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Influence of a reduced wearing time on the attenuation of hearing protectors assessed via temporary threshold shifts
International Journal of Industrial Ergonomics 23 (1999) 573—584
Influence of a reduced wearing time on the attenuation of hearing protectors assessed via temporary threshold shifts Hartmut Irle, Christian Rosenthal, Helmut Strasser* Institute of Production Engineering/Ergonomics Division, University of Siegen, Paul-Bonatz-Str. 9-11, D-57068 Siegen, Federal Republic of Germany Received 10 June 1997; received in revised form 3 November 1997; accepted 3 November 1997
Abstract 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 h shift. Thus, the actual influence of a shortened wearing time on the protection of earmuffs was tested in a laboratory study using audiometric measurements of the temporary threshold shift (TTS ) 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 earmuffs. Ten test subjects (Ss) participated in three 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 earmuffs with an attenuation of 30 dB. The Ss were exposed to the same sound pressure in TS II; however, after 30 min, the earmuffs were removed for a duration of 3 min. Mathematically, this reduced the sound attenuation of the earmuffs to 12 dB, i.e., the average noise level over 1 h is 94 dB, which is 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 earmuffs 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 earmuffs 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 earmuffs predicted from the energetic point of view must be regarded as exaggerated. The TTS values show that TS II — which, according to the principle of energy-damage-equivalence, should result in the same effects as TS III — represents significantly less auditory fatigue. Thus, if the earmuffs are taken off briefly, a drastic reduction in the protection — as predicted in DIN EN 458 — does not result.
* Corresponding author. 0169-8141/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 8 ) 0 0 0 2 2 - 5
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H. Irle et al. / International Journal of Industrial Ergonomics 23 (1999) 573—584
Relevance to industry The results of this study demonstrate that the standards and regulations for noise rating 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 not only for rating noise. This principle also leads to an essential underestimation of the attenuation of hearing protectors when these devices are taken off for only a short time in a noise-filled area. 1999 Elsevier Science B.V. All rights reserved. Keywords: Attenuation of hearing protectors; Wearing time; Temporary threshold shift; Energy equivalence principle; Noise rating
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 (cf. DIN EN 458, 1993; DIN EN 352-1, 1993; DIN ISO 4869-1, 1990). Yet, when considering the wearing time of a hearing protector, the standard DIN EN 458 (1993) assumes a 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 by 18 to 12 dB if it were unused for only 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 Figs. 1 and 2. According to the upper part of Fig. 1, e.g., the acoustic situation at a workplace with a high continuous noise level may be represented by 106 dB. If an earmuff 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 h as it is shown in the front part of Fig. 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—76 dB) are lost, i.e., a sound attenuation of only 12 dB (rather than 30 dB) results. According to the 3 dB rule which is generally used in acoustics, an assumed doubling of the expo-
sure time corresponds with a 3 dB reduction of the noise level, i.e., 106 dB for 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 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 7 h, the hearing is exposed to a noise of 76#94 dB over 8 h, each, the latter originating from the 106 dB for h. Since 76#94 dB"94 dB according to the rules of the rating level calculation, the protector seems to have substantially decreased by 94!76 dB"18 dB due to the h during which it was not worn. Even more curious is the case represented in the front part of Fig. 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 may all seem plausible and reasonable according to the laws of energy equivalence, however, it seems as if such calculations according to DIN EN 458 (1993) 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 (cf. Strasser, 1995, 1996; Strasser and Hesse, 1993) — once again goes too far, since it is at least difficult to
H. Irle et al. / International Journal of Industrial Ergonomics 23 (1999) 573—584
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Fig. 1. 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 h, when applying the energy equivalence principle.
Fig. 2. 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, h, and 4 min when applying the energy equivalence principle.
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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).
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 (cf. N.N., 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 Fig. 3, an experimental, practicable exposure could also be chosen using this configuration, keeping in consideration the fact that — according to previous experiments (cf. 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 (cf. front part of Fig. 3), the same Ss — acting as their own control — were exposed to 106 dB for 1 h but in this case they were protected
Fig. 3. Schematic representation of the three exposures.
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by noise-insulating earmuffs with a noise reduction of 30 dB, so that with 106!30 dB"76 dB only very minimal threshold shifts could be expected. Whether or not the earmuffs actually provided the protection promised was also to be assessed using threshold measurements. Finally, in TS II (cf. middle part of Fig. 3), the Ss removed the earmuffs for the short time of 3 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. During and after the exposure of TS I where the Ss were safely protected by wearing the earmuffs 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 Fig. 4 — which again can be interpreted as physiological cost brought upon the human organism by the energy-
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equivalent noise situations. However, if the test series TS II in which the earmuffs 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 scepticism reflected in DIN EN 458 (1993) 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 workphysiological human characteristics. 2.2. Hearing protective device The earmuff Optac Vario VOL SD was used for the experimental investigation. This hearing protector — as can be seen in Fig. 5 — has proven a protective device with a SNR-value 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 DIN ISO 4869-1 (1990).
Fig. 4. Schematic representation of the three exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shifts (TTS).
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Fig. 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 yr; height: 185$5 cm; weight: 80$10 kg) could — according to DIN ISO 4869-1 (1990), — only exhibit hearing threshold shifts of no more than 15 dB as compared to persons with normal hearing (lower line in Fig. 6) 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 Fig. 7) and the TTS values after the exposures (dark area in Fig. 7) 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 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.
Fig. 6. Criteria for the selection of the test subjects according to DIN ISO 4869-1 (1990).
2.4. Schematic test set-up and noise load As can be seen in Fig. 8, 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.
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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 was chosen
Fig. 7. Selection of the frequency with maximum threshold shift during the first 2 min after the exposure.
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as 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 3 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 3 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. Fig. 9 represents the frequency analysis, i.e., the spectral energy distribution in the middle 8 octaves of the acoustic load with A-weighting, C-weighting
Fig. 8. Schematic test set-up.
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Fig. 9. Frequency analysis of the sound exposure.
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).
3. Results Fig. 10 shows all measured values of the hearing threshold shifts (differences between the TTS values and the individual resting threshold at the respective frequency of maximum threshold shift) as determined for the 10 Ss for the three 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 h (cf. N.N., 1988, 1990) lead to essential physiological cost to the hearing which may not be underestimated. The lower section of Fig. 10 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 3 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 Fig. 10 shows that the majority of the Ss experienced absolutely no threshold shift when exposed to 106 dB while wearing the protectors continuously. In the case of the Ss who did experience a hearing threshold shift, the shift was hardly objectifiable (usually less than 4 dB), and the Ss recovered fully within mere minutes. Thus, the earmuffs 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 (TTS ) 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) according to Fig. 11 is the area that is bordered by
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Fig. 10. 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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the time axis, the TTS axis at 2 min, 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 Fig. 12 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 (1993) is based would have us believe. The physiological cost in TS II is not, as prognosticated identical with those of TS III but
Fig. 11. Exemplary representation of the IRTTS.
Fig. 12. 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.
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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 earmuffs 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.
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 (1993) 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 h dur ing an 8 h shift, i.e. it is reduced by 8 dB. Earmuffs whose insulation values of, e.g. 30 or 35 dB are also reduced to the same 12 dB are even more underrated. The loss of 18 or 23 dB simply makes no sense. 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 earmuffs 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 earmuffs 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).
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In order to obtain information about the maximum 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 3 min on the same Ss was measured as a control. The average of the TTS 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 3 min within 1 h, which is approximately equivalent to a 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., 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 (cf. VDI 2560, 1983) when personal hearing protective devices are selected. Finally, if suitable earmuffs 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., cf. Hesse et al., 1996b; Irle et al., 1997); thus, the standards and regulations have been built upon an untrustworthy foundation.
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5. For further reading This reference also be of interest to the reader: EN-ISO 4869-2, 1994. References DIN EN 352-1, 1993. Hearing Protectors; Safety Requirements and Testing; Part 1: Ear Muffs. DIN EN 458. 1993, Hearing Protectors; Recommendations for Selection, Use, Care and Maintenance; Guidance Document. DIN ISO 4869-1, 1990. Acoustics; Hearing Protectors; Part 1: Subjective Method for the Measurement of Sound Attenuation. EN ISO 4869-2, 1994. Acoustics; Hearing Protectors; Part 2: Estimation of Effective A-weighted Sound Pressure Levels when Hearing Protectors are Worn. Hesse, J.M., Irle, H., 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., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH,, USA, pp. 627—632. Hesse, J.M., Vogt, E., 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., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH, USA, pp. 639—644.
Irle, H., Hesse, J.M., Strasser, H., 1997. 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, in press. Miller, J.D., 1974. Effects of noise on people. Journal of Acoustics Society of America 56 (3), 729—764. N.N., 1988. German Working Places Regulations. Arbeitssta¨tten — Vorschriften und Richtlinien, m 15 Schutz gegen La¨rm. N.N., 1990. Accident Prevention Regulation-Noise, UVV-La¨rm, Unfallverhu¨tungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Ko¨ln. Strasser, H., 1995. Dosismaxime und Energie-A®quivalenz — Ein Kernproblem des pra¨ventiven Arbeitsschutzes bei der ergonomischen Beurteilung von Umgebungsbelastungen. In: Strasser, H. (Hrsg.), Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen — Anspruch und Wirklichkeit des pra¨ventiven Arbeitsschutzes. Ecomed Verlag, Landsberg/Lech, pp. 9—31. Strasser, H., 1996. Curiosities of conventional noise rating procedures. In: Mital, A., Krueger, H., Kumar, S., Menozzi, M., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH, USA, pp. 619—626. Strasser, H., 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., Irle, H., 1995. Hearing threshold shift after energy equivalent exposure to impulse and continuous noise. In: Bittner, A.C., Champney, P.C. (Eds.), Advances in Industrial Ergonomics and Safety VII. Taylor and Francis, London, pp. 241—248. VDI 2560, 1983. Perso¨nlicher Schallschutz. VDI-Verlag, Du¨sseldorf.
Influence of a reduced wearing time on the attenuation of hearing protectors assessed via temporary threshold shifts Hartmut Irle, Christian Rosenthal, Helmut Strasser* Institute of Production Engineering/Ergonomics Division, University of Siegen, Paul-Bonatz-Str. 9-11, D-57068 Siegen, Federal Republic of Germany Received 10 June 1997; received in revised form 3 November 1997; accepted 3 November 1997
Abstract 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 h shift. Thus, the actual influence of a shortened wearing time on the protection of earmuffs was tested in a laboratory study using audiometric measurements of the temporary threshold shift (TTS ) 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 earmuffs. Ten test subjects (Ss) participated in three 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 earmuffs with an attenuation of 30 dB. The Ss were exposed to the same sound pressure in TS II; however, after 30 min, the earmuffs were removed for a duration of 3 min. Mathematically, this reduced the sound attenuation of the earmuffs to 12 dB, i.e., the average noise level over 1 h is 94 dB, which is 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 earmuffs 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 earmuffs 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 earmuffs predicted from the energetic point of view must be regarded as exaggerated. The TTS values show that TS II — which, according to the principle of energy-damage-equivalence, should result in the same effects as TS III — represents significantly less auditory fatigue. Thus, if the earmuffs are taken off briefly, a drastic reduction in the protection — as predicted in DIN EN 458 — does not result.
* Corresponding author. 0169-8141/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 8 ) 0 0 0 2 2 - 5
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H. Irle et al. / International Journal of Industrial Ergonomics 23 (1999) 573—584
Relevance to industry The results of this study demonstrate that the standards and regulations for noise rating 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 not only for rating noise. This principle also leads to an essential underestimation of the attenuation of hearing protectors when these devices are taken off for only a short time in a noise-filled area. 1999 Elsevier Science B.V. All rights reserved. Keywords: Attenuation of hearing protectors; Wearing time; Temporary threshold shift; Energy equivalence principle; Noise rating
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 (cf. DIN EN 458, 1993; DIN EN 352-1, 1993; DIN ISO 4869-1, 1990). Yet, when considering the wearing time of a hearing protector, the standard DIN EN 458 (1993) assumes a 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 by 18 to 12 dB if it were unused for only 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 Figs. 1 and 2. According to the upper part of Fig. 1, e.g., the acoustic situation at a workplace with a high continuous noise level may be represented by 106 dB. If an earmuff 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 h as it is shown in the front part of Fig. 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—76 dB) are lost, i.e., a sound attenuation of only 12 dB (rather than 30 dB) results. According to the 3 dB rule which is generally used in acoustics, an assumed doubling of the expo-
sure time corresponds with a 3 dB reduction of the noise level, i.e., 106 dB for 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 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 7 h, the hearing is exposed to a noise of 76#94 dB over 8 h, each, the latter originating from the 106 dB for h. Since 76#94 dB"94 dB according to the rules of the rating level calculation, the protector seems to have substantially decreased by 94!76 dB"18 dB due to the h during which it was not worn. Even more curious is the case represented in the front part of Fig. 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 may all seem plausible and reasonable according to the laws of energy equivalence, however, it seems as if such calculations according to DIN EN 458 (1993) 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 (cf. Strasser, 1995, 1996; Strasser and Hesse, 1993) — once again goes too far, since it is at least difficult to
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Fig. 1. 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 h, when applying the energy equivalence principle.
Fig. 2. 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, h, and 4 min when applying the energy equivalence principle.
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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).
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 (cf. N.N., 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 Fig. 3, an experimental, practicable exposure could also be chosen using this configuration, keeping in consideration the fact that — according to previous experiments (cf. 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 (cf. front part of Fig. 3), the same Ss — acting as their own control — were exposed to 106 dB for 1 h but in this case they were protected
Fig. 3. Schematic representation of the three exposures.
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by noise-insulating earmuffs with a noise reduction of 30 dB, so that with 106!30 dB"76 dB only very minimal threshold shifts could be expected. Whether or not the earmuffs actually provided the protection promised was also to be assessed using threshold measurements. Finally, in TS II (cf. middle part of Fig. 3), the Ss removed the earmuffs for the short time of 3 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. During and after the exposure of TS I where the Ss were safely protected by wearing the earmuffs 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 Fig. 4 — which again can be interpreted as physiological cost brought upon the human organism by the energy-
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equivalent noise situations. However, if the test series TS II in which the earmuffs 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 scepticism reflected in DIN EN 458 (1993) 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 workphysiological human characteristics. 2.2. Hearing protective device The earmuff Optac Vario VOL SD was used for the experimental investigation. This hearing protector — as can be seen in Fig. 5 — has proven a protective device with a SNR-value 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 DIN ISO 4869-1 (1990).
Fig. 4. Schematic representation of the three exposures and hypothetical physiological responses, i.e., growth and restitution of the temporary threshold shifts (TTS).
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Fig. 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 yr; height: 185$5 cm; weight: 80$10 kg) could — according to DIN ISO 4869-1 (1990), — only exhibit hearing threshold shifts of no more than 15 dB as compared to persons with normal hearing (lower line in Fig. 6) 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 Fig. 7) and the TTS values after the exposures (dark area in Fig. 7) 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 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.
Fig. 6. Criteria for the selection of the test subjects according to DIN ISO 4869-1 (1990).
2.4. Schematic test set-up and noise load As can be seen in Fig. 8, 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.
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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 was chosen
Fig. 7. Selection of the frequency with maximum threshold shift during the first 2 min after the exposure.
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as 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 3 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 3 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. Fig. 9 represents the frequency analysis, i.e., the spectral energy distribution in the middle 8 octaves of the acoustic load with A-weighting, C-weighting
Fig. 8. Schematic test set-up.
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Fig. 9. Frequency analysis of the sound exposure.
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).
3. Results Fig. 10 shows all measured values of the hearing threshold shifts (differences between the TTS values and the individual resting threshold at the respective frequency of maximum threshold shift) as determined for the 10 Ss for the three 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 h (cf. N.N., 1988, 1990) lead to essential physiological cost to the hearing which may not be underestimated. The lower section of Fig. 10 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 3 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 Fig. 10 shows that the majority of the Ss experienced absolutely no threshold shift when exposed to 106 dB while wearing the protectors continuously. In the case of the Ss who did experience a hearing threshold shift, the shift was hardly objectifiable (usually less than 4 dB), and the Ss recovered fully within mere minutes. Thus, the earmuffs 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 (TTS ) 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) according to Fig. 11 is the area that is bordered by
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Fig. 10. 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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the time axis, the TTS axis at 2 min, 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 Fig. 12 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 (1993) is based would have us believe. The physiological cost in TS II is not, as prognosticated identical with those of TS III but
Fig. 11. Exemplary representation of the IRTTS.
Fig. 12. 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.
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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 earmuffs 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.
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 (1993) 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 h dur ing an 8 h shift, i.e. it is reduced by 8 dB. Earmuffs whose insulation values of, e.g. 30 or 35 dB are also reduced to the same 12 dB are even more underrated. The loss of 18 or 23 dB simply makes no sense. 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 earmuffs 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 earmuffs 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).
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In order to obtain information about the maximum 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 3 min on the same Ss was measured as a control. The average of the TTS 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 3 min within 1 h, which is approximately equivalent to a 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., 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 (cf. VDI 2560, 1983) when personal hearing protective devices are selected. Finally, if suitable earmuffs 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., cf. Hesse et al., 1996b; Irle et al., 1997); thus, the standards and regulations have been built upon an untrustworthy foundation.
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5. For further reading This reference also be of interest to the reader: EN-ISO 4869-2, 1994. References DIN EN 352-1, 1993. Hearing Protectors; Safety Requirements and Testing; Part 1: Ear Muffs. DIN EN 458. 1993, Hearing Protectors; Recommendations for Selection, Use, Care and Maintenance; Guidance Document. DIN ISO 4869-1, 1990. Acoustics; Hearing Protectors; Part 1: Subjective Method for the Measurement of Sound Attenuation. EN ISO 4869-2, 1994. Acoustics; Hearing Protectors; Part 2: Estimation of Effective A-weighted Sound Pressure Levels when Hearing Protectors are Worn. Hesse, J.M., Irle, H., 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., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH,, USA, pp. 627—632. Hesse, J.M., Vogt, E., 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., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH, USA, pp. 639—644.
Irle, H., Hesse, J.M., Strasser, H., 1997. 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, in press. Miller, J.D., 1974. Effects of noise on people. Journal of Acoustics Society of America 56 (3), 729—764. N.N., 1988. German Working Places Regulations. Arbeitssta¨tten — Vorschriften und Richtlinien, m 15 Schutz gegen La¨rm. N.N., 1990. Accident Prevention Regulation-Noise, UVV-La¨rm, Unfallverhu¨tungsvorschrift der gewerblichen Berufsgenossenschaften (VBG 121). C. Heymanns Verlag, Ko¨ln. Strasser, H., 1995. Dosismaxime und Energie-A®quivalenz — Ein Kernproblem des pra¨ventiven Arbeitsschutzes bei der ergonomischen Beurteilung von Umgebungsbelastungen. In: Strasser, H. (Hrsg.), Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen — Anspruch und Wirklichkeit des pra¨ventiven Arbeitsschutzes. Ecomed Verlag, Landsberg/Lech, pp. 9—31. Strasser, H., 1996. Curiosities of conventional noise rating procedures. In: Mital, A., Krueger, H., Kumar, S., Menozzi, M., Fernandez, J.E. (Eds.), Advances in Occupational Ergonomics and Safety I. ISOES. Cincinnati/OH, USA, pp. 619—626. Strasser, H., 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., Irle, H., 1995. Hearing threshold shift after energy equivalent exposure to impulse and continuous noise. In: Bittner, A.C., Champney, P.C. (Eds.), Advances in Industrial Ergonomics and Safety VII. Taylor and Francis, London, pp. 241—248. VDI 2560, 1983. Perso¨nlicher Schallschutz. VDI-Verlag, Du¨sseldorf.