Asymptotic Threshold Shift (ATS) in Man from 24 Hour Exposure to Continuous Noise

Abstract

Ten men were exposed to a 300–600 Hz band of noise at an octave-band-level of 90 dB in a sound field for a period of 24 hours. Hearing thresholds were measured in one ear at 11 test frequencies ranging from 125 to 8000 Hz prior to exposure and at selected time intervals during and after exposure. Temporary threshold shift (TTS) appeared to reach asymptotic levels by 12 hours of exposure. Maximum TTS was approximately 11 dB and occurred at 500, 750 and 1000 Hz. TTS was appreciable at 1500 Hz amounting to 7 dB and was less than 5 dB at other frequencies. The growth pattern of TTS was triphasic; slow development during the first two hours of exposure, a rapid increase from 2 to 8 hours, and then apparently reaching an asymptote by the twelfth hour. Recovery was prolonged for the relatively small magnitude of TTS, requiring 24 hours before most of the subjects returned to preexposure threshold levels. Asymptotic TTS (ATS) showed dependence on preexposure threshold hearing levels.