Modes and waves in a cochlear model.

A simplified model hydroelastic system having many features in common with the cochlea is studied theoretically and experimentally. The system consists of a slender rigid tube filled with a viscous incompressible fluid. The tube is divided lengthwise into two chambers (scala vestibuli, scala tympani) by an interior surface, part of which is rigid and part elastic. The elastic portion is an isotropic plate having variable width and is clamped on its edges. The system is driven by a sinusoidal imput at the stapes end. The mechanical model is 24 times life size and dynamical similarity is maintained. Wave patterns in the plate are measured using time-averaged and stroboscopic holographic interferometry. At low frequency (corresponding to less than 500 cycles/s in the cochlea) the response can be adequately predicted by a one dimensional theory. The waves are a combination of traveling and standing waves. The traveling component is solely a dissipative effect. Discrete resonances are readily observed in the low frequency range.