Impedance at the eardrum, middle-ear transmission, and equal loudness.

Abstract

Acoustic impedance at the eardrum was measured on three subjects as a function of frequency (20 cps to about 1000 cps) and intensity (minimum 65 dB SPL; maximum 150 dB SPL, at eardrum) of a sustained ipsilateral sinusoidal stimulation by means of an inversely driven Zwislocki acoustic bridge. At lower intensity levels, acoustic impedance was independent of level, resistance and reactance varying with frequency according to R = af−1/2 and X = bf−3/4, respectively, from 20 cps to about 300 cps; above 300 cps, resistance remained essentially constant, and negative reactance decreased rapidly. At higher levels, one subject showed variations in impedance attributable to the acoustic reflex. At levels above 120 dB SPL, all subjects showed a rapid decrease of both impedance components at all frequencies. Middle-ear attenuations calculated from the magnitude of the acoustic impedance, together with previously obtained equal-loudness contours for the participating subjects, support the hypothesis that equality of loudness between two tones is attained when both generate the same total number of neural impulses within about 200 msec. The decrease in impedance at the highest levels is attributed to subharmonic radiation from the eardrum and/or a shift in the axis of rotation of the ossicles.