High-frequency soundfield microphone for the analysis of bat biosonar

Numerous bat species emit wideband frequency-modulated signals for echolocation to hunt prey and avoid obstacles. Research investigating the behavioral and physiological responses of bats to echoes typically includes analysis of acoustic signals from microphones and/or microphone arrays, using time difference of arrival between array elements or the microphones to locate flying bats (azimuth and elevation). This has provided insight into transmission adaptations such as pulse duration and duty cycle with respect to target distance, clutter, and interferers. Microphones recording transmitted signals and echoes near a stationary bat provide sound pressure as a function of time but no directional information. In this work, the authors propose a spatial audio/soundfield microphone array to both track bats in flight and pinpoint the directions of echoes received by a bat. The authors introduce an ultrasonic (20-80 kHz) tetrahedral soundfield microphone to capture bat sounds up to 80 kHz. A spatial audio decoding technique called high angular resolution planewave expansion (HARPEx) supplies angle and elevation estimates, either for a flying bat based on the bat pulses or for targets based on echoes. Experiments using the soundfield microphone and HARPEx show that the approach accurately estimates the sound direction of arrival in both scenarios.