Long-duration exposure to intermittent noises.

The effects were examined of various patterns of interruption--of continuous noise by periods of quiet--on the growth and recovery of temporary threshold shift of hearing over an exposure period of 24 h. Monaural thresholds of hearing were measured prior to, during, and following exposure to a pink noise at a level of 85 dBA and to four conditions in which the pink noise was interrupted with various on-off ratios. The interrupted exposure patterns and levels were adjusted to make their average levels equivalent to 85 dBA. Among the results: 1) the growth of TTS clear.ly reached an asymptote for all interrupted exposure conditions, even when the TTS was as small as 5 dB, 2) the interrupted exposures produced lower asymptotic levels than the continuous exposure with the same amount of energy, and 3) the TTS recovery patterns were essentially the same at 1 h and beyond, for all conditions. These data support the concept that, following a long exposure, one should be provided at least as much time for recovery as the duration of the exposure.