Nonlinearities in cochlear hydrodynamics.

Cochlear traveling waves show properties of capillary waves progressing along interfaces between two nonmixing fluids. Such wave motion is basically nonlinear. Mechanical cochlear models display at least two types of nonlinearities that may explain a number of auditory and electrophysiological phenomena. (1) The displacement of the cochlear fluids and of the partition is asymmetrical, larger amplitudes being directed away from the input toward scala tympani. The asymmetry is also present in the shearing displacement within the cochlear duct. These phenomena were first observed with the aid of beating signals and tone pips. The asymmetrical displacement of the partition lets the model detect the signal envelope due to its lowpass filter action. The degree of asymmetry is independent of signal amplitude. There is good indication that the asymmetrical displacement is caused hydrodynamically, specifically, that it is a boundary‐layer effect. The latter appears to involve vorticity, a phenomenon that, by defin...