Temporary threshold shift by low and medium intensity exposures.

Temporary threshold shift was measured by Bekesy audiometry in subjects with normal hearing and sensorineural hearing impairment. The 20‐dB SL exposure of 1‐min duration at 4000 Hz resulted in similar shift for the same exposure frequency as 80‐dB SL exposure in a normal subject. This finding is in good agreement with results of Lierle and Reger, Hirsh, and Bilger. When the exposure level was higher than 90 dB SPL, shift at a frequency 1/2 octave above the exposure frequency exceeded that produced at the exposure frequency. The 20‐dB SL (77‐dB SPL) and 30‐dB SL (87‐dB SPL) exposures of 1 and 3 min durations at 4000 Hz for a subject with sensorineural loss showed similar shift as in a normal subject. It is assumed that the area of basilar membrane affected by adaptation remainder is smaller after an exposure to 20 dB SL than to 80 dB SL. Since adaptation is maximal for a given neurone, those neurones concerned with threshold intensities are adapted equally for both 20 and 80 dB. During adaptation remainder...