A parametric study of cochlear input impedance

In this paper various aspects of the cat cochlear input impedance Z_c(ω) are implemented using a transmission 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. [J. Acoust. Soc. Am. 72, 108–130 (1982) ]. From the model, the following observations are made about the cochlear input impedance: (a) Scalae area variations significantly alter the model Z_c(ω); (b) the use of anatomically measured areas improves the fits to the experimental data; (c) improved agreement between model and experimental phase is obtained when perilymph viscosity and tapering are included in the cochlear model for frequencies below ≈150 Hz; (d) when model scalae tapering and perilymph viscosity are chosen to match physiological conditions, the effect of the helicotrema impedance on Z_c(ω) is insignificant; and (e) the cochlear map, which is defined as the position of the basilar membrane peak displacement as a function of stimulus frequency, can have an important effect on Z_c{o>) for frequencies below 500 Hz. A nonphysiological cochlear map can give rise to cochlear standing waves, which result in oscillations in Z_c(ω). Scalae tapering and perilymph viscosity contribute significantly to the damping of these standing waves. These observations should dispel the previous notion that Z_c(ω) is determined solely by parameters of the cochlea close to the stapes, and the notion that Z_c(ω) is dominated by the helicotrema at low frequencies.