally, the average values, that is, the mean measured values TTS2 real and t(0 dB)real averaged over the 10 Ss, are shown. As can be seen in Figure 6A, systematic threshold shifts with individual variations occurred immediately after the exposure; the mean of the threshold shifts was somewhat above 20 dB (e.g., TTS2 reg. = 23.7 dB). The threshold shifts subsided over time following the course of a decreasing exponential function and completely dissipated after at most approximately 21/2 h (e.g., t(0 dB)reg. = 154 min). Figure 6B shows that after industrial noise which was reduced in level by 3 dB to 91 dB but exposed for 2 h instead of 1 h (i.e., an energy equivalent exposure), almost identical physiological responses were measured. The regression-analytically determined TTS2 reg.-value of 24.8 dB amounts to practically the same level as that of TS I. Differences in the range of 1 dB – which are within the sensitivity of the measurement method – may not be interpreted. The time needed for a complete restitution also was almost exactly the same, that is, the t(0 dB)-value amounts to 153 min. Finally, Figure 6C shows the effects of an energy equivalent exposure to classical music on the hearing. Already at first glance it can be seen that loud classical music at a level of 91 dB / 2 h leads to substantially smaller threshold shifts. The threshold shifts with respect to magnitude and duration differ clearly. For example, mean maximum threshold shifts of 9.7 dB are less than half as large as after the other exposures, and instead of hours, it only took about 1/2 h (t(0 dB)reg. = 37 min) for these threshold shifts to completely subside.

H. Strasser et al. / Temporary hearing threshold shifts and restitution

Figure 6:

Individual and mean restitution time course TTS(t) for all 10 subjects after the exposures “TS I – 94 dB(A) / 1 h Industrial Noise” (A), “TS II – 91 dB(A) / 2 h Industrial Noise” (B), and “TS III – 91 dB(A) / 2 h Classical Music" (C). All exposures are equivalent to a rating level LArd = 85 dB(A)

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In order to allow an overall assessment and a comprehensive statistical analysis another global characteristic value for the aural effects of sound exposures was calculated. For this calculation (similar to the determination of the restitution (recuperation) of the cardiovascular stimulation after dynamic muscle work which becomes evident in the sum of work-related increases of heart rate which still exist after finishing work), the area under the regression line was determined. This Integrated Restitution Temporary Threshold Shift (IRTTS) is computed as the integral of the regression function TTS(t) from 2 min after the exposure to the point t(0 dB). The IRTTS is a numeric value for the total threshold shift (in dB x min) which has to be “paid” by the hearing in physiological costs for the exposure. Figure 7 in summarized form shows all the physiological responses to the 3 energy equivalent exposures. The regression analytical characteristic values TTS2 reg. and t(0 dB)reg. are shown for the test series TS I, TS II, and TS III, each, at the beginning and the end of the smoothed courses of restitution. In the upper right corner of the figure, the IRTTS-values with which the physiological costs of the exposure can be described are shown, too. Industrial noise, that is, exposures to 94 dB for 1 h in TS I and to 91 dB for 2 h in TS II cause almost identical global physiological costs (i.e., IRTTS-values of 774 dBmin and 800 dBmin), which could already be expected from the TTS2- and the t(0 dB)-values. However, the characteristic values of these two test series are substantially larger than that of TS III. The IRTTS-value of classical music, namely 98 dBmin, is only a fraction of the values from the other 2 test series.

Figure 7:

Restitution time course TTS(t) with values of TTS2 reg., t(0 dB)reg., and physiological cost IRTTS with symbolic labeling of the significance level (two-tailed WILCOXON-test)

As can be seen, statistically significant differences between the results of the test series can be shown, even if the (more appropriate) two-tailed WILCOXON test is used. The differences between the test series become more pronounced when a one-tailed test is used. Compared to both industrial noise exposures, classical music leads to significantly (’’) or even highly significantly (’’’) smaller physiological responses with respect to all parameters. This is true for the real and the regression-analytically determined TTS2, t(0 dB), and IRTTS-values. The results of the test series TS I and TS II did not show any differences (…) in all three parameters, therefore, absolutely, no significant differences do exist.

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As a global means of evaluation, the ratio of the IRTTS-values from the 3 test series was calculated in such a way that the IRTTS-values of industrial noise in TS I and of classical music in TS III were related to the IRTTS-value of industrial noise in TS II (cp. numbers of “0.97” and “0.12” in upper right block of Figure 7). Thus, classical music leads to substantially lower physiological costs, namely just 12% of those resulting from industrial noise at the same level and exposure duration. Due to 0.97, physiological costs of TS I may be regarded the same as those of TS II.

4 Discussion 4.1 On the validity of the equal energy concept The hearing threshold shifts and their restitution after the exposures of test series TS I and TS II are not significantly different from, but rather practically identical to, each other. This suggests that the principle of energy equivalence can be assumed to be valid even for the physiological responses to noise exposures whose duration varies in the range of hours. It is admittedly true that this alone does not positively prove that the effects of real-working-life noise exposures (which last up to 8 h) are identical to the reference load of 94 dB / 1 h. However, in conjunction with previous experiences of MILLER (1974) which are depicted in Figure 2, there is substantial evidence that the reference load which was chosen for this and numerous other studies (cp. e.g., HESSE et al. 1994; IRLE et al. 1998; IRLE et al. 1999; IRLE and STRASSER 1998; STRASSER et al. 1999) is justified for audiometric studies. Of course, exposure times of 1 h help to keep audiometric measurements in the laboratory manageable. 4.2 On the significance of hearing threshold shifts The threshold shifts experienced in TS I and TS II (approximately 20 dB) again show that the hearing pays considerable physiological costs for sound exposures which in most countries are permissible in the production area without hearing protection (cp. N.N. 1997). This necessarily leads to a reduction in the ability to hear acoustic signals which shall be explained as follows: If the level of a sound event is increased or decreased by 10 dB, respectively, the subjectively experienced loudness of such an acoustic event (measured in Sone) is increased or reduced by the factor 2, respectively. This has been shown empirically by STEVENS (1957) with a psycho-physical law (cp. STRASSER and IRLE 2001). As is well-known, a sound event which is 10 dB louder or quieter, respectively, is felt as twice or half as loud, respectively. Another 10-dB difference corresponds with another change in loudness by the factor 2 according to the Sone scale. If this principle is applied to the loss (which is caused by the threshold shift) in the hearing’s ability to hear sound events of a certain loudness, the following important conclusion results: A temporary threshold shift of, e.g., 20 dB ultimately reduces the loudness with which the individuals hear sound events in their leisure time (such as music or spoken words) to 1/4 of the level which would result if no threshold shift had occurred. During the time of the threshold shift, this means a loss in the quality of life of the affected individuals. Additionally, there is the safety concern that warning signals may not be heard properly. 4.3 Possible reasons for the better tolerability of classical music The two sound exposures “industrial noise” and “classical music” were – as mentioned in chapter 2.1 – put together in a way that there were no substantial and systematic differences in the distribution of frequencies (cp. Figure 8), which otherwise could affect the hearing variously.

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Figure 8:

H. Strasser et al. / Temporary hearing threshold shifts and restitution

Frequency analysis of the sounds

But a possible explanation for the extremely different effects of the two exposures may be seen in differences in the time structure and level distribution of the sound exposures. It must be mentioned in this context, however, that it is apparently not the peak levels which are responsible for the hearing’s metabolic fatigue. After all, the “classical music” exposure contained higher unweighted levels Lpeak (123 dB) than the “industrial noise” (117 dB). The left part of Figure 9 shows 10-min plots of level-time signals of the exposures “industrial noise” and “classical music.” When plotting these time series (levels in dB(AF) over time), the above-mentioned peak levels (Lpeak) cannot be detected due to the smoothing of short-lasting peaks by the utilized time constant (F = Fast) of 125 ms. As can be seen, classical music exhibits substantially broader variations in level, that is, more dynamic sound events also varying in time. On the contrary, industrial noise is characterized by smaller amplitude changes and a short-cycled regular time series. The probability density functions which in addition to the level-time signals are shown in the right part of Figure 9 make clear that the level distribution of the sounds of classical music represents more a “normal distribution” than is the case with industrial noise. When, as can be seen from the table of Figure 9, the level distribution is categorized in 2-dB steps, e.g., with 34.9% and 31.2% for 90 and 92 dB, industrial noise is strongly concentrated on a few classes. On the other hand, the distribution curve of classical music is much more flat and broader with a peak concentration of just 18% for 94 dB. Finally, if one considers that the sound exposure from “classical music” to a large extent consists of sine-shaped time signals, it may become even easier to understand why this type of exposure is associated with less strain for the human hearing. As is well known, a single note is represented by a sinusoidal oscillation of a single frequency while a chord is represented by several simultaneous tones (notes) whose ratio to each other are whole numbers. This is not the case for noises with a – typically – broad-band mix of stochastic, often not sine-shaped time signals. While it is clear that very loud “classical music” poses certain dangers to the hearing as well (cp. e.g., SATALOFF and SATALOFF 1993; ISING et al. 1996; MERCIER et al. 1998; WEGNER et al. 2000), there is no doubt as to which kind of music can be better tolerated by or is more compatible with the human hearing. The threshold shifts caused by heavy metal-music – which is not unlike industrial noise which often results from banging metal on metal – are similar to the threshold shifts caused by industrial noise, as has already been shown in an earlier study (cp. STRASSER et al. 1999).

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Figure 9:

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Level-time signals (left part) and sound level analysis (probability density function in the right part and table)

5 References HESSE, J.M. und STRASSER, H. (1990) Hörschwellenverschiebungen nach verschieden strukturierter energieäquivalenter Schallbelastung. Z.Arb.wiss. 44 (16 NF) 3, 169-174 HESSE, J.M.; IRLE, H. und STRASSER, H. (1994) Laborexperimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Z.Arb.wiss. 48 (20 NF) 4, 237-244 IRLE, H. and STRASSER H. (1998) On the Effects of Dynamic Muscle Work on Noise-Induced Hearing Threshold Shifts. Proceedings of the 7th International Congress on Noise as a Public Health Problem, organized by the International Commission on the Biological Effects of Noise (ICBEN). In: CARTER, N. and JOB, R.F. (Eds.) Noise Effects ’98 (Sydney/Australia) 51-54 IRLE, H.; HESSE, J.M. and STRASSER, H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. Int. Journal of Industrial Ergonomics 21, 451-463 IRLE, H.; ROSENTHAL, Ch. and STRASSER, H. (1999) Influence of a Reduced Wearing Time one the Attenuation of Hearing Protectors Assessed via Temporary Threshold Shifts. Int. Journal of Industrial Ergonomics 23, 573-584 IRLE, H.; HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposures: Temporary Threshold Shifts. In: KARWOWSKI, W. (Ed.) Int. Encyclopedia of Ergonomics and Human Factors. Volume II, Part 7, Environment, Taylor & Francis, London/New York., 1050-1056 ISING, H.; SUST, C.A. und PLATH, P. (1996) Gehörschäden durch Musik. Gesundheitsschutz 5, Bundesanstalt für Arbeitsschutz und Arbeitsmedizin, Dortmund MERCIER, V.; WÜRSCH, P. und HOHMANN, B. (1998) Gehörgefährdung Jugendlicher durch überlauten Musikkonsum. Zeitschrift für Lärmbekämpfung 45 (1) 17-21 MILLER, J.D. (1974) Effects of Noise on People. J. Acoustics Soc. America 56 (3) 729-764 N.N. (1997) Technical Assessment of Upper Limits on Noise in the Workplace. Final Report, Approved by the International Institute of Noise Control Engineering. Noise/News International, 203-216 N.N. (1998) NIOSH Criteria for a Recommended Standard – Occupational Noise Exposure Revised Criteria 1998. DHHS (NIOSH) Publication No. 98-126. US Department of Health and Human Services, Cincinnati, OH

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SATALOFF, R.T. and SATALOFF, J. (1993) Hearing Loss in Musicians. In: SATALOFF, R.T. and SATALOFF, J. (Eds.) Occupational Hearing Loss. Second Edition, Revised and Expanded, Marcel Dekker Inc., New York, Basel, Hong Kong, 583-594 STEVENS, S.S. (1957) On the Psychophysical Law. Psychol. Review 64, 153-181 STRASSER, H.; IRLE, H. and SCHOLZ, R. (1999) Physiological Cost of Energy-Equivalent Exposures to White Noise, Industrial Noise, Heavy Metal Music, and Classical Music. Noise Control Engineering Journal 47 (5) 187-192 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: KARWOWSKI, W. (Ed.) Int. Encyclopedia of Ergonomics and Human Factors. Volume I, Part 3, Performance Related Factors, Taylor & Francis, London/New York, 516-523 WEGNER, P.; WENDLAND, R.; POSCHADEL, B.; OLMA, K. und SZADKOWSKI, D. (2000) Untersuchungen zu Wirksamkeit und Akzeptanz von Gehörschutzmaßnahmen bei Orchestermusikern. Arbeitsmed.Sozialmed.-Umweltmed. 35 (10) 486-497 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin

Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 12

Comparative Investigations into Physiological Responses to Heavy Metal, Techno, and Classical Music H. Irle, F. Körner and H. Strasser

0 Summary Three different kinds of music were utilized to address the question whether sound exposures with different frequency and time structures may differ in their potential danger to the human hearing. In Test Series I (TS I), 10 test subjects (Ss) were exposed to a medley of typical Heavy Metal Music with an exposure level of 94 dB(A) for 1 hour (h). This exposure had been used in a previous study and served as reference or basis for comparison. The exposure in TS II was also 94 dB(A) for 1 h and consisted of a compilation of so-called Techno Music from the 2001 “LOVEPARADE” in Berlin. In TS III, the test subjects were exposed to representative Classical Music at 94 dB(A) for one hour. Contrary to previous studies in which Classical Music containing passages with string instruments had already been compared to Heavy Metal Music, in the Classical Music exposure of this study compositions with dominant brass passages were used. The exposures’ physiological responses were measured via the hearing threshold shifts within 2 minutes after the end of the exposure (TTS2) and during the restitution course until the resting hearing threshold was once again reached (t(0 dB)). Additionally, the area underneath the restitution curve, the Integrated Restitution Temporary Threshold Shifts (IRTTS), was determined as a summary measure of the “physiological costs.” Consistent with previous studies, it could once again be shown that an energy equivalent rating of sound exposures can lead to dangerously wrong assessments. For example, while Techno Music led to IRTTS-values which were comparable to those from Heavy Metal Music as reference exposure, the characteristics of the strain level and the restitution course were completely different. Techno Music caused significantly lower TTS2-values. This positive effect, however, was completely negated by a substantially prolonged restitution time (t(0 dB)). With respect to Classical Music, the results of the previous study could be confirmed despite the compositions’ different instrumentation. Again, the IRTTS-values as indicator of the physiological costs of Classical Music amounted to only 1/4 of those from Heavy Metal Music, even though none of the Ss had indicated Classical Music as his favorite kind of music. Of course, the risk of long-term hearing damage increases if the hearing experiences daily threshold shifts due to noise in the workplace which coincide with restitution processes which have not yet completely subsided. Furthermore, the acoustic stress from Techno and Heavy Metal Music is typically much higher than 94 dB / 1 h, the limit in this test which was chosen for ethical reasons.

1 Introduction Noise has a negative impact on more and more areas of human life. In addition to the workplace and traffic, noise is also increasingly experienced at home and during leisure activities. Rather than being an objective term, noise describes sound events, which have a negative effect on the human body. The effects range from pure nuisance to actual health damages. In addition to the subjective evaluation, the effects mainly depend on the intensity, the sound pressure's time course, the frequency composition, and the exposure time to the sound events (cp. STRASSER and IRLE 2001). One reason why noise is problematic for the human body is the fact that the human ear, “by default,” is always operational, i.e., it is exposed to acoustic stress without protection. Therefore, in order to avoid health risks or even damage in increasing parts of the population, it is necessary to design the technical environment according to biological human properties. Among the possible health damages, partial loss of hearing due to noise (a kind of damage to the inner ear) is the main concern. It results from permanent stress due to intensive noise and, without precautions, can lead to a total loss of hearing. Noise, therefore, deserves special attention also in the discussion about environmental stress and preventative methods.

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In the workplace, noise prevention regulations are in place such as the Ordinance on Working Places (1988) or the Accident Prevention Regulation “Noise” (1990) since, e.g., approximately 5 million workers in the Federal Republic of Germany are exposed to noise with a rating level of more than 85 dB(A) which is considered a health risk, especially to the hearing. Prescribed level restrictions in noise prevention regulations are based on the assumption of “recovery periods” for the hearing. If individuals are additionally exposed to noise during their leisure time, however, substantial risks to the hearing may result. Considering that there often is “overlay” of noise exposures at the workplace and during leisure time, an increased potential risk to the hearing must be assumed since sound pressure levels during leisure time exceeding 70 dB(A) do not allow the hearing to recover from workrelated noise (cp. IRLE et al. 1998). Damages to the hearing are mainly caused by high noise levels of machines, facilities, and work processes at the workplace to which workers are exposed over years. However, leisure activities such as concerts and discotheques or listening to music over headphones also lead to levels which pose a risk to the hearing (cp. ISING 1996). Many youths – as well as adults – spend a considerable amount of their leisure time listening to music. High-level stress comes from powerful stereo systems (at home or in the car), Walkmans (on the way to work or school or during breaks), and – mainly – the large variety of discotheques, techno parties, live concerts, and festivals with substantial changes over the years in the typical style of music. Undercover measurements of sound pressure levels on the dance floors of 29 discotheques in Berlin in 1988 showed mean levels over time between 92 dB(A) and 110 dB(A). Measurements in 1994 and 1997 – in 14 discotheques – resulted in almost identical music levels between 89 dB(A) and 110 dB(A). The following energetically averaged values were determined: • Lm = 102.3 dB(A) in 29 discotheques (1988) • Lm = 102.1 dB(A) in 14 discotheques (1994, 1997). Additionally, according to ISING and BABISCH (2000), the music level increased along with the number of visitors by approximately 2 dB(A) / h. This result is consistent with other studies which found music levels between 90 dB(A) and 110 dB(A) (cp. e.g., AXELSSON 1996; DAVIS et al. 1985). Despite cautionary advice to the contrary, music levels did not decrease over time. Furthermore, various studies have found that the frequency and duration of visits of discotheques has been increasing over time (cp. e.g., BICKERDIKE and GREGORY 1980; BABISCH und ISING 1994; HOFFMANN 1997). The 10 to 15 % of individuals who indicated that they frequent a discotheque once or twice a week for up to 6 hours per visit are of special concern. Studies concerning the use of headphones found that approximately 10 % of interviewees chose levels of at least 100 dB(A) with a duration of at least 3 hours per day (cp. e.g., DAVIS et al. 1985; ISING et al. 1994; ISING et al. 1998). BICKERDIKE and GREGORY (1980) found levels between 89 dB(A) and 119 dB(A) for big concerts. The results in AXELSSON (1996) are similar (97 dB(A) to 110 dB(A)). Studies by KÖRPERT (1992) and BORCHGREVINK (1993) indicate that the number of youths who were never exposed to noise in the workplace, but have irreversible damage to the inner ear has been increasing over time. Exposure to noise during their leisure time can be expected to be the cause of such diagnoses of preventative medical exams. This trend is clearly not only due to the widespread use of electronically amplified music since according to FLEISCHER (1990) extremely loud toys and fireworks are other potential contributors. Studies examining patterns in youths, however, show that approximately 10 % of them must be considered at risk for damages to the hearing from loud music in discotheques, with another 10 % who are at risk due to the use of headphones. It must be kept in mind that because of various additional exposures (e.g., due to a combination of visits to discotheques and the use of headphones, loud music in the car, concert visits, or noisy hobbies such as target shooting or

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home improvement), the actual share of persons at risk is probably higher than that. The results of a study by JANSEN et al. (1994) with 1,814 young males, 16 to 24 years of age, show that the hearing of 24 % of the test subjects was measurably affected in the frequency range between 3 and 6 kHz which is especially important for the understanding of spoken language. Hearing loss due to non-workplace-related noise is referred to as “sociacusis,” analogous to “presbyacusis” (age-related loss of hearing). The Federal Board of Medical Doctors (Bundesärztekammer) (cp. N.N. 1999), the Federal Environmental Agency (Umweltbundesamt) (cp. N.N. 2000) as well as various authors (cp. MERCIER et al. 1998; MAASSEN et al. 2001) stress that sociacusis may become a serious health problem in the next few years. They call for initiatives to reduce the sound pressure levels in discotheques, at concerts, and for walkmans and similar devices. Additionally, the comprehensive distribution of information regarding the risks of hearing damage due to loud music is recommended. In other European countries, several changes have already taken place. In Switzerland, an ordinance regarding the protection of the audience from harmful exposure to noise and laser beams went into effect on April 1, 1996. Accordingly, there is a noise immission limit of 93 dB(A) for events with electronically amplified music. Exceptions for a maximum mean level of 100 dB(A) can be granted, however, if an application is well founded. Since 1996, France also has a law concerning a maximum level for portable electronic music playback devices. Firm maximum levels do not yet exist, however. Regulations, at least, similar to those in Switzerland or France would also be desirable for Germany. The main goal should be to protect children and youths along with adults. The Youth Protection Act (Jugendschutzgesetz) (§ 1 Sentence 1) states that the respective government agencies must take the necessary measures to protect individuals' health and physical well-being. The following limits on noise levels are thus well-founded (cp. e.g., ZENNER et al. 1999) and should be enacted: The energy equivalent sound pressure level in discotheques should be limited to 90 to 95 dB(A), measured at the loudest point according to DIN 15905, Part 5 (“Acoustics in theatres and multipurpose halls; measures to avoid impairing the audience's hearing by high sound pressure levels from loudspeaker reproduction”). The continuous sound pressure level for portable music playback devices with headphones should be limited to 90 dB(A) according to recommendations of the ad hoc workgroup “Limitation of the sound pressure level during use of headphones” of the German ElectroTechnical Commission in the German Institute of Standardization (DIN) and the Association of the German Electrical Engineers (VDE). Youths, as well as managers of discotheques and concerts, should be targeted in order to convince them that music is still enjoyable at reduced volume. Interviews with youths in Switzerland (after the introduction of the above-mentioned limit of noise levels; cp. MERCIER and HOHMANN 2000) as well as in Germany (concerning the use of a music playback device with built-in maximum sound pressure level, cp. ISING et al. 1998) show that level limitations are typically widely accepted. In both cases, only approximately 5 % of youths were not satisfied with the reduced volume. In Switzerland, half of the interviewed discotheque patrons felt that the music was still too loud, despite the level reduction. In Germany, 92 % of interviewed youths would agree with a level limitation (cp. ISING et al. 1998). In light of the above-mentioned leisure-time-related noise problems, the value of legally binding level limitations in the workplace is questionable. Employers spend a lot of money to invest in noise protection while, at the same time, employees voluntarily endure oftentimes much higher sound exposures in their leisure time. Therefore, administrative noise protection measures for leisure activities are a necessity. In addition to individuals who are – voluntary – exposed to noise during leisure activities, professional musicians are exposed to high sound pressure levels due to the often daily practices with their bands and orchestras or live concerts. Similarly, the staff of discotheques is at risk due to the existing high music levels.

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Youths, young adults, and amateur musicians often are exposed to noise in the workplace. That is, they are exposed to acoustic stress in their leisure time during which their hearing is supposed to recover from the exposures in the workplace. The resulting impact on the restitution process can increase the risk of damage to the hearing.

2 Experimental examination methods and test design With the exception of a study by STRASSER et al. (1999a), there is very little experimental research on the effect of different kinds of music on the hearing. Based on the issue outlined above (music also as cause of long-term aural effects), reversible short-term effects of acoustic stress from different types of music on the hearing were examined. Generally, noise exposures in real life are assessed according to the energy equivalence principle (cp. STRASSER and IRLE 2001). This principle is based on the so-called dose maxim which states that the effects of stress on a person's health only depend on the dose, i.e., the product of concentration (stress intensity) and exposure duration. For the purposes of this study, that means that energy equivalent sound exposures should cause comparable physiological cost to the hearing once they exceed a critical threshold. Energy equivalence only considers the effects of exposure level and exposure time. Based on a rating level of LAeq, 8h = 85 dB(A), the maximum permissible critical threshold value for acoustic stress results in permissible peak levels of up to even 140 dB for the unprotected hearing. Previous studies (cp. HESSE 1994; STRASSER et al. 1999a) have shown, however, that the energy equivalence principle should not be used since it is flawed: The level distribution and time structures of the noise exposure – which are not considered in this purely physically oriented principle – do have a very real impact on the “physiological cost” which must be “paid” by the human hearing for the acoustic stress. In order to examine the acute hearing-physiological effects of music exposures containing identical energy, but different time and level structures, three different exposures with a mean level Lm of 94 dB(A), respectively, over an exposure time of one hour were created for the examination of hearing threshold shifts (cp. Figure 1). According to the 3-dB exchange rate, the rating level (LArd) of each exposure was 85 dB / 8 h. Since the maximum unweighted peak level Lpeak of 124.6 dB in Test Series II (Techno Music) was substantially below the permissible peak level for exposures without hearing protection of 140 dB, the test parameters for the subjects were ethically justifiable. In order to allow comparisons to previous studies (cp. STRASSER et al. 1999a), the same typical Heavy Metal Music was used in Test Series I as in the earlier study (three songs by Guns 'n Roses and one song by AC/DC which all consist of relatively constant levels as well as intensive drum parts). In Test Series II, typical Techno Music was chosen in the form of a one-hour mix of pieces from the 2001 LOVE PARADE in Berlin. Low-frequency bass notes which are characteristic for Techno Music were present in all songs in various intensities. For the Classical Music in Test Series III, pieces by Johann Sebastian Bach and Georg Friedrich Handel were chosen (cp. Figure 2). They all featured pervasive passages that were dominated by brass and woodwind instruments. All three sound exposures were played on a CD player and were transmitted via an amplifier to two loudspeakers in a sound proof cabin in which the test subject was sitting in a standardized position.

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Figure 1:

Schematic representation of the 3 energy equivalent exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shift

Figure 2:

Presentation of the 3 different used sound and music exposures

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The physiological responses immediately after the exposure, i.e., the temporary hearing threshold shifts which can be measured in the form of TTS2 values were expected to depend on the preceding exposure. Similarly, the restitution, especially the restitution time t(0 dB), i.e., the time duration until the TTS has completely subsided, was expected to be a function of the preceding acoustic stress in Test Series I through III. In terms of the acoustic stress on the human hearing, Techno Music (in TS II) is comparable to Heavy Metal Music (in TS I) in the

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sense that they both include a lot of impulsive noise. The newly assembled Classical Music in TS III could be expected to lead to similar physiological responses as the Classical Music used in previous studies (cp. STRASSER et al. 1999a). Based on the existing knowledge, different aural effects (hearing threshold shifts and their restitution) were expected in the three test series. They were quantified via the physiological indicators “TTS2” (hearing threshold shift 2 minutes after the end of the exposure), “t(0 dB)” (time at which the resting hearing threshold is once again reached), and “IRTTS” (Integrated Restitution Temporary Threshold Shift).

3 Test subjects and audiometric selection methods Each S was exposed to all three exposures in randomized order on different days, thus acting as their own control. Only individuals with no previous damage to the hearing were considered as test subjects. Ten Ss (four females and six males, age 31.5 ± 4.3 years) were selected according to DIN ISO 4869-1. The individual resting hearing threshold, which was determined before every test, served as a basis for subsequent measurements and analyses. For each S, the frequency of the maximum hearing threshold shift TTS2 was determined via multiple measurements during the first two minutes after the end of the sound exposure. This frequency was characterized by a substantially higher TTS2 value and a substantially longer restitution course relative to the lower and upper neighboring frequencies. At the frequency of the maximum threshold shift (between 3 and 6 kHz), the hearing threshold shift's restitution was measured at predetermined times until the resting hearing threshold was once again reached. The temporary hearing threshold shift (TTS) was determined as the difference between the hearing level during resting and the hearing threshold during restitution.

4 Results Figure 3 shows the results of the three test series for Heavy Metal Music (top), Techno Music (middle), and Classical Music (bottom) with a level of 94 dB(A) for 1 h. All hearing threshold shifts of the 10 Ss after the three exposures are plotted in a TTS-time graph. Additionally, the three diagrams show the arithmetic means over the 10 Ss as well as the regression lines for the test series. For the purposes of a comprehensive assessment of the results and a statistical test of the observed differences, the area under the regression line was determined. This IRTTS value is calculated as the integral over the regression function TTS(t) from 2 minutes after the exposure to t(0 dB). The IRTTS is a numerical value for the entire threshold shift in dB x min. It can be interpreted as a value for the “physiological cost” which the hearing must “pay” for the preceding sound exposure (cp. STRASSER et al. 1999b; IRLE et al. 2001). The top section of each part of the figure also summarizes the mathematical-numerical analysis: First, the regression function itself – which is the basis for the determination of the audiometrically relevant values TTS2 and t(0 dB) – is shown. In addition to the regressionanalytically determined maximum threshold shift two minutes after the exposure (TTS2 reg.) and the time t(0 dB)reg. until the threshold shift has completely subsided, the range of the actually measured values and the average values (i.e., the means of the measured values TTS2 real and t(0 dB)real over 10 Ss) are shown. Finally, the IRTTS value and the coefficient of determination r2 for the means (and for all measured values, in parentheses) are given. It can be seen in Figure 3 that despite some dispersion, the average value of the threshold shifts immediately after Heavy Metal Music (cp. top part of Figure 3) is approximately 20 dB (e.g., TTS2 reg. = 23 dB). Following a decreasing exponential function, they subside over time which can take more than two hours (e.g., t(0 dB)reg. = 145 min).

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Figure 3:

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Individual and mean restitution time course TTS(t) over all 10 test subjects after exposures to 94 dB(A) / 1 h from “Heavy Metal Music” (top), “Techno Music” (middle), and “Classical Music” (bottom)

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The middle part of Figure 3 shows that Techno Music of 94 dB(A) for one hour results in substantially lower maximum temporary hearing threshold shifts (e.g., TTS2 reg. = 14.3 dB). However, the restitution time shows values of up to 4 hours (t(0 dB)reg. with 298 min is up to almost 5 h). Finally, it can be seen in the bottom part of Figure 3 that an exposure to energy equivalent Classical Music also leads to only minor threshold shifts. While the maximum threshold shifts (TTS2 reg. = 13.4 dB) are comparable to those after Techno Music, the restitution time is less than one hour (t(0 dB)reg. = 53 min) rather than several hours. Figure 4 summarizes the results of the three energy equivalent test series for comparison purposes. At the beginning and the end of the “smoothed” restitution courses of TS I to TS III, the regression-analytical characteristic values TTS2 reg. and t(0 dB)reg. are shown.

Figure 4:

Restitution time courses TTS(t) with values of TTS2 reg., t(0 dB)reg., and physiological cost IRTTS with symbolic labeling of the significance level (according to the two-tailed WILCOXON Test)

Furthermore, the IRTTS values are shown as a measure for the physiological costs of the respective exposure. The exposures to Heavy Metal Music and Techno Music in TS I and TS II lead to comparable IRTTS values of 721 and 803 dBmin, respectively, and thus similar overall physiological costs. The respective value after Classical Music (182 dBmin) is just a fraction of the value after either of the other two exposures. For reasons of not overestimating the results, the value t(0 dB)real instead of t(0 dB)reg. has been used for plotting the restitution graph of TS II. From the analysis of the averaged measuring results of all test series, it can be concluded that acoustic stress with differences in the time and level structures results in substantial differences in hearing fatigue, even if there are no differences in sound energy.

5 Discussion The differences in all characteristic values in Test Series I and III are highly statistically significant. A possible explanation for that is the difference in the level structures, as determined by a level frequency count. From the relative level frequency distribution in Figure 5 it can be seen that in TS I (i.e., Heavy Metal) 39.0 % + 46.2 % = 85.2 % of all levels are between 92 and 96 dB, while the respective figure for TS III (i.e., Classical Music) is only 20.8 % + 18.9 % = 39.7 %. Together with higher TTS2, t(0 dB), and IRTTS values from exposure to Heavy Metal, this result confirms earlier studies (cp. STRASSER et al. 1999a)

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Figure 5:

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Sound level analysis of the exposures TS I, TS II, and TS III. (Probability distribution function of the level classes of the 3 exposures)

The fact that there is no statistically significant difference between the IRTTS values of TS I und II, along with the statistically significantly higher IRTTS values of TS I and TS II versus TS III, cannot be explained by the level structure. In TS II (i.e., Techno Music) 22.9 % + 22.8 % = 45.7 % of all levels are in the range from 92 to 96 dB which is similar to TS III, but different from TS I. This is probably due to the substantially higher sound pressure levels in the low frequency ranges in TS II, which are also considered a risk to the hearing. On the one hand, this can be quantified with the results of a frequency analysis (cp. Figure 6) since Techno Music in TS II exhibits substantially higher levels (e.g., 95.6 dB vs. 89.0 dB, 105.3 dB vs. 100.9 dB, 96.3 dB vs. 91.5 dB) than Heavy Metal and also Classical Music in the low frequencies (63 to 250 Hz).

Figure 6:

Frequency analysis of the exposures. (Sound pressure levels of TS I, TS II, and TS III determined for the mid-frequency of the octave bands 63 Hz through 8,000 Hz)

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On the other hand, the exposure in TS II also reached the highest levels in the measurements, which were taken using a C-weighting network (cp. Figure 7). For example, the level which was 3.9 dB(C) higher than the one in TS I corresponds with an intensity or energy to which the human ear is exposed which is more than 2 times higher.

Figure 7:

Differing linear weighted levels (right) and dB(C)-values (middle) of the 3 musical exposures, each with a level of 94.0 dB(A) (left)

This once again raises the question whether the sole use of an A-weighting network is appropriate for the evaluation of levels of noise exposures. This frequency evaluation curve was initially only intended for level ranges below 60 dB. In the meantime, however, it is applied to any dynamic range, partially for reasons of convenience. The data presented above clearly show that, at relatively high noise levels, the use of an A-weighting network – contrary to the course of the applicable phon lines – results in much lower values than the actual effect experienced by the human hearing (cp. STRASSER 1981 and 1995). A comparison of the respective restitution courses in Figure 4 shows that the exposures lead to different start- and endpoints of the restitution graphs. Even though the mean exposure level with 94 dB(A) was the same for all three test series, substantial differences in terms of real TTS2 and t(0 dB) values were typically observed. For example, the TTS2 values were high in TS I, and low in TS II. While the t(0 dB) values in TS I were medium, they were extremely high in TS II. Since the IRTTS values for TS I and TS II are not statistically significantly different from each other, the IRTTS seems to represent the most meaningful characteristic value for the determination of total physiological costs of acoustic stress. The reason for these results is that with respect to time and level structures, different exposures can cause different strain mechanisms. This makes the IRTTS particularly well suited to describe the risk to the hearing of various sound exposures. For the determination of a Hearing Risk Factor (HRF), IRTTS values can be used to obtain the relative risk of a particular acoustic stress: IRTTS values of TS II (Techno Music) and TS III (Classical Music) are expressed relative to the IRTTS value from TS I (Heavy Metal Music) (cp. Figure 8). With TS I as a reference (1.00), the resulting numerical values for TS II and TS III are 1.11 and 0.25, respectively. Hearing Risk Factors greater than 1.00 indicate increased risk while factors less than 1.00 indicate decreased risk to the human hearing. In this study, exposure to Techno Music (TS II) led to physiological costs, which were 11 % higher than exposure to Heavy Metal Music (TS I). Exposure to Classical Music (TS III) led to a completely different result with a factor of 0.25, respectively. The “physiological costs” are only approximately 1/4 of those in the reference exposure (TS I).

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Figure 8:

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"Relative Physiological Cost" of the energy equivalent exposures (IRTTSTS i / IRTTSTS I)

This result is very similar to the results in STRASSER et al. (1999a). Because of the substantially higher Hearing Risk Factors (physiological costs) in Test Series I and II, the unconditional use of the energy equivalence principle with a 3-dB exchange rate (as well as the frequency weighting network A) for the evaluation of sound exposures must be questioned, since differences in the time and level structures are not taken into consideration at all.

6 Conclusions The results (in the form of the characteristic values TTS2, t(0 dB), and IRTTS) have shown that energy equivalent music exposures with different time and level structures have different effects on the hearing. Contrary to the other music exposures, the effects in Test Series II (Techno Music) are clearly of importance in the time course of the strain compensation rather than for the temporary threshold shift (TTS2). This emphasizes the importance of focussing on the restitution process after the exposure, especially the t(0 dB) values, and the IRTTS as an integral characteristic value over strain height and duration which allows the identification of sound exposures with hearing-risk potential. The use of the strain height exclusively in the form of the TTS2 would have assessed Techno Music as much less harmful than it actually is. Physiological responses to Heavy Metal Music, all in all, lead to physiological costs, which are comparable to those from Techno Music. Both musical styles, however, cause substantially greater costs than Classical Music. An earlier study (cp. STRASSER et al. 1999a) found that exposure to Heavy Metal Music identical to the one used in this study leads to the same physiological costs as typical industrial noise, but to much higher threshold shifts than Classical Music. Taking those results into consideration, it can be concluded that industrial noise, Heavy Metal Music, and Techno Music cause similar physiological costs for the hearing. Due to the substantially slower restitution course after Techno Music, however, a higher long-term risk potential can be assumed for Techno Music than for Heavy Metal Music. In light of the test results and the introductory issues with regard to leisure activities, it seems imperative that rules be enacted concerning sound protection during leisure time. This

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is especially true since today's youths typically prefer Heavy Metal or Techno Music to Classical Music (cp. Table 1). None of the 10 Ss of this study who were all in favour of Rock Music preferred the Classical Music test exposure whereas 7 out of 10 Ss appreciated listening to the medley of Heavy Metal Music. This subjective evaluation of the test exposures and the physiological responses show that a connection between the subjective sensation of noise and its impact on the aural effects – which is often believed to be implied in audiological studies – must be questioned. Finally, the question rises, what good can comprehensive rules do with respect to noise reductions in the industrial sector in order to prevent hearing damage for the employees if especially young people voluntarily endure much higher exposures in their leisure time? And what about those individuals who do not work in noise areas, but whose hearing must endure too much strain from frequent and loud music? Friendly reminders to the youths themselves will be insufficient since it is just a part of their daily leisure activities to frequent discotheques and concerts. Table 1: Subject-specific characteristic data and details of their tastes in music

An important task of government agencies must be the widespread dissemination of information regarding the effects of noise, especially in the form of music exposures. Only if affected individuals are aware of possible consequences, can they reduce the risk potential. Such activities should begin early on in school so that youths are responsive to this issue. In addition to the impact of a person's private life, hearing damage from leisure activities can have negative effects professionally. Very few jobs nowadays do not require the use of a telephone. Many “dream jobs” (e.g., airline pilot) are not an option if the hearing is anything short of excellent. Occupations, which require a good deal of customer interaction, pose difficulties. Similarly, jobs with high noise exposures (e.g., locksmith, carpenter, or auto mechanic) may not be possible because of health issues that will be raised if preventative medical exams detect noticeable hearing loss. Interaction between individuals, exchange of information, opinions, and feelings require the ability to speak and to hear and understand language. If this ability is limited or even lost due to hearing loss, the possibility of personal development in social relationships is severely limited which can substantially reduce the quality of life. However, neither in schools nor in the media does this issue receive sufficient attention.

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7 References AXELSSON, A. (1996) Recreational Exposure to Noise and its Effects. Noise Control Eng J 44 (3) 127-134 BABISCH, W. und ISING, H. (1994) Musikhörgewohnheiten bei Jugendlichen. Zeitschrift für Lärmbekämpfung 41, 91-97 BICKERDIKE, J. and GREGORY, A. (1980) An Evaluation of Hearing Damage Risk to Attenders at Discotheques. Leeds Polytechnical School of Constructional Studies. Dept. Environment Report BORCHGREVINK, H.M. (1993) Music-Induced Hearing Loss > 20 dB Affects 30 % of Norwegian 18 Year Old Males Before Military Service – The Incidence Doubled in the 80's, Declined in the 90's. Proceedings of the 5th International Congress on Noise as a Public Health Problem. In: Noise and Man '93. Vol. 2, Nice, France, 25-28 BÜRCK, W. (1981) Lärm - Messung, Bewertung und Wirkungen auf den Menschen. Kap. 4.3. In: SCHMIDTKE, H. (Hrsg.) Lehrbuch der Ergonomie. 2., bearb. und erg. Aufl., Carl Hanser Verlag, München - Wien, 199-221 DAVIS, A. C.; FORTNUM, H.M.; COLES, R.R.A.; HAGGARD, M.P. and LUTMAN, M.E. (1985) Damage to Hearing Arising from Leisure Noise: A Review of the Literature. In: Report Prepared for the Health & Safety Executive by the MRC Institute of Hearing Research, Nottingham. London FLEISCHER, G. (1990) Lärm, der tägliche Terror. TRIAS Thieme Hippokrates Enke Verlag, Stuttgart HESSE, J.M. (1994) Theoretische und experimentelle Untersuchungen zur Gehörschädlichkeit von Impulsschall. Dissertation im Fachgebiet Arbeitswissenschaft/Ergonomie, Universität-GH Siegen HOFFMANN, E. (1997) Hörfähigkeit und Hörschäden junger Erwachsener. Median-Verlag, Heidelberg, 1997 IRLE, H.; HESSE, J.M. and STRASSER, H. (1998) Physiological Cost of Energy-equivalent Noise Exposures with a Rating Level of 85 dB(A): Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. Int. Journal of Industrial Ergonomics 21, 451-463 IRLE, H., HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposure: Temporary Threshold Shifts. In: W. KARWOWSKI (Ed.) International Encyclopedia of Ergonomics and Human Factors. Volume II, Part 7, Environment, Taylor & Francis, London and New York, 1050-1056 IRLE, H. and STRASSER, H. (2001) Methods for Determining Physiological Costs of Sound Exposures to the Hearing. In: PRASHER, D. (Ed.) Abstract Book of NOPHER 2001. An International Symposium on Noise Pollution & Health, Cambridge/United Kingdom, 78 ISING, H. (1996) Gehörschäden durch Musik. Informationsbroschüre der Bundesanstalt für Arbeitschutz und Arbeitsmedizin. Dortmund ISING, H.; BABISCH, W.; HANEL, J.; KRUPPA, B. und PILLGRAMM, M. (1994) Empirische Untersuchungen zu Musikhörgewohnheiten von Jugendlichen. HNO 43, 244-249 ISING, H.; BABISCH, W.; HANEL, J.; KRUPPA, B. und PILLGRAMM, M. (1998) Untersuchung der Hörfähigkeit und Musikhörgewohnheiten von Jugendlichen sowie der Akzeptanz eines pegelbegrenzten Kassettenabspielgerätes. Zeitschrift für Audiologie Supplement I, 195-201 ISING, H. und BABISCH, W. (2000) Hörschadensrisiken durch Freizeitlärm. Fortschritt und Fortbildung in der Medizin Vol. 24, 31-40 JANSEN, G.; STRUWE, G.; SCHWARZE, S.; SCHWENZER, C. und NITZSCHE, M. (1994) Untersuchung von Hörgewohnheiten und möglichen Gehörrisiken durch Schalleinwirkungen in der Freizeit unter besonderer Berücksichtigung des Walkman-Hörens. Forschungsbericht, Institut für Arbeitsmedizin der Heinrich-Heine Universität Düsseldorf KÖRPERT, K. (1992) Hearing Thresholds of Young Workers Measured in the Period from 1976 to 1991. Swiss Acoustic Society, 181-184 MAASSEN, M.; BABISCH, W.; BACHMANN, K.D.; ISING, H. LEHNERT, G.; PLATH, P. PLINKERT, P.; REBENTISCH, E.; SCHUSCHKE, G.; SPRENG, M.; STANGE, G.; STRUWE, V. and ZENNER, H.P. (2001) Ear Damage Caused by Leisure Noise. Noise & Health 4 (13) 1-16 MERCIER, V.; WÜRSCH, P. und HOHMANN, B.(1998) Gehörgefährdung Jugendlicher durch überlauten Musikkonsum. Zeitschrift für Lärmbekämpfung 45, 17-21

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MERCIER, V. und HOHMANN, B. (2000) Wie laut soll Musik sein? Tagungsband zur DAGA 2000 in Oldenburg N.N. (1999) Gehörschäden durch Lärmbelastungen in der Freizeit. Stellungnahme des Wissenschaftlichen Beirates der Bundesärztekammer. Deutsches Ärzteblatt Heft 16, 836-839 N.N. (2000) Pegelbegrenzung in Diskotheken zum Schutz vor Gehörschäden. 12. Sitzung der Kommission „Soziakusis" des Umweltbundesamtes. Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz Vol. 43 (8) 642-643 STRASSER, H. (1981) Beurteilung des Lärms aus arbeitswissenschaftlicher Sicht. Leistung und Lohn Nr. 105/107, 3-28 STRASSER, H. (1995) Dosismaxime und Energie-Äquivalenz – Ein Kernproblem des präventiven Arbeitsschutzes bei der ergonomischen Beurteilung von Umgebungsbelastungen. In: STRASSER, H. (Hrsg.): Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen – Anspruch und Wirklichkeit des präventiven Arbeitsschutzes. Ecomed Verlagsgesellschaft AG & CO.KG, Landsberg/Lech, 9-31 STRASSER, H., IRLE, H. and SCHOLZ, R. (1999a) Physiological Cost of Energy-Equivalent Exposures to White Noise, Heavy Metal Music, and Classical Music. Noise Control Engineering Journal 47 (5) 187-192 STRASSER, H.; IRLE, H und SIEBEL, T. (1999b) Zur Effektivität persönlicher Gehörschutzmittel bei verkürzter Tragedauer aus energieäquivalenter und arbeitsphysiologischer Sicht. Zeitschrift für Arbeitswissenschaft 53 (25 NF) 2, 95-106 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: W. KARWOWSKI (Ed.) International Encyclopedia of Ergonomics and Human Factors. Volume I, Part 3, Performance Related Factors, Taylor & Francis, London and New York, 516-523 ZENNER, H. P.; STRUWE, V.; SCHUSCHKE, G.; SPRENG, M.; PLATH, P.; BABISCH, W.; REBENTISCH, E.; PLINKERT, P.; BACHMANN, K. D.; ISING, H. und LEHNEN, G. (1999) Gehörschäden durch Freizeitlärm. HNO 47 (4) 236-248 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN 15905-5 (1989) Acoustics in Theatres and Multipurpose Halls; Measures to Avoid Impairing the Audience’s Hearing by High Sound-Pressure Levels from Loudspeaker Reproduction. Beuth Verlag, Berlin DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin Ordinance on Working Places (1988) Arbeitsstättenverordnung (Arb Stätt VO) vom 20. März 1975, in Kraft seit Mai 1976, Stand Januar 1988. Schriftenreihe „Regelwerke Arbeitsschutz“ im Auftrag des Bundesministeriums für Arbeit und Sozialordnung, Bundesanstalt für Arbeitsschutz, Dortmund

Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 13

Effects of Noise Exposures during Physical Rest, Additional Physical Exercises and Combined Exposures to Alcohol and Cigarette Smoke on Hearing Threshold Shifts and their Restitution H. Strasser and H. Irle

0 Summary In two studies, each with 5 test series, physiological costs of the hearing due to legally tolerable noise exposures of 94 dB(A) for 1 h have been measured audiometrically. The temporary threshold shifts (TTS) and their restitution time, as well as cardiovascular responses in work-related heart rate increases, of 10 and 8 subjects (Ss), respectively, could be shown to be modulated by additional physical stress and combined exposure to alcohol (Study 1) and cigarette smoke (Study 2). Moderate dynamic muscle work (50 W) administered via a bicycle ergometer either immediately after noise, or simultaneous to the noise exposure, significantly reduced restitution time as well as the integrated restitution temporary threshold shift (IRTTS). A physical stress to 100 W – which exceeded the endurance level when demanded simultaneously to the noise exposure – did not show any favorable effects. However, if the same physical stress succeeded the noise exposure, and when it was interrupted several times for the audiometric measurements, it also brought about significant accelerations of the restitution processes. Some reductions in physiological costs of the hearing were found due to an intervening alcohol consumption (BAC ~ 0.08 %) prior to the noise exposure and a simultaneous physical load of 50 W. Smoking 10 cigarettes instead of the consumption of alcohol was associated with a reduced TTS, but a prolonged restitution time. IRTTS as total physiological costs of the most unfavorable combination of noise, simultaneous high physical workload, and preceding smoke exposure was increased. The results of the test series with cigarette smoke – probably due to the small group of just 8 Ss and the counteracting effects of the agents carbon monoxide (CO) and nicotine – were not statistically significant, but these exposures were associated with a substantial activation of the cardiovascular system. Significant heart rate increases are evidence that CO and nicotine must not be neglected as influential factors in the context of physiological costs which the organism, and especially the hearing, has to pay for noise exposures.

1 Introduction About 10 years ago, an international scientific periodical was founded to publish original writings and studies on the combined effects and mutual interactions of environmental factors, as well as methodological applications needed in interdisciplinary and multi-variant research. Unfortunately, this journal, entitled “Archives of Complex Environmental Studies,” is not widespread, and research published there focuses both on plants, animals, and man, whereby studies with relevance to the effects of combined work-related stressors on man are rare and / or mostly inconsistent due to incomparable test parameters. Also, in Ergonomics textbooks, comprehensive publications, such as an Encyclopedia (cp. KARWOWSKI 2001), and periodicals, so far, investigations into the effects of combined stress have played a rather minor role. In real life, however, it is very rare that an individual is exposed to only one kind of stress at a time, and it can be assumed that the effects of stress must not simply be added up. On the one hand, synergistic, mutually reinforcing effects are possible. On the other hand, antagonistic, compensatory effects are, at least, conceivable. As is well-known, the specific physiological responses to continuous noise are temporary or even (in the case of exposures over several years) permanent hearing threshold shifts. Even exposures which are permissible in manufacturing without a hearing protection device lead to substantial threshold shifts of approximately 20 dB, whose restitution time may be several hours (cp. IRLE et al. 2001).

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Research into the association between physical exertion, noise and effect on hearing is very limited, though, often, in industry, noisy workplaces require physical activities as well. Previous tests, whose results have been published by IRLE and STRASSER (1998), showed that a moderate physical load significantly shortened the restitution of hearing threshold shifts that had been caused by a preceding and simultaneous noise exposure. Whereas in these studies the maximum threshold shifts TTS2 were not substantially influenced (neither significantly increased nor decreased), LANDSTRÖM et al. (1999) found adverse effects. Subjects riding an ergometer cycle with a sub-maximum load equivalent to 50 % of oxygen uptake capacity simultaneous to a music exposure of 94 dB(A) for 45 min showed a significantly higher TTS at 4 and 6 kHz (15 and 11 dB instead of 8 dB) than after the music exposure alone. But these small negative effects due to the physical activity, which were similar to the results of a study carried out by ENGDAHL (1996), were measured within a time period of approximately 10 min just once after the exposure. Unfortunately, the restitution that revealed the main positive effects of a moderate physical activity on the hearing in the study of IRLE and STRASSER (1998) was not monitored. Since the results of isolated and single studies normally cannot be generalized even though significant effects have been found, further research, e.g., with a varied intensity of physical exercise simultaneously to, and following, the noise exposure seemed to be necessary. Furthermore, other relevant exposures that are common at work or during leisure simultaneously to noise or music – and that are often endured voluntarily – should be investigated. Exposure to noise, physical activities, and also smoking is a usual combination in working life and the effect of the latter on hearing fatigue or hearing loss is not yet clear. According to BARONE et al. (1987), smoking alone does not disturb hearing, but due to vascular effects (vasoconstriction), which can disturb the peripheral circulation, it might be a risk factor in noise-induced hearing loss. VIROKANNAS and ANTONNEN (1995), using audiometric data from more than 400 reindeer herders, showed that the exposure to noise (~100 dB(A) from snowmobiles and chain saws) and smoking had an independent action on hearing loss, but that prevalence of age-adjusted hearing thresholds higher than 30 dB at 4 kHz was the highest in heavy smokers (20 cigarettes/day for more than 20 years). According to ADLKOFER (1991), about 40 % of men and 25 % of women (i.e., altogether 23 million people) in Germany are smokers, who often continue smoking during work, hereby exposing themselves to carbon monoxide and nicotine, besides tar (probably representing the most adverse constituents of tobacco smoke). It is well known that the smoke of, e.g., a filtered red Marlboro cigarette, contains about 12 mg carbon monoxide (CO) and 0.9 mg nicotine. Given that, on average, cigarette smoke contains as much as 4 % volume of CO, some of which is absorbed, smokers (up to 20 cigarettes / day) may have a level of carboxyhaemoglobin (COHb) of more than 5 %. COHb concentrations even exceeding 15 % have been observed in some heavy smokers. Yet, the COHb level depends on various interand intra-individual factors, such as the size and number of puffs and depth of inhalation as well as pulmonary transfer and the CO yield from the cigarette (cp. WALD et al. 1975). All in all, subjects smoking 10 to 12 cigarettes a day may have as much as 5 % COHb in their blood (cp. RODAHL 1989). It should be borne in mind that a blocking of 5 % of the haemoglobin by carbon monoxide, and also the nicotine inherent in tobacco smoke, can produce a perturbation of the cardiovascular system, with interference to noise effects in the inner ear (cp. MANNINEN 1998). Additionally, alcohol consumption is not uncommon, especially in the context of music and sound exposure, and seems, at least, to increase tolerance of noise, subjectively. While consistently an increase in threshold of the stapedius reflex has been described by older studies (e.g., COHILL and GREENBERG 1977; ROBINETTE and BREY 1978), effects on hearing

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thresholds as such are not well established. PETIOT et al (1990) reported on combined effects of a moderate dose of alcohol and of exposure to noise (105 dB / 20 min) upon auditory fatigue. The rate of recovery from auditory fatigue, at this definitely most sensitive frequency, was not modified by alcohol, but due to initially lesser auditory fatigue (significantly lower TTS2 at 4 kHz associated with noise and alcohol), recovery under alcohol was achieved earlier than in a mere noise condition. According to the authors, the cause of reduction in auditory fatigue by alcohol is to be sought in metabolic changes at the organ of Corti (a direct vasodilator effect of alcohol which might be suspected in being responsible for a better resistance to noise). Furthermore, the results, on the one hand, suggest, that when ears are exposed to high noise levels, alcohol reduces the protective action of the middle ear reflex and consequently may increase the risk of damage. On the other hand, alcohol consumption in noisy environments might, according to an operant conditioning paradigm, be reinforced not only by the well-known anxiolytic properties of the drug, but also by reduction of auditory fatigue. Since also the effects reported by PETIOT et al. (1990) are relatively small in intensity, there still is a lack of controlled studies. Therefore, we studied the effects of noise alone, and in combination with preceding alcohol consumption, tobacco smoking, and simultaneous as well as succeeding physical workload.

2 Methods Exposures to noise at a rating level LArd of 85 dB for 8 h or energy equivalent exposures according to the exchange rate of 3 dB, that is, 88 dB for 4 h, 91 dB for 2 h or 94 dB for 1 h, are permissible in the production sector without any restrictions in almost all countries (cp. STRASSER and IRLE 2001). Therefore, they can be regarded as still remaining within the ethically acceptable bounds for investigations into the effects on human beings. Since the hearing threshold shifts as physiological responses to the above-mentioned exposures are comparable, and the time which test subjects (Ss) spent in a laboratory had to be limited for both humane and economic reasons, 94 dB(A) for 1 h was chosen as an acoustic load in these two studies, with 5 test series each. 2.1 Test design of Study 1 with noise exposure, physical workload, and alcohol As can be seen from the middle part of Figure 1, in the first test series, TS I of Study 1, the Ss were exposed to continuous noise of 94 dB(A) for 1 h, as a reference stress. In a second test series, TS II, the Ss had to perform a physical output of 50 Watts (W) on a bicycle ergometer after the same noise exposure. In the third test series, TS III, the physical work was to be carried out simultaneously to the noise exposure. The only difference between two further test series (TS IV and TS V) and TS I and TS III, respectively, was that the noise exposure and combined load of noise and simultaneous dynamic muscle work were superimposed by an additional test parameter. In these two test series, immediately prior to the tests, the Ss received a certain amount of alcohol, which was supposed to lead to a blood alcohol concentration (BAC) of about 0.08 %. As can be seen in the domain of strain, that is, in the back (upper) part of Figure 1, the hypothesis to be tested was that varying threshold shifts (as specific physiological responses to the noise exposure) would be observed during the test series because they would be modulated by the additional parameters. These threshold shifts should be measurable in TTS2-values immediately after the noise exposure. Also, the restitution time t(0 dB), that is, the amount of time necessary for a complete recovery from the threshold shifts, was expected to be a function of the TS I through V.

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Figure 1:

Schematic representation of the 5 exposures of Study 1 and Study 2, respectively (Domain of Stress) and hypothetical physiological responses, that is, growth and restitution of the expected TTS (Domain of Strain)

The left part of Figure 2, shows the general schematic test set-up with the exposure of the test subject to the acoustic load of 94 dB / 1 h via headphones, the dynamic muscle work by a computer-controlled bicycle ergometer, and alcohol as an intervening factor in Study 1. According to the WIDMARK-formula (cp. HETTINGER and MÜLLER-LIMMROTH 1970), the Ss had to drink a specific amount of alcohol that – depending on the individuals body weight and a personal reduction factor – should yield the intended blood alcohol concentration (of 0.08 %). As visualized in the right part of Figure 2, temporary threshold shifts were measured audiometrically in a sound proof cabin, the heart rate was determined, and subjective rating of strain associated with noise and the additional combined load took place via specific questionnaires.

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Figure 2:

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Schematic test set-up of the two studies with single and combined exposures by noise, dynamic muscle work, alcohol (Study 1), and cigarette smoke (Study 2)

2.2 Test design of Study 2 with noise exposure, increased physical workload, and exposure to CO, and nicotine The lower (front) part of Figure 1 shows the 5 exposures chosen in a second study (Study 2) (see also left part of Figure 2). While the noise exposure was the same as in the first study, a clearly higher physical load (of 100 W) was demanded, and prior to the tests during days IV and V, the subjects of this study had to smoke 10 cigarettes instead of consuming alcohol, which for them, as smokers, should not have been unusual. As a result, a moderate, yet not insubstantial, carbon monoxide haemoglobin (COHb) level of about 5 % with a corresponding reduction in the oxygen capacity of blood was expected, as well as a cardiovascular (vasoconstrictive) effect from the nicotine, which presumably interferes with the metabolic processes in the inner ear that are caused by the acoustic and physical stress. 2.3 Determination of TTS2 and the restitution time course First, after the acoustic exposure, the frequency of a test subject's maximum hearing threshold shift (TTS2) had to be determined within the first 2 minutes via several measurements. This maximum in the “Individual Hearing Threshold Shift” above the “Individual Hearing Threshold” prior to the exposure (compare left part of Figure 3) is a classic characteristic value of audiometric examinations. With this frequency of the maximum threshold shift (which usually was 4 or 6 kHz), the restitution of the hearing threshold shift (back to the resting threshold) was measured at exactly predetermined points in time (compare right part of Figure 3). The latest point in time, the restitution time t(0 dB), is also an important characteristic value of the acoustic strain analysis. When a linear time scale is used, the shape of the restitution of a temporary hearing threshold shift resembles an exponential function. If, however, it is plotted against a logarithmic time scale, as graphed in the upper right part of Figure 3, the regression function TTS(t) is a straight line.

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Figure 3:

Selection of the frequency with maximum threshold shift after the noise exposure, as well as typical individual restitution time course TTS(t), with characteristic values TTS2 (temporary threshold shift 2 minutes after the exposure), t(0 dB) (restitution time) in logarithmic and linear time scale (upper half) as well as exemplary representation of the IRTTS (Integrated Restitution Temporary Threshold Shift) (lower half)

In addition to the characteristic values TTS2 and t(0 dB), the IRTTS, as a combined value of both parameters, can be used to quantify the total physiological costs of noise exposures. According to the lower part of Figure 3, this “Integrated Restitution Temporary Threshold Shift” is the area that is enclosed by the time axis, the TTS axis at 2 minutes, and the regression function TTS(t). The unit of this characteristic value is dB x min.

3 Results 3.1 Results of Study 1 with sound exposure, physical workload, and alcohol Figure 4 shows all results of TS I, that is, after the exclusive sound exposure to a level of 94 dB(A) for 1 h. Since the audiometric results from all the other test series are also represented in the same manner in the following figures, the selected layout of the figure will be explained in some detail. Firstly, all the hearing threshold shifts which were determined for the 10 subjects after the exposure were graphed in a TTS-time coordinate system. These threshold shifts are the differences between the measured TTS-values and the respective individual resting hearing threshold before the sound exposure, as shown previously in Figure 3. Furthermore, the arithmetic mean values averaged over the 10 subjects at every measuring point and the regression graph are shown in this diagram. The results of the mathematical-numerical evaluation are also shown in the upper right box of the figure. These are represented in the regression function itself, from which the audiometrically relevant values TTS2 and t(0 dB) were determined. In addition to the regression-analytically determined maximum hearing threshold shift within 2 min after the noise exposure (TTS2 reg.),

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and the time t(0 dBreg.), after which the threshold shifts completely dissipated, the range of the real measured values and, finally, the average values, that is, the mean measured values TTS2 real and t(0 B)real averaged over the 10 Ss, are shown.

Figure 4:

Individual and mean restitution time courses TTS(t) as physiological responses to 94 dB(A) / 1 h (TS I) in Study 1

As can be seen in Figure 4, despite some individual variations, substantial threshold shifts occurred. The mean of the threshold shifts immediately after the exposure was roughly 20 dB (exactly TTS2 reg. = 21.5 dB). The threshold shifts subsided over time following the course of a decreasing exponential function and completely dissipated after at most approximately 2 hours (exactly t(0 dB)reg. = 119 min). When the results from the test series with noise and additional physical workload – averaged over the same Ss in each case – are summarized graphically (cp. Figure 5), the TTS2-values of 21.5 dB in TS I, 23.0 dB in TS II, and 23.7 dB in TS III (cp. upper part of Figure 5) exhibit only minor and insignificant differences in the maximum hearing threshold shifts within 2 min after the noise exposure. With respect to TS II, this could also be expected because the same stress situation had existed until that point in time. The results of TS III, during which the dynamic muscle work of 50 W was required simultaneously to the noise exposure, on overage, the TTS2-values were somewhat higher. Consequently, physical stress had no systematic and substantial influence on the level of the maximum threshold shifts. Yet the results of the restitution time, that is, the individual duration until the resting threshold shifts are reached again, show significant differences. While it takes approximately 2 hours (119 min) for the hearing thresholds to return to their resting level after exposure to noise exclusively, the restitution time for the noise exposure which was followed by physical workload of 50 W in TS II is significantly shorter (83 min). Simultaneous physical workload and noise exposure in TS III led to even shorter, statistically significant restitution times (78 min). Thus, the restitution processes of the hearing after the noise exposure were not delayed by a simultaneous moderate physical load. Instead, they were definitely accelerated.

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Figure 5:

Regression-analytically determined characteristic values TTS2 and t(0 dB), as well as physiological costs IRTTS with significance level for differences in means between the results of the 5 exposures of Study 1, respectively. (Means of 10 Ss, one-sided WILCOXON-test)

Finally, the calculation of the total physiological costs according to the formula in Figure 3 yielded IRTTS-values of 573 dBmin (for TS I), 456 dBmin (for TS II), and 442 dBmin (for TS III). The differences between these are numerically clear, but not statistically significant for all comparisons. From the courses of heart rate – depicted in the upper half of Figure 6 (in beats per minute [bpm]) – it can be seen that, in comparison with TS I, the additional physical load to noise in TS III caused substantial increases (in the range of 30 bpm). Since in the course of time a steady state resulted, the physical workload was below the endurance level of the Ss. During TS II, the increases of heart rate associated with a subsequent physical workload were clearly smaller. This was due to the fact that bicycling had to be interrupted several times for the audiometric measurements.

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Figure 6:

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Courses of Heart Rate (HR in beats per minute, bpm). 5-min mean values for all 10 subjects in TS I, TS II, and TS III of Study 1 (upper half) and HR profiles of corresponding TS IV / TS I and TS V / TS III of Study 1 (lower half)

The lower half of Figure 5 shows the restitution time courses TTS(t) of TS IV and TS V, in which the Ss with a presumed BAC of 0.08 % were exposed to noise and the combined load of noise and physical work. For reasons of comparison, the already interpreted results of TS I with noise exclusively and without the intervening alcohol are shown again. As can be seen, a BAC of 0.08 % caused no significant changes (neither increases nor decreases) in the TTS2values, and, apparently, the influence on restitution time also was restricted. Yet it can be demonstrated that restitution time t(0 dB) of TS IV in comparison with TS I was not

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prolonged by alcohol, but rather shortened. Furthermore, the significant shortening of t(0 dB) as a consequence of physical work in TS V was not disturbed, but also slightly improved. The lower half of Figure 6 contains pairs of heart rate profiles from TS IV and TS I as well as TS V and TS III, which are directly comparable with respect to alcohol as an intervening factor. The results show an increase in heart rate associated with alcohol (in TS IV and also TS V) and reveal an activation of the cardiovascular system, rather than the more likely and expected vasodilatation. If IRTTS-values of the 5 test series are related to that of TS I, when the Ss were exposed exclusively to noise (of 94 dB(A) / 1 h) as a reference, then – as shown in Figure 7 – numbers less than 1 correspond with lower physiological costs that have to be paid by the hearing. According to the results with numbers of 0.80 and 0.77 for TS II and TS III (in the left part of Figure 7), physiological costs could be reduced by the positive effects of a physical load of 50 W (both after and simultaneous to noise). Numbers of 0.93 for TS IV and even 0.70 for TS V (in the right part of Figure 7) which are 7 percentage points lower than 1.0 and 0.77 for comparable TS I and TS III, respectively, surprisingly indicate a mild additional, favorable effect of alcohol on physiological costs to the hearing.

Figure 7:

“Relative physiological costs” (IRTTSTS i / IRTTSTS I) of the exposures of Study 1

3.2 Results of Study 2 with noise exposure, increased physical workload, and exposure to CO and nicotine According to the test set-up – arranged and visualized in Figure 1 and 2 in the same manner as for the first Study – the Ss who participated in a second study were exposed to an identical noise of 94 dB(A) for 1 h and to an increased physical workload of 100 W (both simultaneous to and immediately after the noise exposure). Furthermore, in order to create combined load by carbon monoxide and nicotine, they had to smoke 10 cigarettes prior to the exposures of two test series (out of 5) which was designed to lead to a maximum COHbconcentration of about 5 % in the blood. In an already summarized form, the physiological responses to the exposures of TS I, TS II, and TS III are shown in the upper half of Figure 8. Almost identical TTS2-values had been measured in the three test series. Yet again, restitution time was reduced significantly as a consequence of physical load which followed the noise exposure, so that calculation of IRTTS-values with 292 dBmin also revealed significantly lower physiological costs for TS II than for TS I with 334 dBmin. By increasing physical load to 100 W, administered simultaneously to noise in TS III, the positive effects which could be ascribed to 50 W in the previous study disappeared. Instead, doubling the demanded physical work was associated with a small increase of the IRTTS-value (349 dBmin for TS III versus 334 dBmin for TS I).

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Figure 8:

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Regression-analytically determined characteristic values TTS2 and t(0 dB), as well as physiological costs IRTTS with significance level for differences in means between the results of the 5 exposures of Study 2, respectively. (Means of 8 Ss, one-sided WILCOXON-test)

The upper half of Figure 9 shows profiles of heart rate for TS I, TS II, and TS III based on 5-min mean values from 8 Ss. The work-related increases during TS III are substantially higher than in the first study. Heart rates in the range of 120 bpm, associated with performing physical work at a far higher intensity of 100 W, are more than 40 bpm above the level for TS I and TS II, during which approximately 75 bpm had been registered. This extraordinarily high level of activation of the cardiovascular system and the continuous increase of heart rate during exposure time make clear that physical stress was above the endurance level and exceeded individuals' homeostatic control capacity.

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Figure 9:

Courses of Heart Rate (HR in beats per minute, bpm). 5-min-mean values for all 8 subjects in TS I, TS II, and TS III of Study 2 (upper half) and HR profiles of corresponding TS IV / TS I and TS V / TS III of Study 2 (lower half)

Such an overcharge with a strong intensification and already essential shifting of the blood flow into the active musculature of the extremities might be one of the reasons for the mitigation of (principally) favorable effects of physical activity to the inner ear which had been observed via audiometrically monitored parameters in the first study, when just moderate physical workload was put on the Ss. Such shifting in circulation as a consequence of 100 W (well-known from the basics of physiology for one-sided physical work, cp. RODAHL 1989) may have reduced blood input and, therefore, restricted the supply of the cochlea with oxygen.

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In addition to the already previously presented information, the lower half of Figure 8 shows individual and mean restitution time courses TTS(t) as physiological responses to the exclusive noise exposure of TS I as a reference and to TS IV and TS V. Again it must be stressed that the exposure to 94 dB(A) / 1 h alone (in TS I) was associated with maximum temporary threshold shifts of approximately 20 dB averaged over 8 Ss, which completely dissipated after a period of little more than 60 min. As already mentioned, the regressionanalytically determined values for TTS2 and t(0 dB) were 19 dB and 72 min. Additional smoking prior to the noise exposure in TS IV with an expected level of carbon monoxide haemoglobin of about 5 % and nicotine, on the one hand, seems to have favorable effects on the maximum threshold shifts (with a regression-analytically determined TTS2value that is reduced from 19.0 dB to 16.1 dB). On the other hand, restitution apparently is prolonged. The same is true for the combined effects of noise, simultaneous muscle work (when performing a high physical output of 100 W), and preceding cigarette smoke exposure in TS V. Since physical workload imposed on the Ss simultaneously to noise did fail to exhibit systematic effects on audiometric data, the retardation of the restitution process, which becomes evident in TS V, seems to confirm the effects of the smoke exposure that could be demonstrated by TS IV. However, the increased mean value of 19.1 dB for the maximum temporary threshold shift TTS2 (instead of 16.1 dB in TS IV) may be the result of counteracting the initially favorable effects of smoking on the development of TTS2 by the high physical workload. Despite some evident tendencies (as reported before), the smoothed restitution time courses shown in the lower half of Figure 8, together with the regression-analytically determined TTS2, t(0 dB), and IRTTS, exhibit no significant differences between the physiological responses to the exposures of TS I, TS IV, and TS V. Only a hypothetically expected maximum of total physiological costs to the hearing (namely 390 dBmin) could be found for TS V, that is, the highest average value could be associated with the most unfavorable combination of noise, simultaneous high physical workload, and preceding smoke exposure. The lower half of Figure 9 contains pairs of heart rate profiles from TS IV and TS I, as well as TS V and TS III. According to the chosen test design, disparities in the courses of heart rate based on 5-min mean values from the group of 8 Ss, each, can conclusively be attributed to the smoke exposure. From these results, a clear smoke effect (probably vasoconstriction by nicotine) can be identified in a significant activation of the cardiovascular system both in tests with and without physical work (i.e., TS V and TS IV). The continuously increased level of heart rate, e.g., by the smoke exposure in TS V – which ranged from 5 to 10 bpm in addition to the already exceptionally high level as a consequence of physical load in TS III – may have deteriorated the, at least initially, positive side effect of the smoke exposure which could be seen in reduced TTS2-values during TS IV without physical workload. In the same arrangement of the results as for the first study, all IRTTS-values of this second study were related to the reference value of TS I, that is, the exposure of the Ss to noise alone (cp. Figure 10). When summarizing the outcome of this procedure, it is evident that a value of 0.87 which has been calculated for TS II represents a (significant) reduction of aural strain by physical activity again. It needs to be mentioned that the Ss repeatedly had to interrupt bicycling with an output of 100 W in order to carry out audiometric measurements in the soundproof cabin. Ultimately, this led to a physical workload below endurance level, something with which the Ss could cope. Physical work of 100 W simultaneous to noise and a preceding smoke exposure resulted in relative physiological costs higher than those of the reference test, that is, numbers higher than 1.00. That is, 1.04 for TS III and especially 1.17

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for TS V represent negative effects, mainly due to the additional smoke exposure. The quotients that are slightly higher or lower than 1.00 for TS III (1.04) and TS IV (0.97), respectively, must be interpreted with caution because of their limited physiological relevance. However, the following can be stated with respect to the smoke exposure: Although the results of TS V (1.17) do not differ significantly from those of the directly comparable TS III, or also from the reference measurement of TS I, they proved to be significantly higher than those of TS II. This, at the very least, is evidence that CO and nicotine inherent in cigarette smoke must not be neglected as possible influential factors in the context of physiological costs that the hearing has to pay for noise exposures.

Figure 10: “Relative physiological costs” (IRTTSTS i / IRTTSTS I) of the exposures of Study 2

4 Discussion According to MEHNERT et al. (1994), studies on the effects of combined environmental factors and workload at the workplace represent a challenge for researchers in ergonomics and occupational health and safety. Such studies are complex, time-consuming, and also expensive, especially when they have to be carried out in the field as epidemiologic investigations in order to enable statistically relevant results. In order to improve this situation, it is absolutely necessary to elaborate firstly a reliable theoretical basis via laboratory studies for the hypothetical interactions from which practical concepts for directed studies can be derived. In the present studies, this aim was attempted for exposures to noise, physical workload, alcohol, and smoking which comprised carbon monoxide and nicotine as potential ototoxic agents. There is no question that the influences of threshold shifts and their restitution which have been examined in this study are issues of real life situations. For one, noise exposures are typically not the only influence to which individuals are exposed. It is not uncommon that workers have to carry out substantial physical tasks while being exposed to noise. Also, in leisure time, intensive sound exposures often are accompanied by physical activities. By instinct, stimulating music tends to make individuals move rhythmically or in dancing motions. Additionally, many people associate cigarettes and alcohol with “quality of life.” They do not like not being able to enjoy them at work, even though alcohol at the workplace should be taboo. In contrast to the study of LANDSTRÖM et al. (1999), moderate physical activities simultaneous to or immediately after a noise exposure revealed a positive effect. It could be shown repeatedly that dynamic muscle work below endurance level is beneficial to the recuperation of the hearing from noise-induced temporary threshold shifts. But these clear effects were lacking completely when the individual endurance level of the physical capacity was exceeded, as was the case with a workload of 100 W.

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With respect to the other combined effects, the results presented in this study have some shortcomings for various reasons. Regarding the effects of alcohol and cigarette smoke exposure, they must be considered “initial findings” from “feasibility studies.” In comparison with previous audiometric studies (cp. e.g., STRASSER et al. 1995; 1999; IRLE et al. 1998), the results in this study are less pronounced and exhibit less distinct differences with respect to the effects of test parameters. Based on the physiological effects of the test variables, they need to be interpreted as follows (cp. Figure 11): Due to vasodilatation as a result of alcohol consumption, the peripheral circulation can be expected to increase, which may also be beneficial to the inner ear. On the other hand, a sedative effect on the central nervous system can be expected as well, which improves the psychological ability to endure noise. This has also been shown in subjective interviews, which are not discussed here in more detail, however. It is unclear, however, whether the possibly improved psychological condition can already mitigate the negative aural effects of noise that are located in the inner ear rather than in the brain. It is sometimes claimed – cp. BABISCH et al. (1985) for an example – that the subjective condition is relevant not only for how bothersome noise is perceived to be, but also for differences in the temporary threshold shifts. For physiological reasons, however, it must be assumed that the circulation effects have the greater impact. An analysis of the IRTTS-values after acoustic stress suggests a somewhat positive effect of alcohol consumption. These indicators of the total physiological cost that the hearing must “pay” for the noise exposure are more favorable under the influence of alcohol (measured at 401 / 442 dBmin (TS V / TS III) and 535 / 573 dBmin (TS IV / TS I)).

Figure 11: Summary of combined effects of noise, physical load, alcohol, and cigarette smoking in temporary threshold shifts as physiological costs to the hearing

In light of these results, it appears that, concordantly with the experiments reported by PETIOT et al. (1990), which also aimed at short-term effects of a moderate dose of alcohol on hearing thresholds, alcohol might be the cause for some reduction of auditory fatigue.

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A second study with a rather small sample (8 Ss) almost of necessity showed hardly any significant influences, which remained – at least with respect to the influence of nicotine and COHb – uncontrolled and of low intensity. Slightly smaller TTS2-values after noise and a smoke exposure comparable to this study were also found in the study of MANNINEN (1998), where a reduced increase of TTS2 of the hearing at 4 kHz among smokers was associated with a significant increase in blood glucose. It can be speculated that this might be favourable to the metabolism in the inner ear. With approximately 5 % COHb, the replacement of oxyhaemoglobin by carbon monoxide haemoglobin (and thus a reduction in the O2transportation capacity and the oxygen supply to the organs), which was expected to have effects of approximately the same magnitude, was not particularly strong in this study. The same is true for the not very pronounced increase in the slope of the oxygen dissociation curve, which could be expected from a mild CO-intoxication and the resulting reduction in the O2-utilization in the tissue. Since an exactly controlled smoke exposure in the laboratory is not possible for ethical reasons, the test subjects were instructed to either smoke 10 (light or strong) cigarettes of their preferred brand at their typical intervals before the respective test series or to abstain from smoking on the days of TS I, II, and III. Thus, there was no biological monitoring during the test. Nonetheless, the following effects of the different exposures (which were administered in isolation and in combination) emerged: 1. A noise exposure of 94 dB for 1 h – which is energy equivalent to a level of 85 dB for 8 h – leads to substantial hearing threshold shifts of approximately 20 dB, which persist for hours after the exposure. It must be stressed that such an exposure is permissible in the production sector without hearing protection. 2. As long as it is not too exerting, overall dynamic muscle work that occurs simultaneous to a noise exposure is not detrimental to the restitution; to the contrary, there are some significant benefits. 3. Tasks that are in excess of an individual's physical limits no longer exhibit such a positive effect. With increasing demands, they may compound the effects of the noise exposure. 4. Discontinuous dynamic muscle work after the noise exposure which remains below the endurance level due to the selected work-break structure also has a positive influence on the hearing's restitution. 5. In addition to improved tolerability of noise, moderate alcohol consumption leads to a somewhat faster restitution of hearing fatigue. 6. Smoke exposure leads to measurably lower hearing threshold shifts that are induced by a noise exposure. However, it leads to an increase in restitution time. The overall effects must be considered negative, especially when smoking occurs in addition to high physical stress.

5 References ADLKOFER, F.X. (1991) Smoking at the Workplace (in German). In: OPITZ, K.; SORG, C. and WITTIG, U. (Eds.) Rauchen und Umwelt – Rauchen und Arbeitswelt. Immunity and Environment 7. Gustav Fischer Verlag, Stuttgart/New York, 60-82 BABISCH, W.; ELKE, J.U.; GOOSENS, C.; GRUBER, J.; ISING, H. und WINTER, A. (1985) Beeinflussung der zeitweiligen Hörschwellenverschiebung (TTS) durch psychologische Faktoren. Zeitschrift für Lärmbekämpfung 32 (2) 2-8 BARONE, J.; PETERS, J.; GARABRAND, D.; BERNSTEIN, L. and KREBSBACH, R. (1987) Smoking as a Risk Factor in Noise-Induced Hearing Loss. Journal of Occupational Medicine 29, 741-745 COHILL, E.N. and GREENBERG, H.J. (1977) Effects of Ethyl-Alcohol on the Acoustic Reflex Threshold, Journal of the American Audiological Society 2, 121-123

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ENGDAHL, B. (1996) Effects of Noise and Exercise on Distortion Product Otoacoustic Emission. Hearing Research 93 (1-2) 72-82 HETTINGER, Th. and MÜLLER-LIMMROTH, W. (1970) Gesund und fit am Steuer – Ratgeber und Pausenprogramm für den Autofahrer. Georg Thieme Verlag, Stuttgart IRLE, H.; HESSE, J.M. and STRASSER, H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. International Journal of Industrial Ergonomics 21, 451-463 IRLE, H. and STRASSER H. (1998) On the Effects of Dynamic Muscle Work on Noise-Induced Hearing Threshold Shifts. Proceedings of the 7th International Congress on Noise as a Public Health Problem, organized by the International Commission on the Biological Effects of Noise (ICBEN). In: CARTER, N. and JOB, R.F. (Eds.) Noise Effects ’98 (Sydney/Australia), 51-54 IRLE, H.; HESSE, J.M. and STRASSER, H. (2001) Physiological Costs of Noise Exposure: Temporary Threshold Shifts. In: KARWOWSKI, W. (Ed.): International Encyclopedia of Ergonomics and Human Factors. Vol. II: Part 7. Environment. Taylor & Francis, London and New York, 1050-1056 KARWOWSKI, W. (Ed.) (2001) International Encyclopedia of Ergonomics and Human Factors. Taylor & Francis, London and New York LANDSTRÖM, U.; BYSTRÖM, M. and OLOFSSON, B. (1999) Temporary Impairment of Hearing in Connection with Physical Exercise and Exposure to Noise/Music. Archives of Complex Environmental Studies 11 (1-2) 9-19 MANNINEN, O. (1998) Combined Effects of Noises and Cigarette-Smoking. Archives of Complex Environmental Studies 10 (3-4) 7-19 MARÉES, H. und MESTERS, J. (1990) Sportphysiologie II und III. Verlag Diesterweg, Frankfurt a.M. MEHNERT, P.; FRITZ, M. and GRIEFAHN, B. (1994) Noise-Induced Hearing Loss and Ototoxic Agents. Archives of Complex Environmental Studies 6 (3) 1-6 PETIOT, J.C.; PARROT, J.; LOBREAU, J.P. and SMOLIK, H.J. (1990) Combined Effects of a Moderate Dose of Alcohol and of Exposure to Noise upon Auditory Fatigue. Archives of Complex Environmental Studies 2 (1) 37-41 ROBINETTE, M.S. and BREY R.H. (1978) Influence of Alcohol on the Acoustic Reflex and Temporary Threshold Shift. Archives of Otolaryngology 104, 31-37 RODAHL, K. (1989) The Physiology of Work. Taylor & Francis, London, New York, Philadelphia STRASSER, H.; HESSE, J.M. and IRLE, H. (1995) Hearing Threshold Shift after Energy-Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and CHAMPNEY, P.C. (Eds.) Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, London, New York, Philadelphia, 241-248 STRASSER, H.; IRLE, H. and SCHOLZ, R. (1999) Physiological Cost of Energy-Equivalent Exposures to White Noise, Industrial Noise, Heavy Metal Music, and Classical Music. Noise Control Engineering Journal 47 (5) 187-192 STRASSER, H. and IRLE, H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In: KARWOWSKI, W. (Ed.): International Encyclopedia of Ergonomics and Human Factors. Vol. I: Part 3. Performance Related Factors. Taylor & Francis, London and New York, 516-523 VIROKANNAS, H. and ANTTONEN, H. (1995) Combined Effects of Noise and Smoking on Hearing Loss. Archives of Complex Environmental Studies 7 (1-2) 21-28 WALD, N.; HOWARD, S.; SMITH, P.G. and BAILEY, A. (1975) Use of Carboxyhaemoglobin Levels to Predict the Development of Diseases Associated with Cigarette Smoking. Thorax 30, 133-139

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181

Chapter 14

Quantification of the Insertion Loss of Personal Hearing Protection Devices by means of a Subjective Method and an Artificial Head Measuring System H. Irle, H. Fidan, J.M. Hesse and H. Strasser

0 Summary Currently, the hearing threshold method according to DIN ISO 4869 (1991) is typically used to determine the insulation of hearing protection devices. The subjectively determined insulation values are assumed to be independent of the level, that is, valid for all real life exposure levels. The validity of this assumption was to be objectively tested with the aid of an artificial head measuring system. Four hearing protection devices were exposed to a diffuse sound field at levels of 65, 85, and 105 dB with one-third octave band noise according to DIN ISO 4869, and the effective attenuation values (reduction in noise level provided by the hearing protector measured in dB) were determined. The evaluation of measuring data confirmed that the effectiveness of the examined ear muffs is approximately equal for the different noise levels. In a second step, the attenuation values were compared to the values which were subjectively determined according to DIN ISO 4869. Partially very different courses were observed. If appropriate correction factors are applied, however, the use of the absolutely objective artificial head measuring technique instead of the subjective method according to DIN ISO 4869 seems to be feasible.

1 Introduction Measuring methods for the determination of the insertion loss of hearing protection devices can be divided into subjective and objective methods. Subjective methods require the active participation of test subjects. Such relatively intensive participation is not necessary for the artificial head method. The following theoretical considerations and experimental studies may represent a contribution towards a valid, objective method. In the dominant subjective method according to DIN ISO 4869, the test signals consist of one-third octave band noise in a diffuse sound field. A comparison to the hearing threshold shift method according to DIN 45611 (1965) which used sinusoidal tones in the free sound field and which was used until 1984, the more recent method led to insulation values which were approximately 5 dB lower at almost all test frequencies. That means that the insulation values in older data sheets for personal hearing protection devices are too high (cp. Figure 1). In a 1991 revision of the standardized “hearing threshold shift method,” individual requirements were defined more precisely in order to make measuring results easier to reproduce. This indicates the effort to improve existing methods in order to ensure the prognosis of the protection available from the wearing of personal hearing protection devices. However, as a subjective method, the hearing threshold shift method requires a rather substantial effort and maximum control over the test subjects’ participation. Not only do certain requirements concerning the measuring room need to be satisfied, but many measurements on different individuals must be carried out which is both time-consuming and expensive. Few institutions are in a position to afford such efforts.

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Figure 1:

Comparison of determined sound attenuation curves of a hearing protection device according to DIN 45 611 (1965) and DIN ISO 4869 (1991) (source: TECH 1982; modified)

As a possible alternative, an objective method must satisfy the following requirements: 1. less stringent requirements concerning the measuring room, i.e., a sound proof room is not necessarily required; 2. less effort when the experiment is carried out, i.e., fewer rounds of tests because test results are easier to reproduce and measuring values are registered without the involvement of test subjects; 3. resulting sound attenuation values are identical to those from DIN ISO 4869 method. For a comparison of methods, the issue whether the noise attenuation does not depend on the level (as indicated in DIN ISO 4869) must be addressed in a first step (cp. Figure 2). There are some doubts if attenuation values which were determined via the hearing threshold shift method at very low noise pressure levels are still applicable at high levels. For very high levels, especially with impulse noise in excess of 120 dB, lower attenuation values or at least longer exposure times associated with the impulse impacts can be expected due to the compression wave dependent effects or a partial “fizzling out” of noise in the protection device (cp. BRINKMANN and BROCKSCH 1976; STRASSER 1987; STRASSER and HESSE 1993a; STRASSER and HESSE 1993b).

Figure 2:

Hypothesis about the sound attenuation of personal hearing protection devices in dependence of the sound pressure level

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2 Examined ear muffs The insertion loss of ear muffs of the types Opticom SD, Vario VOL SD, ULTRA 9000, and Bilsom Blau (cp. Table 1 and Figure 3) was determined objectively at levels of 65, 85, and 105 dB in order to test the hypothesis that the attenuation effectiveness is provided even at high levels. As a specialty, the ULTRA 9000 hearing protection device has a patented noise valve which is supposed to become effective once noise levels exceed 120 dB. Since the highest levels in this study did not exceed 105 dB, this fluidic phenomenon, however, could not be examined. Table 1: Characteristic values of the examined ear muffs Ear muff

Single number rating SNR

Weight

Contact pressure

Recommended level range in dB(A) for noise category

in dB

in g

in N

I (high/middle)

Opticom SD

22.0

187

13,7

87-102

85-92

Vario VOL SD

27.0

254

14,0

92-107

85-100

ULTRA 9000

17.0

220

11,7

85-100

85-93

Bilsom Blau

24.4

134

10,0

89-104

85-95

Figure 3:

II (low)

Examined ear muffs

Information according to DIN 32760 (1985) about the ear muffs such as the single number rating SNR, the weight, the contact pressure, and the recommended level range were taken from the product data sheets (cp. Table 1). In the experimental study, an attenuation curve was to be obtained for each hearing protection device and all levels which would allow the detection of a possible level dependency of the attenuation effectiveness. Subsequently, the attenuation curves were to be compared to those which were subjectively determined according to the hearing threshold shift method.

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Following DIN ISO 4869 (1991), continuous noise was used as exposure in this study. Effects which might play a dominant role with exposures from impulse noise were outside the scope of this study. Thus, it must be stressed that the results of this study should not be construed to provide information regarding the attenuation values with respect to an impulse noise exposure.

3 Objective attenuation measurement with an artificial head measuring system The test set-up – shown in Figure 4 and Figure 5 – can be divided into the components noise generation, noise exposure, and noise analysis.

Figure 4:

Test set-up for the determination of the insertion loss of ear muffs via the use of an artificial head measuring system

Figure 5:

Schematic test set-up for the determination of the insertion loss of ear muffs via the use of an artificial head measuring system

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In the “noise generation” component, one-third octave band noise is generated via a combination of a noise generator, an one-third octave band filter, and an amplifier. The exposure is then supplied to the artificial head inside a sound proof cabin via a loudspeaker. The “noise reception and analysis” component utilized the artificial Head Measuring System HMS II.2 and the Binaural Analysis System BAS I by HEAD Acoustics. The artificial head itself consists of a head and shoulder replica whose geometry can be described mathematically as well as the necessary electronics. The replica of outer ear and skin has the same acoustic and mechanical impedance as human skin. According to the hearing threshold shift method, a reference hearing threshold without hearing protection as well as a test hearing threshold with the hearing protection device to be evaluated must be determined audiometrically on at least 10 individuals with normal hearing. The difference of the two hearing thresholds is a measure for the noise attenuation of the hearing protection device. Following this subjective standard, one-third octave band noise in the diffuse sound field was used for the test series which had to be repeated at least 10 times. Not only the subjective hearing threshold shift method, but also a standardized artificial head for quality assurance (cp. DIN EN 24869, 1994) uses the same type of noise to build the noise field. The noise attenuation is obtained as the difference of the two one-third octave band pressure levels at different frequencies with and without hearing protection. The procedure is further illustrated with the following example (cp. Figure 6).

Figure 6:

Determination of one-third octave band pressure level L1 and L2 using the ULTRA 9000 hearing protection device; test No. 8, volume range L1 = 85 dB, one-third octave band noise with fm = 500 Hz

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186

The 8th test with the ULTRA 9000 hearing protection device was chosen. After the level range L1 of approximately 85 dB and the mid-frequency fm = 500 Hz of the test signal had been set, the non-insulated one-third octave band noise was “read” into the computer and shown on the monitor. The artificial head supplies a frequency spectrum for each ear channel. Using the BAS software, the two spectrums are overlaid. Alternatively, they can be viewed separately by using different colors. For the one-third band with the mid-frequency fm = 500 Hz, a level of L1l = 85.9 dB for the left ear channel and L1r = 83.2 dB for the right ear channel were registered. After the hearing protection device had been put on, the levels L2l = 59.8 dB for the left ear channel and L2r = 57.5 dB for the right ear channel were registered. These onethird octave band pressure levels were recorded in a measurement protocol for the L1 = 85 dB volume range (cp. Table 2). Table 2: Measurement protocol of the ULTRA 9000 hearing protection device in the volume range L1 = 85 dB ULTRA 9000 Hearing Protection Device f

125

r

l

r

r

l

r

l

6300 r

l

8000

L2i

78.2 82.5 65.5 72.5 61.7 59.0 65.5 63.6 59.8 54.8 57.5 52.5 56.3 58.6 66.7 62.1 59.0 60.9

2

L1i

87.5 87.5 87.5 87.7 89.0 87.1 88.6 86.7 88.2 87.1 85.2 87.1 81.3 85.9 86.3 86.5 85.9 86.1

L2i

78.2 78.2 67.1 67.1 65.0 64.8 67.8 63.6 62.8 57.8 58.2 55.5 57.8 57.8 67.5 64.8 60.2 62.5

L1i

84.8 84.8 85.2 85.4 87.5 84.8 87.5 86.4 86.4 84.8 83.7 84.8 79.1 83.7 84.5 84.5 83.7 84.5

L2i

76.4 74.8 64.1 64.1 59.5 59.5 66.0 62.2 59.8 54.1 59.8 52.9 54.5 56.8 65.2 63.3 57.9 60.2

4

L1i L2i

84.1 84.1 84.5 85.2 86.0 83.7 86.4 85.2 86.0 84.5 83.3 84.8 79.8 84.1 84.1 84.5 84.1 84.3 76.8 76.0 63.7 66.4 60.2 60.2 65.2 63.7 59.5 53.7 55.6 52.2 56.0 56.4 65.6 62.9 58.7 61.4

5

L1i L2i

85.0 85.0 84.2 85.0 86.5 84.2 86.2 83.8 85.8 83.8 82.7 84.2 81.5 85.8 84.6 84.6 84.6 84.8 78.8 76.9 65.0 65.8 61.2 58.8 65.8 62.3 58.8 55.0 55.8 50.8 56.5 58.5 65.8 65.0 58.5 63.5

6

L1i

85.5 85.5 84.4 84.4 87.1 84.8 87.8 85.2 85.2 84.8 82.9 84.8 81.4 84.8 85.2 85.6 84.5 84.6

L2i

77.5 77.9 62.5 62.5 64.8 61.3 67.1 64.4 59.5 55.6 57.2 52.2 56.4 56.8 65.6 63.7 58.7 61.8

L1i

84.5 84.5 85.6 85.8 86.0 83.7 86.8 85.2 85.2 85.2 83.3 85.2 81.4 84.8 88.3 85.2 84.8 84.8

L2i

78.3 78.0 64.5 64.5 60.6 57.5 65.6 63.3 57.9 54.5 57.5 52.5 57.3 57.5 68.3 64.8 57.5 58.3

L1i

84.0 84.0 85.5 86.7 85.9 83.2 86.7 84.6 85.4 85.2 83.2 84.8 79.8 84.0 87.5 84.4 84.0 84.0

L2i

75.9 76.3 70.9 64.4 59.8 57.5 65.2 62.1 57.8 54.0 57.1 52.1 55.9 57.5 69.0 63.2 58.2 54.5

9

L1i L2i

85.1 85.1 83.2 83.9 83.9 83.5 87.8 85.1 88.1 84.2 84.1 84.3 81.2 84.2 86.2 84.6 81.5 84.2 75.5 75.9 62.4 62.4 60.5 58.9 66.5 62.7 56.5 55.0 55.5 55.5 53.8 54.6 65.8 65.8 55.8 56.9

10

L1i L2i

83.1 83.1 85.0 85.0 86.5 84.2 86.8 84.5 89.1 84.8 84.5 84.8 81.4 84.1 86.5 85.0 81.9 85.0 74.4 75.0 65.0 61.5 63.1 57.7 64.8 61.8 58.3 56.0 56.4 55.2 53.7 52.9 66.2 66.2 55.0 56.9

L1-Range: Range of L1i in dB

l

4000

84.4 84.4 85.2 85.5 86.7 83.3 87.5 85.2 86.7 84.4 82.8 85.2 80.2 84.0 84.4 84.8 84.4 84.8

L2i: Sound pressure level with ear muff in dB

r

3150

L1i

L1i: Sound pressure level without ear muff in dB

l

2000

1

8

l

1000

i

7

R

L1-Range: 85 dB(A)

500

n

3

l

250

r

l

r

i: Ear channel of the artificial head measuring system (i = l, r) n: Test number (n = 1, 2, ... , 10) f: Frequency in Hz

As can be seen from the measurement protocol, 10 test courses for each test frequency between 125 and 8,000 Hz were given. Because two ear channels were analyzed, 20 individual attenuation values were determined at each frequency. Averaging over these 20 individual values in each frequency range yielded the values of the objectively determined insulation curve.

4 Results As mentioned previously, 3 insulation curves in the volume ranges 65, 85, and 105 dB were registered for each ear muff. As shown in Figure 7, in the case of the ULTRA 9000 ear muff, there is little difference between the 3 curves.

H. Irle et al. / Quantification of the effective attenuation of personal hearing protection devices

Figure 7:

187

Determination of the level-dependent maximum difference in attenuation 'Rmax(f) for the ULTRA 9000 ear muff (using the 125 and 2000 Hz frequencies as examples)

In order to quantify the degree to which the curves correspond or vary, the absolute value of the maximum attenuation value difference was introduced. This value indicates the maximum deviation of the 3 insulation curves and is calculated as the difference between the largest and the smallest attenuation value in a frequency range. Since there are 9 one-third octave band frequencies (cp. Table 2), 9 values were determined for each hearing protection device. It was found that all of these maximum attenuation value differences were within 2.5 dB. In 84% of all cases, this difference between the 3 curves was even less than 1.5 dB. This shows that the examined ear muffs’ attenuation effectiveness with continuous noise of up to 105 dB can be considered as independent of the level. Next, the objectively determined curves for the ear muffs were compared to the conventional (i.e., subjectively determined) attenuation values at the hearing threshold. Since the noise attenuation curves almost coincided for all ear muffs at the test levels of 65, 85, and 105 dB (cp. Figure 8), the three curves were collapsed into a mean insulation curve which then was compared to the subjectively determined values according to DIN ISO 4869 (cp. Figure 9).

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Figure 8:

Comparison of attenuation values at different sound pressure levels (65 dB, 85 dB, and 105 dB)

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191

Chapter 15

Substantial Protection Loss Associated with a Minimally Reduced Wearing Time of Hearing Protectors – Fiction or Reality? H. Irle, Ch. Rosenthal and H. Strasser

0 Summary Valuable recommendations for the choice, utilization, care, and maintenance, and for the measurement of sound attenuation of hearing-protective devices have been laid down in international standards. Yet, by considering the wearing time of a hearing protector, the standard DIN EN 458 assumes a scarcely understandable drastic reduction in the effective attenuation even when the device is not used for only a short time in a noise-filled area. A 30-dB sound attenuation of such a protective device would, e.g., decrease to 12 dB if it were unused for only 30 min of an 8-hour shift. Thus, the actual influence of a shortened wearing time on the protection of ear muffs was tested in a laboratory study using audiometric measurements of the temporary threshold shift (TTS2) and its recovery after exposure to noise. For that purpose, the effectiveness of a hearing-protective device depending on the amount of time worn as prognosticated by DIN EN 458 was compared with the actual physiological effect of the ear muffs. 10 test subjects (Ss) participated in 3 test series (TS), each. In the first of the TS, the Ss were exposed to a sound pressure of 106 dB(A) for 1 h, during which the Ss wore noiseinsulating ear muffs with an attenuation of 30 dB. The Ss were exposed to the same sound pressure in TS II; however, after 30 min, the ear muffs were removed for a duration of 33/4 min. Mathematically, this reduced the sound attenuation of the ear muffs to 12 dB; i.e., the average noise level over 1 h should be 94 dB, which would be equivalent to 85 dB(A) over 8 h. In order to evaluate the actual additional physiological cost of TS II, the Ss were exposed to 94 dB(A) / 1 h without ear muffs in a third TS. This acoustic load, which is energy equivalent to the load in TS II, is also equivalent to 85 dB(A) / 8 h. The results show that the continuous wearing of the ear muffs offers secure protection. However, the energetic approach and the levelling of differently structured noise loads according to the principle of energy equivalence leads to misconceiving results. The drastic reduction of the sound attenuation of the ear muffs predicted from the energetic point of view must be regarded as exaggerated. The TTS values show that TS II – which, according to the equal energy concept, should result in the same effects as TS III – represents significantly less auditory fatigue. Thus, if the ear muffs are taken off briefly, a drastic reduction in the protection – as predicted in DIN EN 458 – does not result.

1 Introduction and objectives Valuable recommendations for the choice, utilization, care, and maintenance, and for the measurement of sound attenuation of hearing-protective devices have been laid down in international standards (cp. DIN EN 458, DIN EN 352-1, ISO 4869). Yet, when considering the wearing time of a hearing protector, the standard DIN EN 458 assumes a drastic reduction in the effective attenuation even when the device is not used for only a short time in a noisefilled area. A 30-dB sound attenuation of such a protective device would, e.g., decrease by 18 dB to 12 dB if it is unused for only 1/2 h of an 8-h shift. If the device went unused for just 4 min of a 480-min (8-h) shift, its noise insulation would decrease from 30 dB to only 21 dB, i.e., even then an insulation loss of 9 dB would result. Such losses shall be visualized by the following two Figures. According to the upper part of Figure 1, e.g., the acoustic situation at a workplace with a high continuous noise level may be represented by 106 dB. If an ear muff is worn continually, e.g., for 8 h in that area, then – with an assumed attenuation of 30 dB – the worker’s hearing is subjected to only 76 dB (instead of 106 dB). If, however, the protective device is not worn for only 1/2 h as it is shown in the front part of Figure 1 – whether this occurs all at once or over several short periods of time is irrelevant – the worker is exposed to 106 dB for those 30 min and it is prognosticated by the international standard that 18 dB of the attenuation (94 dB - 76 dB) are lost, i.e., a sound attenuation of only 12 dB (rather than 30 dB) results.

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Figure 1:

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Reduced efficiency 'D of a hearing protector with an insulation value D of 30 dB for a noise exposure of 106 dB / 8 h and a wearing time reduced by 1/2 h, when applying the energy equivalence principle

According to the 3-dB rule which is generally used in acoustics, an assumed doubling of the exposure time corresponds with a 3-dB reduction of the noise level, i.e., 106 dB for 1/2 h are equivalent to 103 dB for 1 h, 100 dB for 2 h, as well as 97 dB for 4 h or, finally, 94 dB for 8 h. The 106 dB over 1/2 h, therefore, seem to be equivalent to 94 dB for 8 h. Energetically, this is completely correct since both exposures involve the same dose of noise. Therefore, when the protective device is worn for 71/2 h, the hearing is exposed to a noise of 76 dB + 94 dB over 8 h, each, the latter originating from the 106 dB for 1/2 h. Since 76 dB + 94 dB = 94 dB according to the rules of the rating level calculation, the protector seems to have substantially decreased by 94 dB - 76 dB = 18 dB due to the 1/2 h during which it was not worn. Even more curious is the case represented in the front part of Figure 2, where the hearing protection is not worn for a full hour. This results in a prognosticated loss of 21 dB, which translates into a sound attenuation of only 9 dB compared to the original 30-dB attenuation. Yet, even a short period of just 4 min without the protective device during an 8-h day is assumed to result in the loss of 9 dB. This all may seem plausible and reasonable according to the laws of energy equivalence, however, it seems as if such calculations according to DIN EN 458 are used to pressure the employees into continuously wearing hearing protectors in order to avoid any risk to their hearing from detrimental noise, instead of taking care of noise control measures. In this case, however, it seems that the energy equivalence which makes such calculations possible – as has repeatedly been shown (cp. STRASSER 1995; STRASSER 1996; STRASSER and HESSE 1993) – once again goes too far, since it is at least difficult to imagine and so far is an “open” question whether the aforementioned calculations are based on secured data of the actual sound attenuation of hearing protectors which are measured depending on the wearing time. Therefore, the goal of this study was to shed some light on these speculations. It seemed reasonable to measure the actual effect of the hearing-protective devices via test exposures using relatively simple audiometric methods. The main idea was that the actual reduced protection which results when a noise-appropriate protective device is not worn for a limited amount of time during exposure to a high level of noise would be indicated by auditory fatigue, i.e., Temporary Threshold Shifts (TTS-values).

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Figure 2:

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Reduced efficiency 'D of a hearing protector with an insulation value D of 30 dB for a noise exposure of 106 dB / 8 h and a wearing time reduced by 1 h, 1/2 h, and 4 min when applying the energy equivalence principle

2 Methods 2.1 Test design and working hypotheses Due to pragmatical and especially ethical reasons, such tests are limited. Therefore, experiments on test persons involving exposures which exceed the noise limits for the workplace (cp. Accident Prevention Regulation “Noise” 1990) cannot be carried out. That is, exposures which are higher than the energy equivalent rating level of 85 dB(A) over an 8-h day (without hearing protection) cannot be utilized in the laboratory. Furthermore, experiments on groups of test subjects (Ss) involving 8-h exposures to noise would require an inappropriately high expense. Therefore, when the ethically still allowable limit of 85 dB(A) for 8 h is utilized as an orientation value, it should also be permissible to experiment using energy equivalent exposures of 88 dB / 4 h, 91 dB / 2 h, or 94 dB / 1 h. As can be seen in Figure 3, an experimental, practicable exposure could also be chosen using this configuration – keeping in consideration the fact that, according to previous experiments (cp. MILLER 1974; STRASSER et al. 1995) – 94 dB for 1 h numerically result in approximately equal threshold shifts as 85 dB / 8 h. So, in one of three test series (TS III), the Ss were exposed to 94 dB / 1 h. In a second test series TS I (cp. front part of Figure 3), the same Ss – acting as their own control – were exposed to 106 dB for 1 h but in this case they were protected by noise-attenuating ear muffs with a noise reduction of 30 dB, so that with 106 dB - 30 dB = 76 dB only very minimal threshold shifts could be expected. Whether or not the ear muffs actually provided the protection promised was also to be assessed using threshold measurements. Finally, in TS II (cp. middle part of Figure 3), the Ss removed the ear muffs for the short time of 33/4 min during a 1-h exposure to 106 dB, such that the sound exposure of 106 dB for that amount of time (taking into account the 3-dB rule) is equivalent to a continuous noise of 94 dB / 1 h, as in TS III.

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Figure 3:

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Schematic representation of the 3 exposures

During and after the exposure of TS I where the Ss were safely protected by wearing the ear muff all the time no hearing threshold shifts should be measurable as a consequence of only 76 dB / 1 h. Yet, according to previous studies with the exposure of 94 dB / 1 h or a rating level of 85 dB(A) in TS III distinct responses in temporary threshold shifts and their restitution were to be expected. Finally, one should expect the test series TS II and TS III to result in identical threshold shifts – as hypothetically represented in the upper part of Figure 4 – which again can be interpreted as physiological cost brought upon the human organism by the energy equivalent noise situations. However, if the test series TS II in which the ear muffs were removed for a short period of time result in a significantly lower threshold shift than the shift resulting from 94 dB / 1 h, then it must be assumed that the loss of sound attenuation during shortened wearing times as propagated by the standard is an exaggeration. Then it seems reasonable to assume that the skepticism reflected in DIN EN 458 is without scientific as well as practicable reasoning. Thus, it would have to be considered an over-subtle broad hint which is not conformable with the work-physiological human characteristics.

Figure 4:

Schematic representation of the 3 exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shifts (TTS)

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2.2 Hearing protective device The ear muff Optac Vario VOL SD was used for the experimental investigation. This hearing protector – as can be seen in Figure 5 – has proven a protective device with a SNRvalue of about 30 dB averaged over the frequency in a previous study (HESSE et al. 1996a), whereby measurements have been taken via an artificial head measurement system and according to the suggested method of ISO 4869-1.

Figure 5:

Sound attenuation characteristics of the hearing protector “Optac Vario VOL SD”

2.3 Test subjects and audiometric methods of selection and evaluation Only hearing-physiologically normal Ss were chosen for the experiments. The 10 male Ss which were selected (age: 26 ±6 years; height: 185 ±5 cm; weight: 80 ±10kg) could – according to ISO 4869-1 – only exhibit hearing threshold shifts of no more than 15 dB as compared to persons with normal hearing for frequencies of up to 2 kHz and threshold shifts of no more than 25 dB for frequencies above 2 kHz. On each testing day, in a sound proof cabin the individual resting hearing thresholds before the exposures (middle line in Figure 6) and the TTS2 values after the exposures (dark area in Figure 6) were measured for the Ss, whereby the frequency during which the highest threshold shifts occurred first always had to be determined. These maximum threshold shifts usually occurred at 4 kHz or 6 kHz and were finally determined over the restitution time until recovery was completed. This time t(0 dB) is also an important characteristic of the responses of the ear to an acoustic load.

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Figure 6:

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Selection of the frequency with maximum threshold shift during first 2 minutes after the exposure

2.4 Schematic test set-up and sound exposure As can be seen in Figure 7, the Ss were exposed to noise via loudspeakers in the same soundproof cabin (right part) where the audiometric measurements prior to and after the exposures took place.

Figure 7:

Schematic test set-up

For reasons of control the exact desired sound pressure levels for both ears were achieved using an artificial head measurement system (middle part). A sound exposure in the form of music (Artists: Genesis; Album: We Can't Dance; Song: Driving the last spike) was chosen as

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the acoustic load for the experiment, since such an exposure seems at least as realistic and valid for the objectives of the study as White or Pink Noise which is usually utilized in laboratory experiments. The exposure during the “leak” time of 33/4 min, however, had to be comparable to the exposure during the remainder of the time with respect to dynamics (e.g., peak and average levels), frequency, and time structure. Thus, a music segment which is exactly 33/4 min long was selected; the selection was then copied and pasted together 16 times resulting in an uninterrupted, continuous hour of music. Furthermore, it had to be ensured that the level distribution of the exposure was as homogeneous as possible over the entire frequency range without distinct bass components and that it was not characterized by a specific content of impulses as is the case in heavy metal music. Figure 8 represents the frequency analysis, i.e., the spectral energy distribution in the middle 8 octaves of the acoustic load with A-weighting, C-weighting and linear recording which via different amplification ended up in 106 dB(A) or 94 dB(A) over 1 h, measured utilizing the time constant “Fast” (125 ms).

Figure 8:

Frequency analysis of the sound exposure

3 Results Figure 9 shows all measured values of the hearing threshold shifts (differences between the TTS values and the individual resting threshold at the respective frequency of maximal threshold shift) as determined for the 10 Ss for the 3 test series. Furthermore, in points and lines the arithmetic means and the regression lines are shown. The upper section contains the physiological responses to TS III. It is noticeable that significant threshold shifts occur after the exposure to 94 dB / 1 h. The average of these shifts is approximately 20 dB and it takes up to 2 h for a full recovery. Thus, legally permissible exposures with a rating level of 85 dB(A) for 8 hours (cp. N.N. 1988; Accident Prevention Regulation “Noise” 1990) lead to essential physiological cost to the hearing which may not be underestimated.

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The lower section of Figure 9 shows the results for the test series in which the Ss were exposed to 106 dB without hearing protection for only a short period of 33/4 min, which is energy equivalent to 94 dB for 1 h. On average, such an exposure results in threshold shifts of approximately only 10 dB; fortunately, recovery lasts approximately only 1 h. The middle section of Figure 9 shows that the majority of the Ss experienced absolutely no threshold shift when exposed to 106 dB while wearing the protector continuously. In the case of the Ss who did experience a hearing threshold shift, the shift was hardly objectiviable (usually less than 4 dB), and the Ss recovered fully within mere minutes. Thus, the ear muffs actually can be considered effective protection for exposures of up to 106 dB(A). Due to the above-described results, it seems sensible to consider the restitution process after an acoustic exposure not only with respect to the strain level (TTS2) but also with respect to the restitution time t(0 dB). Furthermore, as an overall parameter combining both, the integral of the temporary threshold shift over the restitution time can be calculated. This parameter, the so-called Integrated Restitution Temporary Threshold Shift (IRTTS) is the area that is bordered by the time axis, the TTS axis at 2 minutes, and the restitution course TTS(t). It represents the total physiological cost – (for the respective hearing frequency where these data have been measured) – analogous to the sum of heart rate increases above the resting level during the recovery period as a characteristic of the physiological cost of dynamic muscle work. A simplified summary of the test results in Figure 10 again shows that although the removal of the protective device is not without any consequences, the effects are by no means as dramatic as the energy equivalence principle upon which DIN EN 458 is based would have us believe. The physiological cost in TS II is not, as prognosticated identical with those of TS III but significantly lower than in TS III. Finally, integration of the postexposure threshold shifts until their total disappearance results in significantly different values for the three test series. The IRTTS-value for TS I in which the device is continuously worn is only 3 dBmin. The exposure to 94 dB / 1 h in TS III, however, results in about 500 dBmin. The brief removal of the ear muffs in TS II which is energy equivalent to the 94 dB / 1 h of TS III resulted in only about 150 dBmin, i.e., the physiological cost is less than one-third of that in TS III (see also Figure 11).

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Figure 9:

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Individual and mean temporary threshold shifts and their restitution time course after the exposures of the test series TS I (middle), TS II (lower section), and TS III (upper section)

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Figure 10: Restitution time course TTS(t) and physiological cost IRTTS (numbers in the upper right box) with symbolic marking of the significance level of differences between the test series TS I, II, and III

Figure 11: “Relative physiological costs” of the exposures in TS I ((106 dB(A) - 30 dB(A)) / 1 h) and TS II (106 dB(A) / 33/4 min + 76 dB(A) / 561/4 min) energy equivalent to 94 dB(A) / 1 h related to the exposure to music with a level of 94 dB(A) / 1 h (IRTTSTS i / IRTTSTS III)

4 Discussion and conclusions The dramatic decrease in the attenuation of hearing protectors due to shortened wearing time – as it is stated in national and international standards – cannot be supported by the experimental results presented here. Incidentally, the calculations of supposed losses in the insulation in DIN EN 458 are independent of the immission level and lead to losses that greatly vary in size between hearing protective devices whose effectiveness varies by nature. It can be calculated that the insulation of ear plugs with an insulation value of, e.g., 20 dB is reduced to 12 dB due to a “leak” of 1/2 h during an 8-h shift, i.e., it is reduced by 8 dB. Ear muffs whose insulation values of, e.g., 30 dB or 35 dB are also reduced to the same 12 dB are even more underrated. The loss of 18 dB or 23 dB simply makes no sense. Fortunately, instead of reality a substantial protection loss is fiction.

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Finally, the point in time at which the hearing protector is removed is of significance for the protection. In this study, the midpoint of the exposure time was consciously chosen for removal of the device. Removal of the ear muffs towards the end of the exposure would certainly have resulted in higher threshold shifts which can be measured in the time after noise exposure. On the contrary, the removal or the delayed putting on of the ear muffs at the beginning of the exposure should cause scarcely measurable threshold shifts after the exposure since possible hearing threshold shifts can almost disappear when the protective device ensures that the ears are exposed to noise immissions only below 70 dB(A). In order to obtain information about the maximal magnitude of the threshold shift immediately after the hearing protective device was put on again or about the premature removal at the end of the exposure, a further study was carried out in which the hearing threshold shifts after a singular exposure of 106 dB for 33/4 min on the same Ss was measured as a control. The average of the TTS2-values was 14.9 dB, i.e., they were lower than the values after the energy equivalent exposure to 94 dB / 1 h. Concluding it can be stated that a short-term removal of a protector such as 33/4 min within 1 h, which is approximately equivalent to a 1/2-h removal within 8 h, does not end up in the prognosticated drastic reduction in protection. Instead of the essential auditory fatigue represented in an IRTTS-value of 503 dBmin, a reduction to 146 dBmin, i.e., as shown in Figure 11 with 0.29 less than approximately one-third, resulted. While it seems reasonable that the removal of the hearing protective device over an extended period of time should be avoided, the statistically secured results of this study show that the equation “energy equivalence = strain equivalence” cannot be valid, just as it would be senseless to assume that the equation “energy equivalence = interference equivalence” is permissible in the context of the psychological (extra-aural) effects of noise. Thus, predictions such as the ones cited in the standard should not be used to put pressure on the employees that are subjected to a noisy environment. However, greatest care should be exercised with respect to wearing comfort and sufficiently effective insulation values (cp. VDI 2560) when personal hearing protective devices are selected. Finally, if suitable ear muffs are taken off briefly, a drastic reduction in the protection – as predicted in DIN EN 458 – does not result. Again, this is a classic example that the standards and regulations for noise immission on man do not correspond with the actual physiological facts and, therefore, can only be used in a very limited manner. Utilization of the principle of energy equivalence has proven problematic in numerous studies (e.g., cp. HESSE et al. 1996b; IRLE et al. 1998); thus, the standards and regulations have been built upon an untrustworthy foundation.

5 References HESSE, J.M.; IRLE, H. and STRASSER, H. (1996a) Objective Measurement of Hearing Protection Provided by Earmuffs Versus Subjectively-Determined Sound Attenuation at the Hearing Threshold. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, 627-632 HESSE, J.M.; VOGT, E.; IRLE, H. and STRASSER, H. (1996b) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Restitution of a Continuous Noise-Induced Temporary Threshold Shift Under Resting Conditions and Under the Influence of an Energetically Negligible Continuous Noise Exposure of 70 dB(A). In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, 639-644 IRLE, H.; HESSE, J.M. and STRASSER, H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. International Journal of Industrial Ergonomics 21, 451-463 MILLER, J.D. (1974) Effects of Noise on People. J. Acoustics Soc. Am. 56 (3) 729-764

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STRASSER, H. (1995) Dosismaxime und Energie-Äquivalenz – Ein Kernproblem des präventiven Arbeitsschutzes bei der ergonomischen Beurteilung von Umgebungsbelastungen. In: STRASSER, H. (Hrsg.) Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen – Anspruch und Wirklichkeit des präventiven Arbeitsschutzes. Ecomed Verlag, Landsberg/Lech, 9-31 STRASSER, H. (1996) Curiosities of Conventional Noise Rating Procedures. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, 619-626 STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Work Load. Archives of Complex Environmental Studies 5 (1-2) 9-25 STRASSER, H.; HESSE, J.M. and IRLE, H. (1995) Hearing Threshold Shift after Energy Equivalent Exposure to Impulse and Continuous Noise. In: BITTNER, A.C. and CHAMPNEY, P.C.(Eds.) Advances in Industrial Ergonomics and Safety VII. Taylor & Francis, 241-248 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN EN 352-1 (1993) Hearing Protectors; Safety Requirements and Testing; Part 1: Ear Muffs. Beuth Verlag, Berlin DIN EN 458 (1993) Hearing Protectors; Recommendations for Selection, Use, Care and Maintenance; Guidance Document. Beuth Verlag, Berlin DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin DIN ISO 4869-2 (1994) Acoustics; Hearing Protectors; Part 2: Estimation of Effective A-weighted Sound Pressure Levels when Hearing Protectors are Worn. Beuth Verlag, Berlin German Working Places Regulations - § 15 Noise Abatement (1988) Arbeitsstätten – Vorschriften und Richtlinien, § 15 Schutz gegen Lärm VDI 2560 (1983) Personal Noise Protection (Guidance Document of the Association of German Engineers). Beuth Verlag, Berlin

Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 16

Influence of Reduced Wearing Time on the Attenuation of Earplugs – Prognosis by the 3-dB Exchange Rate versus Audiometric Measurements H. Strasser, H. Irle and T. Siebel

0 Summary If hearing protectors are unused for only a short time, their attenuation according to international standards is reduced drastically Whether this actually occurs was to be investigated in a second study during which also 10 test subjects (Ss) were exposed to noise at a level of 94 dB(A), continuously, for 1 h. In a further test series (TS), during which earplugs instead of ear muffs in the study of chapter 15 with an attenuation of 30 dB were provided, the Ss were exposed to noise at a level of 106 dB(A) for 1 h. The protectors were inserted just 33/4 min after the noise exposure began. According to the equal energy concept and the 3-dB exchange rate, this constellation leads to an equivalent noise exposure of 94 dB(A) / 1 h. Therefore, the attenuation of the earplugs which went unused for 33/4 min during 1 h seems to deteriorate by 18 dB to only 12 dB. Furthermore, the influence of several short-time removals of the earplugs on the attenuation was simulated. The actual effects of the shortened wearing time on the protection of the earplugs were evaluated via audiometric measurements. According to the results, the only slight physiological responses to short periods of earplug removal cannot be interpreted as representing drastic reductions of the attenuation which were predicted in the standards.

1 Introduction and goal of the study The standard DIN EN 458 gives recommendations for the choice, utilization, care, and maintenance of personal hearing protectors in order to provide protection against noise and to promote occupational safety. Furthermore, this guidance document stresses that the effectiveness of earplugs and earmuffs is not only dependent on the specific attenuation but also is decisively determined by the wearing time in a noisy area. If hearing protectors are unused for only a short time, their attenuation according to a given formula in the standard seems to be reduced drastically. The attenuation losses which can be calculated by the 3-dB exchange rate, e.g., amount to 18 dB if a protector is not used for only 1/2 h in an 8-h shift. The losses are even more drastic when hearing protection is unworn for longer periods of time. Despite a wearing time of 7 h while exposed to noise, a doubled time of 1 h reduces the effectiveness by another 3 dB. But even an extremely short break of just 4 min during which no protection is worn still reduces the efficiency by 9 dB. These almost unbelievable hearing protection losses which are prognosticated by the international standard due to blind faith in the validity of the principle of energy equivalence needed to be scrutinized with experimental studies during which test subjects (Ss) were exposed to acoustic exposures with and without hearing protectors. It was to be expected that a reduced wearing time would not only result in mathematically determinable protection losses but would also cause corresponding audiometrically measurable temporary hearing threshold shifts.

2 Methods and materials 2.1 Test design and working hypotheses Since experiments with a duration of 8 h would have caused a number of problems, and since aural effects of continuous noise for 8 h are quite similar to those of energy equivalent continuous noise for 1 h at a correspondingly higher level (cp. MILLER 1974), a more practical exposure constellation was created (cp. Figure 1).

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Figure 1:

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Schematic representation of the 4 energy equivalent exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shift

In a first test series (TS I), the Ss were exposed to White Noise at a level of 94 dB(A), continuously, for 1 h. This acoustic load is energy equivalent to a rating level LArd of 85 dB(A) for 8 h, which is still tolerable for workplaces in the production sector in almost all countries (cp. N.N. 1997). This series was carried out as a reference in order to enable the comparison of the results with those of preceding studies during which White Noise had also been used (cp. IRLE et al. 1998). In a second test series (TS II), typical industrial noise (continuous noise with some impulsive components) at the same level and for the same duration was used. In test series III (TS III), during which earplugs (see Figure 2) with a SNR value (single number rating-value) of 30 dB (cp. N.N. no year, DIN ISO 4869-1) were provided, the Ss were exposed to the same industrial noise for 1 h; however, the noise level was increased to 106 dB(A). Rather than being inserted prior to the exposure, the earplugs which should reduce the noise level reaching the ear by 30 dB(A) to 76 dB(A) were inserted just 225 s (33/4 min) after the noise exposure began. As can be calculated by the energy equivalence principle, a high exposure of 106 dB(A) / 33/4 min and a low level of 76 dB(A) during 561/4 min lead to an equivalent exposure of 94 dB(A) / 1 h, the same as in TS I and TS II. Therefore, the attenuation of the earplugs which went unused for 33/4 min during 1 h seems to deteriorate by 18 dB to only 12 dB. The noise immission of the hearing resulting from the short period in which no protection was provided seems to be the same as that which stems from a continuous energy equivalent exposure to 94 dB / 1 h. Compared with an 8-h shift, similar is true if hearing protection is not worn for 1/2 h. In reality, this may be the case when hearing protectors are not worn immediately upon or even before entering a noise area, but are utilized belatedly instead. In comparison to this situation, in a further test series (TS IV), the influence of several short-time removals of the earplugs on the attenuation – e.g., when smoking a cigarette or talking to a colleague or to the foreman – was simulated. For this reason, the earplugs were removed three times for 75 s (11/4 min), each, after 15 min, 30 min, and 45 min during exposure to 106 dB(A) / 1 h.

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Figure 2:

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Sound attenuation characteristics of the earplug “AIR Soft” by Howard Leight

If the principle of energy equivalence is valid for the evaluation of noise exposures or the dose maxim on which the calculations in DIN EN 458 are based, the effects on the hearing should be identical. Hypothetically, however, it could be expected that the hearing threshold shifts as a measure for auditory fatigue would not necessarily be the same. As shown in the back of Figure 1, the aural effects immediately after the exposure, measurable as Temporary Threshold Shifts (the TTS2 values), should at least be a function of the previous exposure. It was also expected that the restitution in general and the restitution time t(0 dB) in particular would be a function of the preceding sound exposure of test series I through IV. All 4 exposures were transmitted as prepared sound sources from a DAT-recorder via an amplifier to loudspeakers in a sound-insulated test booth. 2.2 Test Subjects and Audiometric Selection Procedures In a cross-over test design each S was exposed to the 4 TS in a random sequence on different days, thus acting as his or her own control measure. Only persons with normal hearing (according to DIN ISO 4869-1) were used as test subjects. Ten Ss (4 women and 6 men) aged 28 ± 7.3 years were selected based on these criteria. Before each test, the individual resting hearing threshold of each S was determined. This was the basis for subsequent measurements and analyses. Threshold shifts within 2 min after the exposures and then during the whole time of the restitution process until complete recovery to the resting hearing threshold were measured at the frequency where the maximum shifts occurred (regularly at 4 or 6 kHz). Herewith, also the time t(0 dB) it took to restore the resting hearing level was determined.

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3 Results Figure 3 visualizes the results of the 4 test series with detailed information on the physiological responses of all Ss and regression analytical calculations. Figure 4 contains the results in one graph in summarized form. The characteristic values TTS2 reg. and t(0 dB)reg. are shown for TS I through TS IV (in compatible association with the sound exposures in the insert in the middle) at the beginning and the end of the smoothed restitution courses. Immediately after the exposure to White Noise, threshold shifts were roughly 20 dB (TTS2 reg. = 18.8 dB) and subsided over time following the course of a decreasing exponential function. They completely dissipated after at most approximately 100 min (t(0 dB)reg. = 97 min) (see Figure 3A and Figure 4). After the exposure to industrial noise of also 94 dB for 1 h, i.e., a realistic sound exposure, the situation is similar to that which occurs after exposure to White Noise. However, as shown in Figure 3B and Figure 4, the average threshold shifts are somewhat higher (TTS2 reg. = 22 dB) and the restitution time is longer (t(0 dB)reg. = 130 min). The effects on the hearing of TS III with a slightly delayed wearing of the earplugs are quite different from those which result from an exposure to 94 dB(A) for 1 h despite the equal energy. The average threshold shift is only approximately 10 dB, and it completely subsided already after 40 min (cp. Figure 3C and Figure 4). Taking off the hearing protection 3 x 75 s, each, during an exposure to 106 dB (in TS IV) leads to similar results. The average TTS2 value of 8.6 dB is even slightly less than 10 dB. Again, the restitution time (t(0 dB) = 43 min) is fairly short (cp. Figure 3D and Figure 4). In order to allow an overall assessment and a comprehensive statistical analysis, the area under the restitution course TTS(t) as a global characteristic value for the aural effects of sound exposures was calculated. This Integrated Restitution Temporary Threshold Shift IRTTS (computed as the regression function’s integral from 2 min after the exposure to t(0 dB)) is a numeric value for the total threshold shift (in dBmin) which has to be “paid” by the hearing in physiological costs for the exposure. As can be seen in the upper right corner of Figure 4, White Noise and industrial noise as exposures in TS I and TS II with IRTTS values of 424 and 631 dBmin, respectively, cause somewhat differing but fairly high global physiological costs. Both values are several times higher than those of TS III (106 dBmin) and TS IV (98 dBmin). They are only a fraction of those of the energy equivalent exposures in TS I and TS II. Differences between the results of TS I and TS II (White Noise and industrial noise) on the one hand and TS III and TS IV (in both of which hearing protection was worn with only short interruptions during an exposure to industrial noise) on the other hand are statistically significant even when a two-tailed WILCOXON-test is used. Finally, the IRTTS values of industrial noise in TS II, of industrial noise without hearing protection for 1 x 225 s in TS III, and of industrial noise without hearing protection for 3 x 75 s in TS IV were related to the IRTTS value of White Noise. Compared to the reference value of 1.0 in TS I, the values for the three other test series of 1.49, 0.25, and 0.23 differed quite substantially (cp. rear part of Figure 5). This means that industrial noise causes threshold shifts which are approximately 50% higher than White Noise. If hearing protection is temporarily not worn, the physiological costs of energy equivalent noise is substantially lower, with approximately 25% of the reference value. The IRTTS values related to those of TS II (with a quotient of 0.17 and 0.16) are even less than 20% of the reference value for a realistic exposure to industrial noise (cp. front part of Figure 5).

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Figure 3:

Individual and mean restitution time course TTS(t) for all 10 subjects after the exposures “TS I – 94 dB(A) / 1 h White Noise” (A), “TS II – 94 dB(A) / 1 h Industrial Noise” (B), “TS III – 106 dB(A) / 225 s Industrial Noise” (C), and “TS IV – 106 dB(A) / 3 x 75 s Industrial Noise (D). All exposures are equivalent to a rating level LArd = 85 dB(A)

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Figure 4:

Restitution courses TTS(t) with the regression analytical characteristic values TTS2 reg., t(0 dB)reg., and IRTTS of all energy equivalent noise exposures (Significance level for differences: ’’’ – D d 0.001 ’’ – D d 0.01 ’ – D d 0.05)

Figure 5:

“Relative physiological costs” of the energy equivalent exposures based on White Noise (IRTTSTS i / IRTTSTS I) (rear) and Industrial Noise (IRTTSTS i / IRTTSTS II) (front)

4 Discussion and conclusions This study confirmed the earlier results (cp. among others IRLE et al. 1998; 1999) that the principle of energy equivalence or the 3-dB exchange rate misses the reality of aural strain by far. It was empirically shown in this study that short periods without hearing protection in a noise area by no means lead to the immense loss of protection which is prognosticated by the international standard DIN EN 458. Thus, standards cannot prevent folly; that is, blind faith in

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the validity of the principle of energy equivalence for the evaluation of environmental stress seems to lead to standards which reduce the purpose of work safety (cp. Accident Prevention Regulation “Noise” 1990) to absurdity. While it makes sense that removing hearing protection for extended periods of time should be avoided, the statistically significant results in this study show that the equation “energy equivalence = strain equivalence” is not valid. Using this equation is just as unreasonable as claiming the validity of the equation “energy equivalence = annoyance equivalence” for the psychological effects of noise (see STRASSER 1996; STRASSER and HESSE 1993 for more details). Therefore, predictions which were mentioned in the introduction should not be used to exert pressure on employees who are subject to noise. Moreover, since virtually every hearing protection device is uncomfortable to wear, tolerating the employees’ brief removal of the hearing protectors instead of insisting on unreasonable standards may increase the employees’ general willingness to wear the protection. However, personal hearing protection devices still need to be selected very carefully with respect to comfort and “effective” noise reduction at least satisfying the demands of VDI 2560.

5 References IRLE, H.; HESSE J.M. and STRASSER H. (1998) Physiological Cost of Energy-Equivalent Noise Exposures with a Rating Level of 85 dB(A) – Hearing Threshold Shifts Associated with Energetically Negligible Continuous and Impulse Noise. International Journal of Industrial Ergonomics 21, 451-463 IRLE, H.; ROSENTHAL, Ch. and STRASSER H. (1999) Influence of a Reduced Wearing Time on the Attenuation of Hearing Protectors Assessed via Temporary Threshold Shifts. International Journal of Industrial Ergonomics 23, 573-584 MILLER, J.D. (1974) Effects of Noise on People. J. Acoustics Soc. Am. 56 (3) 729-764 N. N. (no year) HOWARD LEIGHT – Hearing Protectors, SNR Values of Personal Hearing Protectors Measured According to EN 352-2 (1993) / ISO 4869-1 (1990) by Inspection Lab. Ltd., University Salford/England. Brochure of Optac GmbH, Rödermark/Germany N.N. (1997) Technical Assessment of Upper Limits on Noise in the Workplace – Final Report. Approved by the International Institute of Noise Control Engineering. Noise/News International, 203-216 STRASSER, H. (1996) Curiosities of Conventional Noise Rating Procedures. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M., and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, 619-626 STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Work Load. Archives of Complex Environmental Studies 5 (1-2) 9-25 Standards, Guidelines, Regulations Accident Prevention Regulation “Noise” (1990) UVV Lärm, Unfallverhütungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Köln DIN EN 458 (1993) Hearing Protectors; Recommendations for Selection, Use, Care and Maintenance; Guidance Document. Beuth Verlag, Berlin DIN ISO 4869-1 (1991) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation. Beuth Verlag, Berlin VDI 2560 (1983) Personal Noise Protection (Guidance Document of the Association of German Engineers). Beuth Verlag, Berlin

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Traditional Rating of Noise Versus Physiological Costs of Sound Exposures to the Hearing H. Strasser (Ed.) IOS Press, 2005 © 2005 The authors. All rights reserved.

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Chapter 17

Dubious Risk Prevention via Traditional Rating of Whole-Body Vibrations, UV Radiation, and Carbon Monoxide J.M. Hesse and H. Strasser

0 Summary Based on specific stress-strain relations, deficits are shown to exist in the energy or dose equivalent rating of mechanical whole-body vibrations, UV radiation, and carbon monoxide exposures. Due to the fact that repair mechanisms are time-dependent, it must be assumed for the upper and lower rating range that effects on the human body cannot be approximated via an equivalency relationship. To summarize the current state of the ergonomic rating of environmental exposures which is of high importance for preventative work safety, the rating of mechanical whole-body vibrations can be seen in a positive light. Substantial work remains to be done in the rating of UV exposures in the workplace. The rating of CO exposures in the workplace can be considered ergonomic if the tolerable exposure time is not exceeded. If, however, a prolonged exposure time in high concentration occurs, the traditional rating implies even risks of a fatal organ concentration.

1 Introduction In general, environmental stress such as mechanical whole-body vibrations, UV radiation, and exposure to carbon monoxide represent an unplanned effect at the workplace with a typically negative impact on individuals. Their effect can be undesirable, annoying, performance-reducing, and possibly detrimental to health. In the sense of future-oriented management, the ergonomic evaluation of environmental factors is of special relevance. The minimum requirement is the prevention of negative effects on the health of individuals in accordance with laws, rules, and regulations. In principle, each evaluation and final rating of work or environmental conditions requires detailed knowledge of the connection between the initial stress, i.e., the exposure, the resulting strain, and the – possibly irreversible – long-term effects of strain. Then, and only then, can the ultimate goal “health” be evaluated by comparing the physical stress (which can be determined relatively easily) to certain threshold values which are based on relevant knowledge and assumptions. For this connection, the traditional approach uses the energydamage-equivalence hypothesis for the rating of exposures by mechanical whole-body vibrations, UV radiation, and carbon monoxide. This hypothesis claims that, in the long run, the same amount of energy causes the same amount of damage once a certain threshold has been exceeded. Differences in the structural timing of the exposures are not considered to be relevant with respect to the damage caused. It has been mentioned frequently that such a strategy is not appropriate for the evaluation of the real effects on a human being and that it cannot be called “ergonomic” (cp. STRASSER and HESSE 1988; STRASSER 1990; HESSE and STRASSER 1991; ECKERT et al. 1993; STRASSER and HESSE 1993). This will once again be demonstrated in the following review.

2 Evaluation and rating of whole-body vibrations and problem areas from an ergonomics point of view The evaluation and rating of whole-body vibrations which affect a human body is traditionally based on the VDI guideline 2057 (1987) (cp. Figure 1).

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Figure 1:

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Evaluation and rating of whole-body vibrations according to VDI guideline 2057 (1987) and associated exposure and strain parameters

The amplitude of the vibration signal (in this case, the acceleration signal) in an electronic network is weighted differently, dependent on the normalized, frequency-dependent human reactions. Due to the frequency weighting, a transformation of measuring data into effectrelevant stress data takes place. The successive normalization serves the purpose of obtaining a dimension-less measuring value which is independent of the measurement type and which is approximately comparable with respect to the effect. This measuring value corresponds to the level of exposure, evaluated or weighted by the general human characteristics or properties. The following floating effective value determination over the time constant W (for whole-body vibrations: W = 125 ms) yields the so-called “evaluated vibration intensity K” which is comparable to the instantaneous sound pressure level in dB(A) in noise measurements. After another effective value determination over the exposure time Te, the “energy equivalent average Keq” results as the individual vibration exposure which is comparable to the sound pressure level LAeq. This Keq-value is the basis for the determination of the individual-specific “rated vibration intensity Kr” which is analogous to the rating level. The left part of Figure 2 shows in a simplified manner, however, that the rated vibration intensity Kr is equivalent to the stress which has been averaged or evenly spread out over the workday, i.e., the energy dose. The right part of Figure 2 clearly depicts that any information about the structure of the exposure which might be of importance for the effect of the vibration is lost in this single value Kr. It must be noted that the Keq-values are already averaged values themselves, and that the “filling up” of vibration-free work sections with low vibration stress is hardly represented at all in the rated vibration intensity Kr. One example is the comparison between the values Kr = 12.5 resulting from Keq = 50 for 30 min (in the left part of Figure 2) and Kr = 15 resulting from additional substantial Keq-values (in the right part of Figure 2). The rating of the criterion “health” is carried out by comparing the observed energy to immission guidance values which were determined in a laboratory setting and were deemed to be still tolerable. The recommended stress threshold is approximately half of the level which previously – based on a relatively small sample of healthy pilots – was determined to be the pain threshold for exposure to harmonic vibrations. Based on this method, the maximum permissible Keq-value is 112 for 10 minutes. The standard value curve of 1979 starting at Kr = 112 / 1 min was (for reasons of mathematical practicability) replaced by a straight line in the 1987 version (cp. Figure 3).

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Figure 2:

Two examples for the calculation of the rated vibration intensity Kr out of Keq-values and correspondent exposure times

Figure 3:

Standard value curves for the rating criterion “Health” according to VDI 2057 draft (1979) and VDI 2057 (1987)

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As a result, individuals may be exposed to the maximum Keq-value for up to 10 minutes. The rated vibration intensity (for 8 hours) was increased from 12.5 to 16.2. The possibility of an energy equivalent trade-off between Keq-values and exposure times is represented by the straightening of the curve. The method for the evaluation and rating presented so far can supposedly be applied to periodic, stochastic, and impulsive vibrations. In real life, however, only the vibration direction with the largest evaluated vibration intensity is taken into consideration. It has been found, however, e.g., in drivers of large construction machines, that vibration portions in transversal direction are of approximately the same magnitude. A therefore necessary vectorial addition of K-values to a total value shows that the maximum permissible value of K = 112 for one-dimensional vibration exposures is reached at K-values of 79 and 65, respectively, if instead vibration exposures are two- or three-dimensional (cp. Figure 4).

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Figure 4:

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Consideration of various vibration directions with equal K-value based on a maximum permissible value of 112 for 10 minutes (source: HESSE and KLUTH 1992)

Coming back to the rating according to VDI, the conversion of individual exposures with exposure times of less than 10 minutes and substantially higher K-values than 112 is considered to be dangerous (cp. Figure 3). The presented standard value curve is based on permissible exposure values in the most sensitive frequency range, respectively (e.g., 4 - 8 Hz for vertical vibration immissions). The main resonance range between 4 and 8 Hz can be clearly seen from the reduced acceleration values of vibrations in this range when curves of equal “evaluated vibration intensity” are shown (cp. Figure 5). The curves range from the perception threshold (K = 0.1) to the threshold of health risks (K = 112). For the rated vibration intensity Kr, however, only the values in the upper third (from K = 16.2 to K = 112) are relevant.

Figure 5:

Curves with equal evaluated vibration intensity (K-values) for vibrations in z-direction according to VDI guideline 2057 (1987)

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Compared to the also possible rating method according to ISO 2631-1 (1985), the rating can – dependent on the frequency composition – be more stringent. According to the current state of scientific knowledge, an increased risk can be expected with coordinates for vibration exposures in excess of the standard value curve (cp. Figure 6).

Figure 6:

Area of health risks from whole-body vibrations (according to ISO 2631-1 (1985), VDI 2057 (1987), and DUPUIS 1993)

The straightening of the curve has the desirable feature that it allows a clear-cut decision whether an exposure is dangerous or not. Such a “jackknife” decision process, however, is not adequate from an ergonomics point of view, a fact of which the decision-makers – who sometimes lack scientific training – are often not aware. It must be noted with respect to the straightening of the curve that, with respect to human issues, it has led to worsening rather than an improvement towards the ends of the rating continuum. Due to the fact that repair mechanisms are time-dependent, it must be assumed – especially for the rating levels towards the ends – that the effects on the human body cannot be approximated via the equivalency relation. The potentially damaging effects should be based on the actual exposure rather than a daily average (which has not actually been experienced). This becomes especially clear with exposure to impulse vibration which has been shown to increase the risk of health damage in drivers of work machines in epidemiological studies. Fortunately, there have been attempts to take that fact into consideration via an over-energetic evaluation on the basis of a vibration rating level Kr = 12.5 (cp. the lower standard value curve in Figure 6, or, in more detail, DUPUIS 1993). The stricter evaluation of impulse vibration is also reflected in the determination of professional requirements for the new Occupational Disease 2110 “Damage to the lumbar intervertebral discs due to long-term, mainly vertical whole-body vibrations in a sitting position.” It also needs to be applied to unfavorable body postures (e.g., twisted, bent, or tilted torso posture). Despite this positive trend towards a more ergonomic rating – which, by the way, includes the differentiation between standing and sitting (cp. SCHNAUBER and TREIER 1994) – there remain several deficits such as: • the still missing simultaneous consideration of different vibration directions, • the determination of the impact of rotatory vibration components, • the determination of the importance of gender differences, and • the lacking of an agreement on the effects of intermittent breaks.

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After these considerations concerning mechanical whole-body vibrations, in the following chapter it will be roughly outlined to what extent individuals’ exposure to ultraviolet (UV) radiation at the workplace has been evaluated and rated traditionally and where deficits seem to exist from an ergonomics point of view.

3 Evaluation and rating of UV radiation and unanswered questions from an ergonomics point of view UV radiation and associated health risks have received growing attention among the general public. The reason is the increasing damage to the protective ozone layer due to human-made air pollution and the resulting exposure from outdoor activities. A further increase in UV radiation from the sun and a progression in skin damage up to and including tumors in the next few decades can be expected (possibly not yet in Germany, but certainly in countries such as Australia). Compared to this increased awareness in the collective consciousness, work-related exposure to UV radiation has yet to receive much more attention. This is surprising since exposure to UV radiation occurs in a variety of industrial, medical, and cosmetic settings. Figure 7 lists a few examples in which an exposure to UV radiation of employees must be assumed. Some jobs involve the direct use of UV radiators (examples include the bacterial disinfecting of tools and appliances, the disinfecting of rooms, the drying of paint, the hardening of varnishes and plastic, the material testing, the registration of markings, the production of vitamin D as well as medical/therapeutic and cosmetic applications). In addition to the explicit use of UV radiation, arc welding and cutting methods lead to an accidental residual radiation which also affects individuals at a distance, either directly or indirectly via reflections. Additionally, all outdoor workers are exposed to the sun’s natural UV rays to varying degrees.

Figure 7:

Examples of exposure to UV radiation at the workplace

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Just like visible light and infrared radiation, ultraviolet radiation is electromagnetic radiation with a wavelength in the range from 100 and 400 nm (cp. Figure 8). Based on known dermatological causal relationships from exposure to the sun, electromagnetic radiation is classified into several spectral ranges.

Figure 8:

Classification of electromagnetic radiation into typical spectral ranges

While UV-A1 radiation (between 400 and 340 nm) is unlikely to damage connective tissue, chronic UV-A2 exposures (between 340 and 320 nm) can lead to erythema. Intense erythemic (skin-reddening) and, long-term, even carcinogenic effects in the skin are caused by UV-B radiation components between 320 and 290 nm. The long-term effects of short-wave UV-B radiation (below 290 nm) or UV-C radiation (between 280 and 200 nm) on the human body are largely unknown (cp. BERGNER and PRZYBILLA 1990a; 1990b; WISKEMANN 1988). So far, the earth’s surface has not been exposed to these two radiation components because of the intact ozone layer’s filtering effect. Vacuum-UV with wavelengths between 100 and 200 nm is almost completely absorbed in the air and does not pose a risk at the workplace. Only with the development of artificial UV sources did humans get exposed to short-wave UV-B and UV-C radiation. DIN 5031-7 (1984) provides a division into 3 biology-based ranges which is valid for artificial UV sources (cp. Figure 9).

Figure 9:

Classification of UV radiation according to DIN 5031-7 (1984)

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According to that standard, the spectral range between 380 and 315 nm is classified as UVA radiation which is responsible for the tanning of the skin. UV radiation with wave lengths between 315 and 280 nm is called UV-B radiation with skin-reddening effects. Emissions in the range from 280 - 100 nm are called UV-C radiation and have germ- and bacteriadestroying properties. This classification scheme has been criticized repeatedly from a dermatology point of view. Since emissions between 315 and 319 nm still clearly have skin-reddening (and, longterm, probably even carcinogenic) effects, this range is considered internationally to be UV-B radiation. According to the German standard, however, a UV source with the mentioned spectral components can still be classified as UV-A radiator and can be used, e.g., in tanning salons. Without going into biological or phototoxic connections of the effects of radiation (cp. ECKERT et al. 1993 for more information), UV radiation can lead to skin damage as well as damage to the eyes. In this context, it must be distinguished between acute and chronic effects. Examples for acute damages are the skin erythema and the inflammation of the eye’s cornea or connective tissue (photo keratitis and photo conjunctivitis). These damages to the eye in welders are known as “eye-flash.” Effects due to chronic exposure to UV radiation include premature skin photo-aging (“sailor’s skin”), changes in the skin’s pigmentation (e.g., aging pigments), and the forming of skin tumors. It is not known yet to what extent the eye’s lens can be clouded from long-term UV exposure. Whether damage occurs or not mainly depends on the radiation’s dose and time course. Binding dose thresholds to avoid chronic effects from UV radiation in the workplace exist neither in Germany nor elsewhere. For the avoidance of acute UV damage in the workplace, dose thresholds of the American Conference of Governmental Industrial Hygienists (ACGIH) for UV exposure are used. They were adopted by the German main association of employer’s liability insurance associations (Hauptverband der gewerblichen Berufsgenossenschaften) without changes. The table in Figure 10 shows the maximum permissible radiation doses during a work shift. The effects of radiation depend heavily on the wavelength which is expressed via the relative effectiveness (in the right column). This factor shows the reduced sensitivity – relative to the maximum UV effectiveness at 270 nm – in dependence of the wavelength. The spectral evaluation is also represented in the graph of the relative effectiveness plotted against the wavelength (in the right part of the figure) which is based on UV effects on the human organism. Wavelengths between 200 and 300 nm can be expected to cause photo keratitis whereas wavelengths between 300 and 320 nm can be expected to have skin-reddening effects in the human skin. Wavelengths in excess of 320 nm lead to immediate tanning of the skin. It is assumed that radiation with wavelengths in excess of 320 nm do not cause acute damage. Therefore, the thresholds for this UV-A range are several times higher than those for exposure to UV-B or UV-C radiation, respectively. Individual characteristics such as increased photo sensitivity are not considered. The radiation dose H is assumed to be the decisive variable for the photo-biological effect of UV radiation (BUNSEN-ROSCOE’S Law). Analogous to other kinds of exposure, the dose of the radiation can be obtained by multiplying the intensity with the duration of the exposure. According to Figure 11 equal radiation doses are assumed to have equal effects. Based on the minimum threshold dose for light-related inflammations of the eye of 3 mJ / cm2, the maximum permissible effective (i.e., weighted) radiation intensity Eeff can be determined with the help of the presented mathematical relationships in dependence on the exposure duration.

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Figure 10: Thresholds for UV radiation dose during one shift (left) and spectral weighting function of UV-B and UV-C radiation (right) (according to American Conference of Governmental Industrial Hygienists (ACGIH) 1984/85)

Figure 11: Energy equivalent rating of UV radiation based on the maximum permissible effective radiation dose of Heff, perm. = 3 mJ / cm2 or the effective radiation intensity of Eeff = 0.1 PW / cm2 for 8 h

With an exposure time of 1 s, for example, the permissible radiation dose of 3 mJ / cm2 corresponds with a permissible radiation intensity of 3,000 PW / cm2. Due to the multiplication of radiation intensity and exposure duration, higher effective radiation intensities are permissible with shorter exposures. For the rating of UV exposures of less than 0.1 s (i.e., less than a tenth of a second), the method must not be applied. Also, it is not meant for the rating of deliberate therapeutic and cosmetic radiations.

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Judging from the current state of knowledge, there is no foundation for the purely physically grounded equalization of the presented energy or dose equivalent UV exposures with equal effects on the human body. It must be noted that the assumed adaptation of the organs “skin” and “eye” to the type and intensity of physical radiation is at least questionable due to the limited physiological possibilities to react and the limited defense mechanisms. For the upper area of the exposure continuum, there is the latent danger of burns of the skin due to extremely high radiation intensities. It has been shown in tests on animals that radiation at lower intensities for longer time spans has a stronger carcinogenic effect than radiation of equal UV dose at higher intensities for shorter time spans. However, the simplistic mutual exchange of exposure intensity and duration is hardly appropriate for this problem. Also, it must be assumed that long-term (chronic) UV exposure can already be health-damaging at substantially lower doses. The presented threshold doses may only be used for the rating of acute effects of unintentional UV exposure. Since no threshold doses for chronic effects exist, any repetitive long-term UV exposure should be avoided, even if the UV doses are quite small. It is not always possible to ensure compliance with existing threshold doses for unintentional UV radiation in the workplace. Furthermore, excessive UV exposures were found, for example, when UV-emitting tools had design flaws, were handled improperly, or were defective. Therefore, routine checks and appropriate training along with primary, secondary, and tertiary protective measures, i.e., activities in the spirit of preventative work safety, must gain in importance in the future. The issue of rating UV exposures leads to a number of questions which – from an ergonomics point of view – must be addressed: • Does the emission spectrum of artificial UV sources contain ionizing radiation? • Why does the relative effectiveness according to ACGIH decrease extremely from 270 nm to shorter wavelengths even though there is evidence that this kind of radiation exhibits increasing ionization ability? Is only the sun’s effective radiation taken into consideration? • Is pure UV-A radiation completely without risk in its biological effectiveness? • Who can guarantee under which conditions an 8-hour UV exposure is risk-free? • With carcinogenic effects in mind, is it justifiable to separate possible UV exposures in the workplace from those encountered during leisure time? • Can short-wave UV radiation of low dose be neglected with respect to its ionization ability and the resulting mutation-causing effects? • Does the variety of stress and tasks (multiple stress and general environmental exposures) put too much strain on the body’s own repair mechanisms? • Can the rating sequentially focus on individual stress factors which occur simultaneously in real life? • Does the organism respond in an energy equivalent way to different exposure times of immissions of equal dose? As long as there is no secured scientific knowledge about the dose-dependence of acute and long-term damaging UV effects, an overly cautious approach must be preferred to a lack of acknowledgment of a potential problem. Ultraviolet radiation in the workplace should not be dealt with in the form of threshold values. Instead, it should be pointed out – similar to the German technical exposure limits for hazardous substances (Technische Richtkonzentrationen für gefährliche Stoffe (TRK)) – that even adherence to published values cannot guarantee that there will be no hazardous effects to the health of exposed individuals (cp. ECKERT et al. 1993).

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While the discussion so far focused on the energy/damage equivalent rating of mechanical stress (such as noise, whole-body vibrations, and UV radiation), the next chapter examines the rating of the inhalation of the toxin “carbon monoxide.”

4 Rating of carbon monoxide Carbon monoxide (CO) is a toxic, colorless, odorless, and tasteless gas which cannot be detected by a human’s senses. Because carbon monoxide results from the incomplete burning of carbon, the human organism is exposed to it in many situations in the workplace as well as in everyday life (cp. Figure 12).

Figure 12: Examples of work areas with potential exposure of humans to CO

Combustion engines of motor vehicles are the most frequent source of carbon monoxide, thus affecting employees in car repair shops or vehicle inspection sites as well as personnel and users of parking garages or loading decks on ships. Intersections with heavy traffic and tunnels sometimes exhibit concentrations of up to 300 ppm (parts per million) CO, i.e., 300 ml / m3 or 0.03 percent by volume. Exposure to carbon monoxide is also experienced in workplaces in the proximity of industrial furnaces. A routine exposure in the military sector is the breathing of firearm exhaust gases. For example, extremely high concentrations can occur inside a battle tank with short-term peaks of 10,000 to 50,000 ppm (i.e., ml / m3). Carbon monoxide poses a threat because it hinders the oxygen transportation in blood. It exhibits an affinity to attach to the blood’s hemoglobin, which is 240 times higher than that of oxygen (with respect to a normal oxygen concentration in the alveolar air of 15%), resulting in carbon monoxide hemoglobin (COHb). The share of carbon monoxide which is passed on via diffusion from the alveoli into the blood is, on average, approximately 50 % of the amount present in the alveoli, but it can be as high as 70 % (mainly if the gradient is high). The level of COHb in each particular case is influenced substantially by several intervening variables (cp. Figure 13). It depends, e.g., on the ventilation of the alveoli, the pre-load, and substantial kinetic transformation processes (cp. STRASSER 1992). The only physiologically relevant factor for the organism and thus for a human-oriented rating is the blood COHb level, not the dose (i.e., the product of C x T). In addition to the already mentioned reduced oxygen transport capacity, with COHb-carrying blood – put somewhat simplified – the remaining oxygen cannot be utilized as usual. That is, the effects of insufficient supply of tissue with oxygen (hypoxia) due to CO poisoning are more detrimental than a hemoglobin reduction due to anemia. The right side of Figure 13 shows, in simplified form, the effects of certain COHb concentrations on the organism. They range from headaches at COHb concentrations of 5 - 10 % to acute danger to life at COHb values in excess of 50 %. Semi-saturation of hemoglobin (i.e., 50 % COHb) can be caused by prolonged exposure to a CO concentration of 600 ppm in the inspired air. There are different models for the calculation and approximation of the blood COHb level based on the CO concentration in the inspired air.

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Figure 13: Causal relationship between CO exposure and blood COHb level (source: STRASSER 1989)

In order to avoid health risks from the inhalation of toxins, the German Ordinance on Hazardous Substances (Gefahrstoffverordnung) specifies maximum permissible workplace concentrations. The relevant 8-hour average for carbon monoxide is currently 30 ppm. Based on the assumption of no more than 4 short-term exposures during one shift, the permissible 30-minute average is a CO concentration of 60 ppm. The determination of a threshold ensures that the German Biological Tolerance Value for Occupational Exposures (Biologischer-Arbeitsstoff-Toleranzwert (BAT-value)) of 5 % COHb (for nonsmokers) is not exceeded. However, it is not indicated anywhere on which ventilation (for medium physical stress or during resting) the determination is based. Based on the permissible 30-minute short-term average of 60 ppm, Figure 14 shows the equivalent short-term CO exposures according to the German Ordinance on Hazardous Substances (Gefahrstoffverordnung) (bottom) and the respective values for the military sector (top). The latter are approximately three times as high than the former. In the military sector, a ventilation of 40 - 60 l / min is assumed which corresponds with heavy physical work. The respective values may only be experienced once a day. In the military sector, a COHb concentration of 10 % is tolerated. A mathematical model was developed for the determination of the rating-relevant blood COHb level (cp. Figure 15) which – contrary to the frequently used nomogram according to FORBES et al. (1945) – allows an approximation of the expected COHb saturation from a short-term CO exposure. This new method takes essential values of the previously mentioned intervening variables (such as tidal volume, alveolar volume fraction, breathing frequency, and oxygen consumption) into consideration, and the instantaneous blood COHb concentration after each respiration cycle is determined.

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Figure 14: Permissible short-term CO exposures in the military sector according to BMVG-In San 4 (1972) (top) and according to the German Ordinance on Hazardous Substances (Gefahrstoffverordnung) (bottom)

Figure 15: Mathematical method for the determination of COHb values from short-term exposure (according to ECKERT et al. 1991)

As can be seen in Figure 16, the application of this method to three equivalent short-term exposures of 60 ppm for 30 minutes, 1,800 ppm for 1 minute, and 9,000 ppm for 12 seconds results in equal values for the blood COHb saturation of 0.55 % (assuming resting ventilation, i.e., ventilation of approximately 7.5 l / min).

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Figure 16: COHb dissociation curve for dose equivalent short-term exposures according to the German Ordinance on Hazardous Substances (top) and COHb dissociation curve for short-term exposure for different ventilation (bottom) (according to ECKERT et al. 1989)

The lower part of the figure shows the influence of an increased ventilation of 60 l / min on the blood COHb level for the short-term exposure of 9,000 ppm for 12 seconds. Heavy physical work results in a BAT-value of 5 % COHb which is approximately 10 times as high as for resting ventilation. It must be concluded that the dose-equivalent conversion of carbon monoxide exposures results in equal effects on the human body only if the respiratory conditions are equal as well. In this sense, even the highest concentrations pose no health risk if the permissible exposure time is not exceeded. Failure to adhere to the permissible exposure time, however, can lead to a fatal organ concentration within very short periods of time, often just a few minutes. Repeated exposures, of course, can lead to higher organ concentrations even though the half-life of CO in the human body decreases from the initial value of approximately 4 h with increasing physical activity or intensified ventilation (mainly CO2 inhalation). With respect to the equivalency dose of 6,000 ppm for 1 minute which is tolerated in the military sector, COHb values of approximately 18 % were determined with a ventilation of 60 l / min. Even though the previously computed COHb concentrations may be slightly exaggerated due to elimination processes not included in the model, it must be kept in mind for the rating that the permissible thresholds in both the military and non-military sector assume healthy individuals without any pre-load. With this qualification and if the tolerable toxin concentration and exposure time are not exceeded, the rating of carbon monoxide exposures, at least in the workplace, can be called

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ergonomic. In comparison to the more mechanical environmental exposures, the typically higher potential for damage of high intensities has less of a negative effect because of the “buffering” properties of the trachea tract and the lungs. However, it must be stressed once more that exceeding the permissible CO exposure time can very quickly have devastating, i.e., fatal, effects. To summarize the current state of the ergonomic evaluation of environmental exposures which is of high importance for preventative work safety, the rating of mechanical wholebody vibrations can be seen in a positive light. The rating of UV exposures in the workplace requires additional research on the effects on man. The alleged improvement in the rating of impulse noise via LAeq , however, seems to have been a step in the wrong direction.

5 References BERGNER, T. und PRZYBILLA, B. (1990a) Kosmetische Bräunung in Solarien. Münchener medizinische Wochenschrift 132 (5) 43-47 BERGNER, T. und PRZYBILLA, B. (1990b) UV-Expositionen am Arbeitsplatz. Hautarzt 41, Springer Verlag, Berlin, 523-526 DUPUIS, H. (1993) Erkrankungen durch Ganzkörper-Schwingungen. Kap. IV - 3.5. In: KONIETZKO, J. und DUPUIS H. (Hrsg.) Handbuch der Arbeitsmedizin. ecomed-Fachverlag, Landsberg/Lech, 9. Erg. Lfg. 4/1993, 1-24 ECKERT, J.; HESSE, J.M. und STRASSER H.(1991) Kohlenmonoxyd-Expositionen am Arbeitsplatz – Physiologische Grundlagen, Wirkungen und Beurteilungskonzepte. 50 Seiten. 3. Zwischenbericht zum Forschungsvorhaben “Intensitäts-Expositionszeit-Äquivalente von Umgebungsbelastungen”, Fachgebiet Arbeitswissenschaft/Ergonomie der Universität Siegen ECKERT, J.; HESSE, J.M. und STRASSER, H. (1993) UV-Expositionen am Arbeitsplatz, Dosis-WirkungsBeziehungen und Schutzmaßnahmen aus ergonomischer Sicht. Zbl. Arbeitsmed. 43 (3) 78-93 FORBES, W.H.; SARGENT, F. and ROUGHTON F.J.W. (1945) The Rate of Carbon Monoxide Uptake by Normal Men. American Journal of Physiology 143, 594-608 HESSE, J.M. und KLUTH, K. (1992) Ergonomische Überlegungen zur Bewertung und Beurteilung von GanzKörper-Schwingungen. Z. Arb.wiss. 46 (18 NF) 2, 100-105 HESSE, J.M. and STRASSER, H. (1991) Assessment and Rating of Whole-Body Vibration for Occupational Risk Prevention from an Ergonomics Point of View. In: QUÉINNEC, Y. and DANIELLOU, F. (Eds.) Designing for Everyone. Vol. 2. Proceedings of the 11th Congress of the International Ergonomics Association. Taylor & Francis, London/New York/Philadelphia, 1010-1012 N.N. (1993) Merkblätter des BMA zur Anlage 1 der BeKV – Bandscheibenbedingte Erkrankungen der Lendenwirbelsäule. Merkblatt für die ärztliche Untersuchung zu Nr. 2110 (BArbBl. 2 1993, S. 55). Arbeitsmed. – Sozialmed. – Umweltmed. 28, 242-245 SCHNAUBER, H. and TREIER, C. (1994) The Human Response to Vertical Vibration in Standing Posture. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor & Francis, London, 247-252 STRASSER, H. and HESSE, J.M. (1988) Guidelines for Occupational Health Protection and Safety Under Environmental Stress from an Ergonomics Point of View. Proceedings of the International Conference on Ergonomics, Occupational Safety and Health and the Environment, Beijing/China, 24-28 October 1988, 395-405 STRASSER, H. (1989) Zur Problematik der CO-Belastung und ihrer Beurteilung. Tischvorlage zum Workshop “Energie-Äquivalenz von Umgebungsbelastungen aus ergonomischer Sicht” am 26.09.1989 an der Universität Siegen STRASSER, H. (1990) Ergonomische Überlegungen zur Dosismaxime bzw. zur Energieäquivalenz bei Umgebungsbelastungen. Zbl. Arbeitsmed. 40 (11) 338-354 STRASSER, H. (1992) Luftzusammensetzung und Druckänderungen – Anpaßbarkeit und Grenzen respiratorischer Mechanismen. Z. Arb.wiss. 46 (18NF) 1, 8-17

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STRASSER, H. and HESSE, J.M. (1993) The Equal Energy Hypothesis Versus Physiological Cost of Environmental Workload. Archives of Complex Environmental Studies. ACES 5 (1-2) 9-25 WISKEMANN, A. (1988) Langzeitwirkungen optischer Strahlung auf die Haut. Aktuelle Dermatologie 14, 320-322 Standards, Guidelines, Regulations BMVG-In San I 4 (1972) CO-Belastung von Besatzungsmitgliedern im Panzer. Bundesministerium der Verteidigung DIN 5031-7 (1984) Optical Radiation Physics and Illumination Engineering; Terms for Wavebands. Beuth Verlag, Berlin Ordinance on Hazardous Substances (1986) Verordnung über gefährliche Stoffe (Gefahrstoffverordnung – GefStoffV) BGBl. I, Nr. 47. Bundesanzeiger Verlagsgesellschaft mbH ISO 2631-1 (1985) Evaluation of Human Exposure to Whole-Body Vibration – Part 1: General Requirements. Beuth Verlag, Berlin VDI 2057-1 (1987) Effect of Mechanical Vibrations on Human Beings; Fundamentals, Classification, Terms. Beuth Verlag, Berlin

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Author Index Fidan, H. 181 Hesse, J.M. 25, 43, 81, 93, 181, 211 Irle, H. 43, 53, 67, 81, 93, 105, 115, 127, 137, 149, 163, 181, 191, 203 Körner, F. 149 Legler, R. 137 Linke, S. 67 Rosenthal, Ch. 191 Rottschäfer, M. 43 Scholz, R. 127 Siebel, T. 203 Strasser, H. 1, 43, 53, 67, 81, 93, 105, 115, 127, 137, 149, 163, 181, 191, 203, 211

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