Computer modeling of acoustic beam formation in D e l p h i n u s d e l p h i s

Abstract

It has been established that some dolphins possess well-developed acoustic orientation (echolocation) and information gathering abilities, though substantially less is known about the system of sound generation and beam formation. Dolphins use a narrowly focused sound beam that emanates from the forehead and rostrum during echolocation. The primary objectives of this study were to simulate the effects of anatomical structure on beam formation, and to test the viability of various hypothetical sound source locations. Outlines from parasagittal x-ray CT scans were used to construct a 2-D model of the head of the common dolphin, Delphinus delphis. Finite difference techniques were used to simulate sound propagation through tissues modeled as inhomogeneous fluids. Preliminary simulations confirm that beam formation results primarily from reflection off of the skull and the skull-supported air sac surfaces. For the frequencies tested, beam angles best approximate those measured by experimental methods for a source located in a region of the model referred to as the monkey lip/dorsal bursae (MLDB) complex. The results suggest that: (1) the skull and air sacs play the central role in beam formation; (2) the geometry of reflective tissue is more important than the exact acoustical properties assigned; (3) a melon velocity profile of the magnitude tested is capable of mild focusing effects; and (4) experimentally observed beam patterns are best approximated at all frequencies simulated when the sound source is placed in the vicinity of the MLDB complex.