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| Year : 2003
| Volume
: 5 | Issue : 20 | Page
: 75-84 |
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| Temporary hearing threshold shifts and restitution after energy-equivalent exposures to industrial noise and classical music |
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H Strasser, H Irle, R Legler
Ergonomics Division, University of Siegen, Siegen, Germany
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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 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 TTS 2 , 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 energyequivalence can lead to gravely misleading assessments of their actual physiological costs. Keywords: energy-equivalent exposures, industrial noise, classical music, TTS (Temporary Threshold Shift), restitution, IRTTS (Integrated Restitution Temporary Threshold Shift), physiological cost
How to cite this article:
Strasser H, Irle H, Legler R. Temporary hearing threshold shifts and restitution after energy-equivalent exposures to industrial noise and classical music. Noise Health 2003;5:75-84 |
How to cite this URL:
Strasser H, Irle H, Legler R. Temporary hearing threshold shifts and restitution after energy-equivalent exposures to industrial noise and classical music. Noise Health [serial online] 2003 [cited 2023 Dec 6];5:75-84. Available from: https://www.noiseandhealth.org/text.asp?2003/5/20/75/31686 |
| Introduction | |  |
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 TTS 2 -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 TTS 2 -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, i.e., 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 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 metalmusic. 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 the left part of [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.
Based on some measurements in the 1960s, Miller (1974) developed a diagram (cp. right part of [Figure - 1]) which makes it possible to hypothetically estimate the TTS 2 -values from levels (between 70 and 120 dB) after exposures of different duration. According to such calculations, the threshold shifts TTS 2 measured at the testing frequency of 4 kHz, indeed, seem to be similar (~ 30-35 dB) for the abovementioned situations of 85 dB / 8 h, 91 dB / 2 h, or 94 dB / 1 h. For even higher levels (with reduced exposure times in accordance with energy equivalence), decreasing TTS 2 -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, TTS 2 -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. N.N., 1990 or N.N., 1998), is not justifiable from a work-physiological 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 2000 impulses of 5 ms each (i.e., a total duration of 2000 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 L r = 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.
| Methods | | |
Test Design and Test Set-Up
The left part of [Figure - 2] 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.
As shown in the right part of [Figure - 2], 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, i.e., 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, i.e., 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 Haendel'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 8000 Hz.
As can be seen in the back right part of [Figure - 2], the hypothesis to be tested was that different threshold shifts would be observed during the test series. These threshold shifts should be measurable in TTS 2 -values immediately after the sound exposure. Also, the restitution time t(0 dB), i.e., 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. Since interindividual 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.
Test Subjects and Audiometric Selection Procedures
Only persons with normal hearing were used as Ss. According to the standards in DIN ISO 48691 (1990), 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 TTS 2 had to be determined via several measurements within the first 2 minutes (cp. left part of [Figure - 3]).
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 - 3]). 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 - 3]), the regression function TTS(t) is a straight line.
| Results | | |
The results of TS I, i.e., after industrial noise at a level of 94 dB(A) for 1 h which was used as a reference, are shown in the upper part of [Figure - 4]. 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 - 3]. 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 mathematicalnumerical 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 TTS 2 and t(0 dB) were determined. In addition to the regression-analytically determined maximum hearing threshold 2 min after the noise exposure (TTS 2 reg. ) and the time t(0 dB) 2 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 TTS 2real and t(0 dB) real averaged over the 10 Ss, are shown.
As can be seen in the upper part of [Figure - 4], systematic threshold shifts with individual variations occurred immediately after the exposure; the mean of the threshold shifts was somewhat above 20 dB (e.g., TTS 2 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 2½ h (e.g., t(0 dB)reg. = 154 min). The middle part of [Figure - 4] 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 energyequivalent exposure), almost identical physiological responses were measured. The regression-analytically determined TTS 2 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, i.e., the t(0 dB)value amounts to 153 min. Finally, the lower part of [Figure - 4] shows the effects of an energyequivalent 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 ½ h (t(0 dB) reg. = 37 min) for these threshold shifts to completely subside.
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 TTS (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 - 5] in summarized form shows all the physiological responses to the 3 energyequivalent exposures. The regression analytical characteristic values TTS 2 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, i.e., 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 TTS 2 - 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.
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 regressionanalytically determined TTS 2 , t(0 dB), and IRTTS-values. The results of the test series TS I and TS II did not show any differences (fl) in all three parameters, therefore, absolutely, no significant differences do exist.
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 - 5]). 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.
| Discussion | | |
On the Validity of the Principle of Energy Equivalence
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 the right part of [Figure - 1], 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; 1999; Irle & 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.
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 & 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 ¼ 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.
Possible Reasons for the Better Tolerability of Classical Music
The two sound exposures "industrial noise" and "classical music" were - as mentioned earlier - put together in a way that there were no substantial and systematic differences in the distribution of frequencies (cp. left part of [Figure - 6]), which otherwise could affect the hearing variously. 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 L peak (123 dB) than the "industrial noise" (117 dB).
The middle part of [Figure - 6] 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 (L peak ) 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, i.e., 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 - 6] 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 in the right part of [Figure - 6], 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).[18]
| References | | |
| 1. | DIN ISO 4869-1 (1990) Acoustics; Hearing Protectors, Part 1: Subjective Method for the Measurement of Sound Attenuation  |
| 2. | Hesse,J.M. and Strasser H. (1990) Horschwellenverschiebungen nach verschieden strukturierter energie-aquivalenter Schallbelastung. Z.Arb.wiss. 44 (16 NF) 3: 169-174 |
| 3. | Hesse J.M, Irle H. and Strasser H.(1994) Laborexperimentelle Untersuchungen zur Gehorschadlichkeit von Impulsschall. Z.Arb.wiss. 48 (20 NF) 4: 237-244 |
| 4. | Irle H. and Strasser H. (1998) On the Effects of Dynamic Muscle Work on Noise-Induced Hearing Threshold Shifts. In "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), Carter N. & Job R.F., eds., Sydney/Australia, pp51-54 |
| 5. | 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 |
| 6. | Irle H., Rosenthal C. and Strasser H. (1999) Influence of a Reduced Wearing Time one the Attenuation of Hearing Protectors Assessed via Temporary Threshold Shifts. Int. J.Ind Ergon, 23: 573-584 |
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| 14. | Sataloff R.T. and Sataloff J. (1993) Hearing Loss in Musicians. In: Occupational Hearing Loss. Sataloff R.T. & Sataloff J., eds., Second Edition, Revised and Expanded, Marcel Dekker Inc., New York, Basel, Hong Kong, pp583-594. |
| 15. | Stevens S.S. (1959) On the Psychophysical Law. Psychol. Review 64: 153-181 |
| 16. | 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 |
| 17. | Strasser H. and Irle H. (2001) Noise: Measuring, Evaluation, and Rating in Ergonomics. In Int. Encyclopedia of Ergonomics and Human Factors, Volume I, Part 3, Performance Related Factors. Karwowski W., ed., Taylor & Francis, London/New York, pp516-523 |
| 18. | Wegner R., Wendland P., Poschadel B., Olma K. and Szadkowski D. (2000) Untersuchungen zu Wirksamkeit und Akzeptanz von GehorschutzmaBnahmen bei Orchestermusikern. Arbeitsmed.-Sozialmed.-Umweltmed. 35 (10): 486-497 |
Correspondence Address:
H Strasser
Ergonomics Division, University of Siegen, Paul-Bonatz-Str. 9-11, 57068 Siegen
Germany
 Source of Support: None, Conflict of Interest: None  | Check |
PMID: 14558895 
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6] |
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