Sound Localization in the Owl

Abstract

Finding the source of a sound by ear is called sound localization, and most animals, including nocturnal owls, that use sound signals for communication, prey capture, and predator evasion can localize sound. Sound localization and its brain mechanisms are better known in the barn owl than in any other animal. The barn owl can localize sound equally well in two dimensions, azimuth, or the horizontal plane, and elevation, or the vertical plane. Experiments show that the barn owl uses interaural time difference for determining sound azimuth and interaural intensity difference for elevation. When a sound signal reaches one ear before the other ear, an interaural time difference results. Sound travels 1 cm in 29 μsec. The maximum interaural time difference experienced by the barn owl is about 170 μsec when sound propagates along the aural axis; the minimum time difference is zero when sound emanates from a place equidistant to the two ears. Thus, interaural time disparity varies systematically as a function of sound azimuth. Interaural intensity difference is due to the head’s shadow in the sound field. The ear in the shadow registers a weaker signal than the other ear. Given a constant head size, shorter wavelengths and thus higher frequencies produce greater intensity differences than longer wavelengths. The barn owl needs relatively high frequencies (5–8 kHz) for accurate localization in elevation. Because the left ear is situated higher on the head than the right one in the barn owl, interaural intensity disparity varies as a function of sound elevation.