Lloyd Watts Ph.D. Thesis

Cochlear Mechanics: Analysis and Analog VLSI


The cochlea separates sounds based on their frequency content and on their fine time structure, using an active and nonlinear fluid-mechanical traveling-wave mechanism. This dissertation describes a simplified model of the cochlear mechanics problem, and techniques for solving the problem.

The Liouville--Green (LG) method has been used to obtain analytical solutions for the cochlear mechanics problem; however, the failure of the method to agree quantitatively with numerical methods has left doubts about its validity. In this dissertation, it is shown that the LG method fails to solve the problem, and that an additional degree of freedom is required for a consistent solution. The additional degree of freedom corresponds to a second wave mode, which has been observed experimentally in the cochleas of living animals. The new mode-coupling LG solution agrees quantitatively with numerical solutions. This problem has been outstanding since 1971.

In addition to analytical techniques, this dissertation also presents analog circuit techniques, specifically for the medium of analog very-large-scale-integration (VLSI) complementary metal-oxide-semiconductor (CMOS) technology. A silicon cochlea that models the behavior of the passive cochlea has been fabricated and tested. The silicon cochlea operates in real time with 8 mW of power dissipation.

The active and nonlinear behavior of the cochlea is a subject of intense research interest at the present time, and many issues are still unresolved. A preliminary model of active elements in the cochlea is described and characterized, and shown to be consistent with the prevailing views of active cochlear function.