Adaptation of the Ear to Sound Stimuli

Abstract

Adaptation is defined as the elevation of the auditory threshold by a previous sound stimulus. It is detd. by means of a short tone impulse (testing impulse) which follows the sound stimulus (stimulating impulse) causing the adaptation. The testing impulse is adjusted to the threshold value. By altering the interval between the stimulating and testing impulses, the curve of return adaptation, i.e., the return to normal sensitivity, may be ascertained. When plotted on a logarithmic db scale, this curve is approx. a straight line. The entire process of adaptation requires less than 0.4 sec.; the return adaptation is complete within a few tenths of a sec. After stimulation by a tone of 80 db above threshold intensity, return adaptation is complete after 250 sec. The increase in adaptation is approx. proportional to the intensity of the stimulating tone when both magnitudes are expressed in db. With a sound impulse of 80 db above the auditory threshold, the adaptation reaches a value of 40 to 50 db. Binaural detn. of adaptation, by leading the stimulating impulse to one ear and the testing impulse to the other, shows that adaptation is a monaural and therefore a peripheral process. A pure tone induces an elevation of the auditory threshold not only for its own frequency but also for neighboring ones. The adaptation spreads chiefly to higher frequencies than that of the stimulating tone at which it reaches a maximum. The max. is more pronounced at high frequencies of the stimulating tone than at low ones. Comparative expts. between adaptation and masking demonstrate a far-reaching similarity between the 2 processes. These show a similar behavior with respect to intensity and frequency. A quantitative comparison leads to the conclusion that masking depends chiefly upon the adaptation of the ear to sound stimuli.