Frequency characteristics of sound transmission in middle ears from Norwegian cattle, and the effect of static pressure differences across the tympanic membrane and the footplate.

Abstract

For 23 cadaver ears from Norwegian cattle, frequency characteristics for the round-window volume displacement relative to the sound pressure at the eardrum have been measured, and are compared to earlier results for human ears [M. Kringlebotn and T. Gundersen, J. Acoust. Soc. Am. 77(1), 159-164 (1985)]. For human as well as for cattle ears, mean amplitude curves have peaks at about 0.7 kHz. At lower frequencies, the mean amplitude for cattle ears is about 5 dB smaller than for human ears. The amplitude curves cross at about 2 kHz, and toward higher frequencies the amplitude for cattle ears becomes increasingly larger. If amplitude curves are roughly approximated by straight lines above 1 kHz, the slope for cattle ears is about -5 dB/octave as compared to about -15 dB/octave for human ears. The phase of the round-window volume displacement lags behind the phase of the sound pressure at the tympanic membrane. The phase lag is close to zero below 0.2 kHz, but increases to about 3.5 pi at 20 kHz for cattle ears, as compared to less than 2 pi for human ears. Further investigations are needed in order to explain the observed differences. Sound transmission in the ear decreases with an increasing static pressure difference across the tympanic membrane, especially at frequencies below 1 kHz, where pressure differences of 10 and 60 cm water cause mean transmission losses of about 10 and 26 dB, respectively, the losses being somewhat larger for overpressures than for underpressures in the ear canal. At higher frequencies, the transmission losses are smaller. For small overpressures, and in a limited frequency range near 3 kHz, even some transmission enhancement may occur. Static pressure variations in the inner ear have only a minor influence on sound transmission. Static pressures relative to the middle ear in the range 0-60 cm water cause mean sound transmission losses less than 5 dB below 1 kHz, and negligible losses at higher frequencies.