The Cochlear Amplifier

Abstract

The recent observations of sharply-tuned basilar membrane motion and the existence of cochlear acoustic emissions provide evidence that the cochlea is an active mechanical system, capable of generating mechanical vibrations. A model of cochlear mechanics is presented in this paper to support the hypothesis that the primary function of mechanical generators in the cochlea is to amplify displacements of the basilar membrane at sound levels near the threshold of hearing. Some numerical solutions of this model show basilar membrane displacement amplitudes of about 1 angstrom for sound pressures at the eardrum of 0 dB SPL (20 µPa). The rate of energy flow out of the basilar membrane into the cochlear fluid due to the cochlear amplifier is often more than 40 dB greater than the rate of energy flow into the cochlea from the stapes. The action of the cochlear amplifier in this model may be interpreted as a piezoelectric effect which provides a delayed positive feedback to basilar membrane motion.