Validation of a vibroacoustic finite element model using bottlenose dolphin simulations: the dolphin biosonar beam is focused in stages

Abstract

Psychoacoustic laboratory studies with live dolphins require considerable resources and are essential for assessing the validity of our models. Computerized numerical modelling methods are a reasonable approach to simulate the vibroacoustic functions of the dolphin biosonar apparatus. In order to validate this approach, we chose a vibroacoustic finite element model to simulate sound production and sound beam formation in the bottlenose dolphin (Tursiops truncatus), based on computed tomography scans from live and postmortem dolphins. The right and left dorsal bursae were assumed to be potential sound sources. The simulations confirm several hypotheses: (1) the shape of the skull plays a role in the formation of the sound transmission beam; (2) the melon appears to concentrate the acoustic energy by a factor of four in the transmitted beam; (3) focusing the sound beam apparently occurs in a series of stages that include contributions from the skull, nasal diverticula, melon and connective tissue structures. An unexpected result is that adjustments to the focus and direction of the sound beam can result from small (millimetre scale) changes in the relative position of the anterior and posterior bursae within each sound generation complex. Comparing our results with those from dolphin psychoacoustic experiments establishes validation of our vibroacoustic model. The potential for varied effects from anthropogenic sound also emphasizes the importance of developing vibroacoustic modelling. These numerical modelling tools complement experimental data for determining exposure thresholds and may allow us to simulate exposure levels, from moderate to extreme, without impacting live animals.