EFFECTS OF PINNA POSITION ON HEAD-RELATED TRANSFER-FUNCTIONS IN THE CAT

To measure the effects of the pinna position on spectral sound localiz ation cues, the head-related transfer function (HRTF) from the free-fi eld to a point in the ear canal was measured for anesthetized cats wit h their pinnae in three positions: the relaxed, anesthetized position; pulled forward into an approximation of the ''alert cat'' posture; an d pulled back. The general Features of HRTFs are not changed by moving the pinna, although the mapping of particular HRTF Features onto dire ctions in space is changed. As an approximation, the pinna behaves lik e a Fixed-shape sound collector, so that HRTFs shift with the pinna wh en it moves; however, pinna movement changes some quantitative details of HRTFs beyond what is predicted by this approximation. When viewed as directional gain, pinna movements serve to optimize listening condi tions. However, when considering sound localization, pinna movements l ead to ambiguities regarding source location. If pinna position is not incorporated into the computation, and spectral cues alone are used f or localization, the ambiguity is about 60 degrees in azimuth and 30 d egrees in elevation. Pinna movements produce similar azimuthal ambigui ty in interaural level differences. Interaural time difference cues co uld be used to reduce the ambiguity in azimuth, but a knowledge of pin na position seems to be necessary to resolve ambiguities in elevation. (C) 1996 Acoustical Society of America.