The bat head-related transfer function reveals binaural cues for sound localization in azimuth and elevation

TLDR

The study measured head‑related transfer functions (HRTFs) of four intact Eptesicus fuscus bats, re‑measuring them without the pinna and tragus to isolate the contributions of external ear structures and to examine how spatial and spectral features interact. The results show that the bat’s pinna and tragus generate frequency‑dependent gain and directionality, that interaural level differences and monaural spectral cues vary with azimuth and elevation, and that the interaction of these multiple cues improves reliable sound‑source localization in both horizontal and vertical planes.

Abstract

Directional properties of the sound transformation at the ear of four intact echolocating bats, Eptesicus fuscus, were investigated via measurements of the head-related transfer function (HRTF). Contributions of external ear structures to directional features of the transfer functions were examined by remeasuring the HRTF in the absence of the pinna and tragus. The investigation mainly focused on the interactions between the spatial and the spectral features in the bat HRTF. The pinna provides gain and shapes these features over a large frequency band (20–90 kHz), and the tragus contributes gain and directionality at the high frequencies (60 to 90 kHz). Analysis of the spatial and spectral characteristics of the bat HRTF reveals that both interaural level differences (ILD) and monaural spectral features are subject to changes in sound source azimuth and elevation. Consequently, localization cues for horizontal and vertical components of the sound source location interact. Availability of multiple cues about sound source azimuth and elevation should enhance information to support reliable sound localization. These findings stress the importance of the acoustic information received at the two ears for sound localization of sonar target position in both azimuth and elevation.