Measurements of acoustic input impedance of the cochlea in cats

Abstract

Although the middle ear is often called an impedance matching device, published measurements of the load impedance presented by the cochlea are not extensive. In the experiments reported here, sound stimuli were applied directly to the stapes in anesthetized cats in which the malleus and incus had been removed. Measurements were made of sound pressures at the stapes (Ps) and in the vestibule (Pc) with calibrated probe-tube transducers. Stapes velocity Vs was determined with the Mössbauer technique. Magnitude and phase of the total acoustic impedance Zf = Ps/(AfpVs) and the cochlear impedance Zc = Pc/(AfpVs) were obtained at frequencies between 10 and 104 Hz (Afp = footplate area). Zt is approximately characterized by a series circuit with compliance of 3 × 10−10 cm5/dyne, resistance of 1 × 106 dyne-sec/cm5, and mass of 3 × 10 g/cm4 .Zc is approximately equal to Zt from 500 to 5000 Hz, hence for this frequency range force applied to the stapes is all transmitted to the cochlear fluid. At lower frequencies the annular ligament absorbs most of the applied force. [Supported by grants from NIH and the Research Fund of the American Otological Society.]