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Abstract
Harbor porpoises (Phocoena phocoena) use narrow band echolocation signals for detecting and locating prey and for spatial orientation. In this study, acoustic impedance values of tissues in the porpoise's head were calculated from computer tomography (CT) scan and the corresponding Hounsfield Units. A two-dimensional finite element model of the acoustic impedance was constructed based on CT scan data to simulate the acoustic propagation through the animal's head. The far field transmission beam pattern in the vertical plane and the waveforms of the receiving points around the forehead were compared with prior measurement results, the simulation results were qualitatively consistent with the measurement results. The role of the main structures in the head such as the air sacs, melon and skull in the acoustic propagation was investigated. The results showed that air sacs and skull are the major components to form the vertical beam. Additionally, both beam patterns and sound pressure of the sound waves through four positions deep inside the melon were demonstrated to show the role of the melon in the biosonar sound propagation processes in the vertical plane.
Highlights
Numerical acoustic models based on physics and mathematics can be used to gain further understanding of the acoustic processes of sound production and propagation in the head of dolphins and porpoises
The far field transmission beam pattern in the vertical plane and the waveforms of the receiving points around the forehead were compared with prior measurement results
The properties of the beam pattern in the far field and the waveforms of the receiving points around the forehead were compared with prior measurement results, the simulation results were consistent with the measurement results from the same species but different individuals with different head sizes and different species
Summary
Numerical acoustic models based on physics and mathematics can be used to gain further understanding of the acoustic processes of sound production and propagation in the head of dolphins and porpoises. The numerical modeling has been used for investigating sound production, transmission and reception on different species of odontocetes, including short-beaked common dolphin (Delphinus delphis) (Aroyan et al, 1992; Aroyan, 2001), Cuvier’s beaked whale (Ziphius cavirostris) (Cranford, 2000), humpback whales (Megaptera novaeangliae) (Adam et al, 2013), bottlenose dolphin (Tursiops truncatus) (Cranford et al, 2014), baiji (Lipotes vexillifer) (Wei et al, 2014; Wei et al, 2016). The physiological mechanism of the biosonar beam formation in an echolocating harbor porpoise’s head is still not well understood
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