SOUND-POWER COLLECTION BY THE AUDITORY PERIPHERY OF THE MONGOLIAN GERBIL MERIONES-UNGUICULATUS .3. EFFECT OF VARIATIONS IN MIDDLE-EAR VOLUME

The effects of variations in middle-ear cavity size on hearing sensiti vity in the Mongolian gerbil are predicted by computing the effect on acoustic power input to the middle ear. Acoustic power collection from a diffuse sound field was computed from measurements of the middle-ea r input impedance and external-ear radiation impedance and mathematica l models of the middle and external ear presented in the first two pap ers of this series [J. Acoust. Sec. Am. 92, 157-177 (1992); J. Acoust. Sec. Am. 99, 3044-3063 (1996)]. A reduction in middle-ear cavity volu me to 1/4 its normal value is predicted to cause a frequency-selective elevation in auditory threshold of as much as 12 dB, with the largest elevation occuring in the 1-2 kHz range. Greater reductions produce l arger threshold elevations. Increases in cavity volume cause decreases in the predicted threshold of at most 12 dB. Threshold predictions fo r volumes equal to those of the smaller hamster and the larger kangaro o-rat middle-ear cavity volumes resemble threshold functions measured in those animals. Results are consistent with the idea that large midd le-ear cavities evolved in gerbil to improve hearing sensitivity below 3 kHz and thereby improve the animal's chances for survival. (C) 1997 Acoustical Society of America.