Directionality of sound pressure transformation at the pinna of echolocating bats.

The directionality of sound pressure transformation at the pinna of three species of bats was studied by measuring the sound pressure level of a tone (25 45 65 and 85 kHz) at the tympanic membrane as a function of azimuth and elevation of the sound source under free-field conditions. The tympanic sound pressure level varied with location of the sound source. The directionality of sound pressure transformation pattern of the pinna of each bat was studied by plotting isopressure contours. The area within each isopressure contour decreased with increasing tonal frequency. For each tonal frequency, the point of maximal sound pressure was always located in the frontal ipsilateral sound field. This point shifted medially with increasing tonal frequency along the horizontal plane in all species tested, but it shifted in a species-specific manner along the vertical plane. Removal or distortion of the pinna and tragus resulted in either uncircumscribed or irregular isopressure contours for all tonal frequencies tested. Acoustic pressure gain of the external ear reached 16-23 dB for frequencies at 15-18 kHz. The importance of the external ear to the directionality of the bat's echolocation system is discussed.