Holographic Investigation Of Hydroelastic Modes And Waves In A Cochlear Model

The cochlea of the inner ear has attracted the attention of investigators in a wide variety of fields from physics to physiology. Of particular interest is the question of how much signal processing occurs before sound waves become neural codes. Studies with properly scaled mechanical models have revealed much qualitative information which correctly corresponds to physiology observations, such as a place principle and traveling wave motion of the basilar membrane. This membrane, which supports the neural sensors, resembles a slender tapered plate of trapezoidal platform with a length of about 35 millimeters and a width of a few tenths of a millimeter. This divides a tube of variable cross-sectional area into two chambers (scala vestibuli and scala tympani). A mechanical model, twenty-four times life size, that maintains dynamical similarity has been tested using both time averaged and stroboscopic holography. This model has been tested with both viscous and inviscid incompressible fluids. Also the thin plate, which represents the basilar membrane, has been modeled by a lossless material (aluminum) and a material with internal damping (polycarbonate). Results of the holography study are presented and compared to a parallel theoretical study. In the lossless case pure nodal patterns are observed and the prominence of the place principle increases with decreasing fluid density. At low frequency (corresponding to less than 500 Hz in the ear) with a viscous fluid the steady state sinusoidal response can be adequately described by a one dimensional theory. The waves are a combination of travelling and standing waves. The travelling component is solely a dissipative effect. Discrete resonances are readily observed in this frequency range. At higher frequencies dissipation smooths out the resonance effect. Also at higher frequencies three dimensional fluid motion becomes highly significant.