Influence of a reduced wearing time on the attenuation of hearing protectors assessed via temporary threshold shifts

Author(s): Rosenthal, C., Strasser, H.

Journal/Book: Int J Ind Ergonomic. 1999; 23: PO Box 211, 1000 AE Amsterdam, Netherlands. Elsevier Science Bv. 573-584.

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 (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 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 3/4 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.

Note: Article Strasser H, Univ Gesamthsch Siegen, Inst Prod Engn, Ergon Div, Paul Bonatz Str 9-11, D-57068 Siegen, GERMANY

Keyword(s): attenuation of hearing protectors; wearing time; temporary threshold shift; energy equivalence principle; noise rating

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