Sound-pressure measurements in the cochlear vestibule of human-cadaver ears.

The middle-ear pressure gain for the 50-Hz to 12-kHz range was determined from the ratio of sound pressures measured in the vestibule and the ear canal of four human-cadaver ears. The magnitude of the middle-ear pressure gain is 20 dB for frequencies between 500 Hz and 2 kHz. Above 4 kHz, the gain changes as a function of frequency at a rate of approximately -8 dB/octave and below 400 Hz at 4 dB/octave. The standard error of the mean magnitude across the four ears is typically less than 3 dB. The phase angle of the pressure gain also changes with frequency. Interruption of the ossicular chain decreases the vestibule pressure by at least 20 dB. It is shown that air bubbles in the inner ear can diminish the vestibule pressure; procedures are used to remove bubbles. From these pressure measurements and previous measurements of stapes motion, the frequency dependence of behavioral thresholds for tones was tested to discover whether it corresponds to the constancy of a physiological variable at the cochlear input. Among pressure, power, or stapes-motion measures, the vestibule pressure is most nearly constant with frequency at the behavioral "minimum audible pressure."