On the Sound Pressure Transformation by the Head and Auditory Meatus Op the Cat

Abstract

Anesthetized cats were supported by a canvas cradle in a sound-proof chamber and were exposed to a progressive sound wave. A flexible probe microphone was implanted in the external auditory meatus and used to measure the sound pressure in the vicinity of the eardrum. A duplicate probe was used to measure the sound pressure at a point near the center of the cat's head in the absence of the animal. From these measurements the magnitude of the ratio of the sound pressure near the eardrum to that in the free field was determined as a function of frequency. When the ear under study was turned toward the sound source, the sound pressure near the eardrum exceeded the free-field pressure appreciably, except at very low frequencies. By measuring the sound pressure near the tragus, the resonance of the external auditory meatus itself could be determined. The data obtained in this study can be used to compare results of experiments in which the acoustic stimulus is measured in terms of free-field pressures with those in which the stimulus is measured in terms of the sound pressure near the eardrum.