Acoustic effects of middle-ear cavities in the cat family

Abstract

The acoustic effect of the middle-ear air-filled space is to add an impedance in series with that of the tympanic membrane and ossicular chain. In the cat family, a bony septum divides the space into two volumes, coupled through a small opening (foramen); this configuration is well modeled as a lumped circuit of two parallel compliances (the cavities) coupled by an acoustic mass and resistance (the foramen). The model impedance magnitude has a minimum and a maximum, whose frequencies are predictable from structure. A narrow-band notch in hearing sensitivity near 2–4 kHz is caused by the pole. Measurements of cavity impedance in four felid species are consistent with this structure-based model. Acoustic measurements in ears of three species with the septum removed show that the minimum and maximum in impedance magnitude are eliminated, as predicted by the lumped model. However, new impedance maxima appear at 8–12 kHz; these maxima are assumed to result from distributed resonances in the enlarged cavity. Thus the inferred effect of the septum on hearing is to introduce a notch in the midfrequencies, while avoiding such notches at higher frequencies where they could interfere with sound-source localization. [Work supported by NIH and NSF.]