Acoustical Aspects of Hearing and Echolocation in Bats

Abstract

The structure of the external ear of Microchiropteran bats shows a remarkable diversity, particularly in the shape and size of the pinna. There is also a sharp contrast between the highly mobile pinnae of Rhinolophids and Hipposiderids compared to the almost immobile pinnae of Megadermatids. The presence of "large" ears (pinnae) is often associated with a gleaning type of foraging behaviour, including the ability to detect and capture prey by passive listening (Neuweiler, 1984). Within this context, the Australian false vampire or ghost bat Macroderma gigas, has extremely large pinnae, possibly the largest amongst the Microchiroptera and the acoustical properties of the external ear have been examined. Similar data has been obtained in Gould's long-eared bat Nyctophilus gouldi, which is also probably a gleaning bat similar Antrozous eallidu~ (Bell, 1982). In fresh specimens of ~~igas and N. gouldi, the tympanic membranes were removed and replaced by a small calibrated microphone (Bruel and Kjaer Type 4138) inserted through the bulla, leaving the external ear intact. Under normal conditions in M. gigas, there is a rapid increase in the amplification of sound pressure at the tympanic membrane, relative to the free field, for frequencies above 3kHz. For speaker positions on the acoustic axis of the ear, peak amplification of 20-30dB occurs between 5-12kHz, gradually declining to less than 10dB above 40kHz. In comparison, the pressure gain in the ear canal of N. gouldi rises rapidly above 5kHz to peak values of 15-23dB between 7-22kHz. In both species, removal of the pinna results in a loss of pressure in the ear canal and the gain of the pinna can be estimated by the difference curve (Fig. 1). Pinna gain curves resemble the gain characteristics of a finite-length acoustic horn (Beranek, 1954). Expected gain curves were calculated for a series of finite horns as shown in Fig. 1 and the pinnae of M. gigas and N. gou~! are, to a first approximation, similar to conical horns, at least in the low to mid frequency range. The pinnae have similar cone angles (~=28D), although they are irregularly shaped and asymmetrical. The maximum pressure gain at the throat of each pinna, as an efficient horn, would be expected to be 13dB, based on the areal ratio of the pinna mouth and throat (entrance to the ear canal) and is close to the observed values. The larger size of the pinna in M. ~ (for dimensions see Fig.1 legend) produces a gain curve which rises about an octave earlier than N. gouldi. However in both cases, pressure gain starts to increase rapidly as the ratio of the circumference of the pinna mouth to the wavelength approaches unity (ka=1; k=wavenumber 2~/A; a=radius). The peak in pressure gain near 5kHz and 7kHz for M. gigas and 289