Acoustic modelling of dolphin sound reception and implications for biosonar design

Odontocetes use active sonar for echolocation, navigation and socialisation. Their sonar is characterised by narrow transmission and reception directivity patterns, over a variety of ranges and frequencies. Typical echolocation clicks of bottlenose dolphins (Tursiops truncatus) last between 50-200 mu s, with a broad frequency range of similar to 100-170 kHz depending on circumstances. These characteristics are very attractive for the design of bio-inspired sonars, but the actual mechanisms of sound reception are not well understood. Physiological and behavioural evidence suggests that dolphins hear the echoes of their high-frequency clicks through their lower jaws. The angular precision predicted by this theory is however much less than dolphins have been observed to display. A recent hypothesis is that the teeth also play a part in sound reception, acting as a passive beam-forming structure. This paper presents 2-D models of acoustic propagation in a dolphin jaw, based on real measurements, and shows the importance of multiple scattering between teeth, potential masking effects, and the match between theoretical directivity and that observed in the field. We use these results to look at the implications for realistic biosonar design.