Acoustic Transmission Through the Head of the Common Mole, Talpa Europaea

Abstract

Cues for directional hearing in mammals are traditionally based on inter-aural intensity differences (IIDs) established as a result of the diffraction of a progressive sound wave by the head and pinna, or on inter-aural time differences (ITDs) established because of the separation of the two ears in space. This ‘dual hypothesis’ has been extended to other animal groups despite the fact that the majority of submammalian animals have head dimensions that are small relative to the wavelengths of the sounds used in communication. Under these conditions, little if any diffraction can occur and time differences become very small. As an alternative to the pressurereceiver principle of the mammalian ear, crickets (Michelsen, 1979) and the quail (Coles et al. 1980) use the pressure-difference principle for directional hearing. In a pressure-difference receiver, sound has access to both surfaces of the tympanic membrane (TM) because of an air-filled cavity between the ears which allows sound transmission through the head or body. When the sound pressure level (SPL) acting on the inner surface of the TM is equivalent to that acting on the external surface, the response of the TM (and, therefore, the cochlea) will be determined by the phase difference between the external and internal components. At any one frequency this phase difference will, in turn, vary with the angle of incidence of the sound.