A Theory of Cochlear Input Impedance and Middle Ear Parameter Estimation

A THEORY OF COCHLEAR INPUT IMPEDANCE AND MIDDLE EAR PARAMETER ESTIMATION by SUNIL PURIA Advisers: Professor Nenad M. Marinovic and Dr. Jont B. Allen In this thesis it is hypothesized that the geometry of the cat cochlea has evolved to maximize sound transmission from the eardrum to the cochlea. This hypothesis is shown to be tenable by modeling the cochlear input impedance and the middle-ear. It is also hypothesized that the geometric configuration of the middle-ear cavities plays an important role in its acoustic properties. Various aspects of the cat cochlear input impedance Zc(lj ) are implemented using a transm ission line model having perilymph viscosity and a varying cross-sectional scalae area. These model results are then compared to the experimental results of Lynch et al. [JASA 72, (1982) pp. 108-130]. From the model, the following observations are made about Zc(u>): (a) The use of anatomically measured scalae cross-sectional areas improves the fits to the magnitude of the experimental data, (b) Improved agreement between model and experimental phase for frequencies below approximately 150 Hz is obtained when perilymph viscosity and tapering are included in the cochlear model, (c) When model scalae tapering and perilymph viscosity are chosen to match physiological conditions, the effect of the helicotrema impedance on Zc(u>) is insignificant. To quantify the effect of Zc{u) on the eardrum impedance, a cat middle-ear model is presented. The parameters of the model were evaluated by modeling the eardrum impedance, measured [J.B. Allen, (1986) in Peripheral Auditory Mechanisms, eds. J.B. Allen, et al.. Springer-Verlag] at each step of various “surgical modifications” to the cochlea and middle-ear. The model indicates that as much as 80% of the incident wave in the ear canal is absorbed by the cochlea in the mid-frequency region and as a result inaccurate representations of Zc(u) will result in inaccurate representatioas of the middle-ear. A physical model for the middle-ear cavities is presented. The model explicitly takes into account effects of non-planar wave propagation due to area discontinuities and visco-ihcrmal effects at the walls. This model is experimentally verified by making impedance measurements on several cylindrical cavities with area discontinuities in its acoustic path. The models for the ossicular path and middle-ear cavities are combined to obtain a compre­ hensive model for the “intact” cat eardrum impedance.