Modeling and interpreting the head-related transfer function to understand directional hearing in bottlenose dolphins

Abstract

Toothed whales have evolved to communicate, forage, and navigate effectively underwater using sound. It is generally accepted that toothed whales receive sounds through their lower mandible and the associated fat body, which guide sound to the tympano-periotic complexes (TPCs) enclosing the cochleae. However, little is known about how the direction of an impinging sound wave affects acoustic interactions with these and other structures in the head to alter the signals driving the left and right TPCs. In this work, we constructed a three-dimensional head model using computed tomography (CT) images of a live bottlenose dolphin. Using a finite-element model to simulate sound-structure interactions, we computed how left and right TPC signals vary with sound direction for multiple frequencies to generate dolphin head-related transfer functions (HRTFs). The simulated HRTFs vary strongly with frequency. Importantly, HRTFs for sources off midline exhibit complex frequency-dependent differences, which are acoustic features that could be used to estimate sound source location. We also observed scenarios where interaural level differences (ILDs) may not be reliable directional cues. Results like these can identify which acoustic cues, at which frequencies, support robust directional hearing in toothed whales. [Work supported by ONR.]