Acoustic input impedance of the stapes and cochlea in human temporal bones

Abstract

The acoustic input impedance of the stapes and cochlea Z SC represents the mechanical load driven by the tympanic membrane, malleus and incus. Z SC was calculated from broad-band measurements (20 Hz to 11 kHz) of stapes displacement made with an optical motion sensor and of sound pressure at the stapes head in a human temporal-bone preparation. Measurements were made in 12 fresh temporal bones with the round window insulated from the sound stimulus. Below 1 kHz, the magnitude of Z SC was approximately inversely proportional to frequency, and Z SC angle was between −0.10 and −0.20 periods. This behavior is consistent with a mixed stiffness and resistance. Between 1 and 4 kHz, Z SC was resistance-dominated with a magnitude between 40 and 100 mks acoustic GΩ that was roughly independent of frequency, and its angle was between −0.12 and 0 periods. Between 4 and 7 kHz, the magnitude of Z SC was either constant or increased with frequency while Z SC angle was near 0. Between 7 and 8 kHz, both Z SC magnitude and angle decreased sharply with frequency, and both increased somewhat at higher frequencies. The input impedance of the cochlea Z C was estimated in one ear from Z SC measurements made before and after draining the inner ear fluids. Z C was stiffness-dominated below 100 Hz, and resistance-dominated from 100 Hz to 5 kHz. The frequency-dependent magnitude of Z SC in our bones is similar to those reported by other investigators in cadaver temporal bones (Nakamura et al., 1992; Kurokawa and Goode, 1995). Our Z SC measurements are qualitatively similar to theoretical predictions (Zwislocki, 1962; Kringlebotn, 1988), but are a factor of 3 greater in magnitude, implying that Z SC may be more resistive and stiffer than previously thought. We found inter-ear variations of a factor of 4 (12 dB), which may explain some of the clinically observed variations in size of the air-bone gap in individuals with middle ear lesions or after middle-ear reconstructive surgery.