Middle-ear mechanics in the CF-bat Rhinolophus ferrumequinum

The acoustic vibrations of the eardrum at the umbo and of the stapes have been measured in the greater horseshoe bat. The displacement amplitude response of the eardrum shows a second-order low-pass characteristic, typical of a lumped mass and stiffness system with a resonance frequency of about 55 kHz. The effective mass was calculated to be about 8 micrograms, and the specific stiffness 40 X 10(6) dyne/cm3, which is one hundred times greater than guinea pig. The measured level ratio appears to be greater (3X - 5X) than the geometric ratio (2X) probably due to flexing of the manubrium. The umbo-stapes phase lag exceeds 1 cycle at high frequencies, suggesting a system of at least four reactances. This is not consistent with the relatively slight change in lever ratio with frequency. One possibility for reconciling the two results is that the distributed mass and stiffness of the ossicles act as a transmission line for transverse vibrations. There is no evidence for a sharply peaked middle-ear response (although it is more sharply tuned than some species), nor for resonant absorption by the cochlea in the region of 83 kHz - the 'constant' frequency of this bat. The eardrum shows theoretically optimal matching to the air at 55 kHz and is reasonably efficient from 15 kHz to at least 110 kHz.