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

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 formulas – 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.

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 energy-equivalence 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 scientific-critical 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 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

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.

Real acoustic exposures are presented on the basis of a classification of impulse noise using different sound pressure time patterns/courses. The collection of examples is supplemented by the presentation and analysis of impulse noise measurements during the use of bolt setting guns under various operation and set-up conditions as well as during the practice use of guns. Additionally, results of studies which were carried out in cooperation with an acoustics measurement division of the German military are presented. Immission levels from impulse noise caused by the repeated use of a typical weapon were measured on a sample of volunteers both outside and inside the ear muffs. The resulting rating of the noise immission leads to the determination of the maximum number of permissible impulse events which vary substantially dependent on the rating method. When compared with the rating based on the equal energy concept, the number of permissible impulse events must be reduced by a factor of 30 when the impulse measurement method is used. Relative to the standardized measurement according to DIN 45635-34 for the noise emission of bolt setting tools, the number of driving processes during the real-life use of these tools should be reduced by a factor of 100, for example, under various operation and set-up conditions of the real.

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.

In order to determine the physiological responses associated with sound exposures or the physiological costs the human hearing has to pay for noise, a comprehensive, reliable method was developed which uses pure-tone audiometry to measure and analyze via regression analysis hearing threshold shifts and their restitution. In audiometry, how loud a sound can be perceived – especially with high measurement frequencies in excess of 4 kHz – depends greatly on the positioning of the headphones over the ear canal. Minor deviations of the headphones from the optimal position already lead to changes in the level of several decibels. Thus, precise measurement of threshold shifts and their restitution requires exact and repeatable positioning over the ear canal. To that end, the test subjects are offered a high-frequency 6-kHz tone approximately 30 dB above their individual hearing threshold. They were instructed to place the headphones over the ear canal in such a manner that they could subjectively hear the tone the loudest. Before each test series, the test subjects' initial hearing threshold has to be determined at several frequency measuring points. During the actual tests, the frequency of the maximum threshold shift is determined within 2 minutes after the end of the exposure (TTS₂). At the frequency of the maximum threshold shift, the remaining threshold shift has to be measured at reasonably spaced time intervals until the initial hearing threshold shift is reached again (i.e., the time t(0 dB)). The so-obtained real values TTS_{2 real} and t(0 dB)_real as isolated measurement values are not sufficient, however, to make well-founded statements about the entirety of audiometric data. Since the restitution of hearing threshold shifts is approximately log-linear with respect to time, the measurement values can be analyzed statistically using linear regression techniques. The determination of the values TTS₂, t(0 dB), and the regression function TTS(t) which are obtained from the regression analysis makes it possible to determine an integral value which as the area under the function TTS(t) allows statements about the risk to the hearing. In order to achieve correct results, the point in time when the initial hearing threshold is reached once again needs to be weighted. The characteristic value IRTTS (Integrated Restitution Temporary Threshold Shift) shows the physiological costs of sound exposures as a numeric value (measured in dBmin). If it is related to reference values for permissible sound exposures of 85 dB(A) / 8 h or 94 dB(A) / 1 h, it can express the relative risk of the studied sound exposures (the so-called risk factor). The real characteristic values of the measurements as well as the regression results have to be tested for statistical significance. Since measurements of the restitution of hearing threshold shifts in principle do not exhibit a normal distribution, a non-parametric test, e.g., the WILCOXON test, has to be applied.

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., ¼, 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.

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.

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 L_Ard 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 L_Ard by merely 0.1 dB to 85.1 dB(A). In all three test series, the noise-induced 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 ≥ 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 ≥ 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 %.

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.

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 L_Ard 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 (L_Ard 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 inter-individual 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.

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.

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 2½ 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.

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 ¼ 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.

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.

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.

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 noise-insulating 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 3¾ 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.

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 3¾ 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 3¾ 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.

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.