抄録
Temporary threshold shift was observed in normal persons at high frequencies above 10 kc when the sustained pure tones were applied. The experimental method was as follows: The threshold of hearing was measured at first by routine audiometry at frequencies ranging from 1 kcs to 14 kcs. When the sustained pure tone became inaudible the intensity of the tone was increased successively by a step of 5 dB. This procedure was repeated until the applied tone become audible over 1 minute. At the frequencies above 10 kcs, the threshold shifts were found in 6 cases out of 9. Particularly in some cases the value of the threshold shifts showed 30 dB during 30 second interval. It was found that this phenomenon was due to the auditory adaptation and not to fatigue. The reason was that when the tone was interrupted, the threshold shift instantly disappeared and did not persisted as in the cases of fatigue. Concerning the origin of this phenomenon, two reasons were considered. First, at high frequencies, the sound pressure of minimal audible threshold was higher than that at middle frequencies. Secondly, the vibrating portion of the basilar membrane in the cochlea became narrower in proportion to frequencies.
