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

Abstract

AbstractPsychoacoustic 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.Keywords:: dolphinbiosonar functional morphologybeam formationsound productionfinite element modelmelon function AcknowledgementsThis work was part of a larger effort. Acquiring specimens is one of the most difficult and important aspects of attempting a project such as this. We are very appreciative for assistance with specimens provided by Dr Judy St Leger, DVM at SeaWorld, San Diego.The CT scan of the live dolphin was conducted at the US Navy Marine Mammal Program, Space and Naval Warfare Systems Center (SSC) Pacific, Code 7151, 53560 Hull Street, San Diego, California 92152, USA. The study followed a protocol approved by the Institutional Animal Care and Use Committee at SSC Pacific. We are indebted to the veterinary staff at the National Marine Mammal Foundation, San Diego for allowing us to use their live animal data to accomplish our research goals. Many thanks to Drs Sam H. Ridgway and Eric Jensen, DVM.The CT scans on the postmortem dolphin were conducted at the University of California, San Diego Medical School, under the supervision of Dr. Robert F. Mattrey and Jacqueline Corbeil.We also thank Dr John Hildebrand from Scripps Institution of Oceanography and Dr Robert Gisiner, currently at the Naval Facilities Engineering and Expeditionary Warfare Center for their initial support and past contributions to this work.We greatly appreciate the efforts of Wim Verboom and two anonymous reviewers who significantly improved the manuscript with their critiques.Supplemental materialSupplemental material for this article is available via the supplemental tab on the article's online page at http://dx.doi.org/10.1080/09524622.2013.843061. This is a high-resolution version of Figure 4.Additional informationFundingThe funding was provided in grants to Dr Ted W. Cranford of San Diego State University (Grant numbers N00244-09-1-0072 and N00244-10-1-0054) by Dr Frank Stone and Capt. Ernie Young (USN Ret.) at the Chief of Naval Operations, Environmental Readiness Division [CNO45], as well as Dr Curtis Collins and Cmdr John Joseph (USN Ret.) from the Naval Postgraduate School.