ACTIVE CONTROL OF WAVES IN A COCHLEAR MODEL WITH SUBPARTITIONS

Multiscale asymptotic methods developed previously to study macromecha nical wave propagation in cochlear models are generalized here to incl ude active control of a cochlear partition having three subpartitions, the basilar membrane, the reticular lamina, and the tectorial membran e, Activation of outer hair cells by stereocilia displacement and/or b y lateral wall stretching result in a frequency-dependent force acting between the reticular lamina and basilar membrane, Wavelength-depende nt fluid loads are estimated by using the unsteady Stokes' equations, except in the narrow gap between the tectorial membrane and reticular lamina, where lubrication theory is appropriate. The local wavenumber and subpartition amplitude ratios are determined from the zeroth order equations of motion. A solvability relation for the first order equat ions of motion determines the subpartition amplitudes, The main findin gs are as follows: The reticular lamina and tectorial membrane move in unison with essentially no squeezing of the gap; an active force leve l consistent with measurements on isolated outer hair cells can provid e a 35-dB amplification and sharpening of subpartition waveforms by de laying dissipation and allowing a greater structural resonance to occu r before the wave is cut off; however, previously postulated activity mechanisms for single partition models cannot achieve sharp enough tun ing in subpartitioned models.