Intensity and Duration of Noise Exposure and Temporary Threshold Shifts

Abstract

What is the “trading relationship” between intensity and duration of exposures to loud sound in producing a given magnitude of temporary threshold shift (TTS)? Examination of the literature led to the hypothesis that duration is about twice as important as intensity: Two exposures should produce the same TTS if one exposure were one-fourth the sound level (6 db less) and twice the duration of the other exposure. The hypothesis was tested on 12 human ears with thermal noise exposures ranging from 130 db SPL for 1 min to 94 db SPL for 64 min. In this series each doubling of duration was associated with a fourfold or 6 db decrease in sound level. TTS was measured at 2000, 4000, 6000, and 8000 cps. If the hypothesis were true, all exposures should have produced the same TTS. Actually the obtained average TTS, 5 min after exposure, described an inverted U-shaped function when plotted by increasing duration of exposure. The conditions from 118 db for 4 min through 100 db for 32 min produced roughly similar amounts of TTS, while the other conditions produced substantially less TTS. The shapes of the recovery curves differed systematically: At 4000 cps, the TTSs 20 sec after exposure were in perfect rank-order agreement with intensity of exposure, while from about 2 min to 10 min the inverted-U rank order was apparent.