A model of the odontocete middle ear

The high acoustic sensitivity of the bottlenose dolphin is physically defin ed and related to the anatomy of the middle car. The paper presents a conce ptual and parametric analysis of the demands imposed by this high sensitivi ty upon the middle ear mechanisms: the head and the middle ear structures m ust collect sound energy from a large area and concentrate it onto the oval window. Assuming that the specific input impedance of the mammalian cochle a is relatively constant, and smaller than the characteristic acoustic impe dance of water, we find that the impedance matching task of the cetacean mi ddle car is very different from that of terrestrial mammals: instead of a l arge pressure amplification, cetaceans need amplification of particle veloc ity. Our mechanical four-bone model of the odontocete middle ear is based o n the anatomy of the tympano-periotic complex and consists of four rigid bo ne units (tympanic bone, the malleus-incus complex, stapes, periotic bone) connected through elastic junctions. The velocity amplification is brought about by lever mechanisms and elastic couplings. The model produced velocit y amplifications ranging from 7- to 23-fold when provided with middle ear p arameters from the sir; odontocete species for which audiograms are availab le. The model reproduces the complete audiograms of these six species fairl y well for frequencies up to about 100-120 kHz. (C) 1999 Elsevier Science B .V. All rights reserved.