Anthony, Adam

Direct measurements were made of the displacement of the guinea-pig eardrum when acted upon by acoustic pressures (200–6000 cps). The mechanical nonlinearities of the ear were studied by introducing a static pressure (0 to ±3000 dyn/cm2) in the middle-ear cavity. Below 1000 cps, the ratio of the tympanum displacement to the sound pressure was frequency-independent, with a value of about 4×10−7 cm3/dyn. Above 1000 cps, this ratio exhibited a series of maxima and minima. A static-pressure difference across the tympanum changed the frequency at which the maximum occurred and generally decreased the displacement/pressure ratio. It was concluded (1) for low frequencies, the lumped-parameter description of the ear as a cavity was valid; (2) the tympanum acts as a flexible, conical plate under slight tension; (3) the measurements were best explained in terms of two nonlinearities: hysteresis and a nonlinearity of the elastic properties of the ear.

By use of a high-power, corona type loudspeaker we have been able to demonstrate behavioral and endocrine responses of laboratory rodents to measured high sound-pressure levels at the frequencies at which their hearing is most sensitive, namely, 10 to 40 kc. The corona type loudspeaker behaves in the fashion predicted by the theoretical analysis reviewed in this paper. It is limited by extremely low efficiency (2.5%) and high production of ozone. At high frequencies it is limited by the volume of the corona, and at low frequencies by the failure of the quasi-adiabatic condition within the corona. Despite these limitations its performance has not been equaled by other types of loudspeakers; it is a unique tool for studying the responses of laboratory rodents to sound fields.