Inner Ear Impedance (Cat) Determined by Time-Averaged Holography of the Round Window

Abstract

After amputation of the middle ear, a small closed system was built over the oval window, isolating it from the round window. Sound pressures applied to the stapes footplate were monitored by a probe microphone placed with its opening a few millimeters above the footplate. The round window was coated with a fine bronze powder to facilitate time-averaged holography. Vibration patterns of this window, obtained holographically, permitted calculating the volume displacement of the window for a given sound-pressure level and several frequencies between 300 and 6500 Hz. The small size of the window presented some initial technical difficulties. Inner-ear impedances were calculated on the assumption that the volume displacement of the round window should equal that of the oval window. Results are compared to those obtained in an earlier study [Tonndorf et. al., Ann. Otolaryng. 75, 752, (1966)] in which a capacitive probe has served for measuring round-window displacements, but the actual displacement pattern was unknown. [Supported by a number of HIH grants.]