Static and Dynamic Impedance of the Chinchilla Middle Ear

Abstract

Two electroacoustic bridges were used to obtain static acoustic impedance measures on 41 chinchilla. These measures were obtained awake and under varying doses of sodium pentobarbital. The mean impedance of the awake chinchilla is 880 acoustic ohms. The effect of sedation is twofold: (1) a negative pressure is developed in the middle ear; (2) the static impedance is decreased. Since a negative pressure is normally associated with an increase in impedance, a possible explanation of the seemingly contradictory results is represented. The acoustic reflex threshold was measured on 27 ears using stimuli of 0.5, 2, 4, and 8 kHz and signal durations of 10, 20, 50, 100, 200, and 500 msec. The acoustic reflex of the chinchilla is approxmately 20 dB more sensitive than man's after the stimulus is corrected for the transmission characteristics of the chinchilla external meatus. The reduction in stimulus duriaton from 500 to 10 msec elevated the reflex threshold by approximately 25 dB.