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

Valuable recommendations for the choice, utilization, care, and maintenance , and for the measurement of sound attenuation of hearing-protective device s have been laid down in international standards. Yet, by considering the w earing time of a hearing protector, the standard DIN EN 458 assumes a scarc ely 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 1 2 dB if it were unused for only 30 min of an 8 h shift. Thus, the actual in fluence of a shortened wearing time on the protection of earmuffs was teste d in a laboratory study using audiometric measurements of the temporary thr eshold shift (TTS2) and its recovery after exposure to noise. For that purp ose, the effectiveness of a hearing-protective device depending on the amou nt of time worn as prognosticated by DIN EN 458 was compared with the actua l 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 expos ed 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 t he same sound pressure in TS II; however, after 30 min, the earmuffs were r emoved 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 9 4 dB(A)/1 h without earmuffs in a third TS. This acoustic load, which is en ergy equivalent to the load in TS II, is also equivalent to 85 dB(A)/8 h. T he results show that the continuous wearing of the earmuffs offers secure p rotection. However, the energetic approach and the levelling of differently structured noise loads according to the principle of energy equivalence le ads to misconceiving results. The drastic reduction of the sound attenuatio n of the earmuffs predicted from the energetic point of view must be regard ed 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 ea rmuffs are taken off briefly, a drastic reduction in the protection - as pr edicted in DIN EN 458 - does not result.