Finite element simulation of broadband biosonar signal propagation in the near- and far-field of an echolocating Atlantic bottlenose dolphin (Tursiops truncatus)

Abstract

Bottlenose dolphins project broadband echolocation signals for detecting and locating prey and predators, and for spatial orientation. There are many unknowns concerning the specifics of biosonar signal production and propagation in the head of dolphins and this manuscript represents an effort to address this topic. A two-dimensional finite element model was constructed using high resolution CT scan data. The model simulated the acoustic processes in the vertical plane of the biosonar signal emitted from the phonic lips and propagated into the water through the animal's head. The acoustic field on the animal's forehead and the farfield transmission beam pattern of the echolocating dolphin were determined. The simulation results and prior acoustic measurements were qualitatively extremely consistent. The role of the main structures on the sound propagation pathway such as the air sacs, melon, and connective tissue was investigated. Furthermore, an investigation of the driving force at the phonic lips for dolphins that emit broadband echolocation signals and porpoises that emit narrowband echolocation signals suggested that the driving force is different for the two types of biosonar. Finally, the results provide a visual understanding of the sound transmission in dolphin's biosonar.

Echolocation in Bats, Odontocetes, Birds, and Insectivores

“…In dolphins and the harbor porpoise, a vestibular air sac (VS) is associated with the melon and also acts like a shield to preventing sound leakage. New results indicate that the melon of the harbor porpoise functions as an acoustic waveguide (Wei et al 2017(Wei et al , 2018.…”

Three-dimensional finite element simulation of acoustic propagation in spiral bubble net of humpback whale

“…This identified the existence of the 'walls of sound' encircling the 'quiet zones' Leighton had predicted in areas appropriate for trapping prey, for both circular 4,8 and spiral 6,13,18 bubble nets, using estimates of the sound speed in bubbly water 18,19 . Finite element methods have been widely applied in the fields of acoustic propagation and bioacoustics 20 , and this approach is suitable for examining the case of the humpback whale call within a bubble net of this size.…”

The Distinctive Forehead Cleft of the Risso’s Dolphin (Grampus griseus) Hardly Affects Biosonar Beam Formation

“…The forehead is composed of various anatomical structures (e.g., the melon and connective tissues) and plays a critical role in echolocation, since it forms a pathway for sound propagation from the phonic lips (i.e., ellipsoid fatty dorsal bursae located near the posterodorsal terminus of the melon) to the surrounding water [ 6 , 9 ]. The role of the forehead structures has been investigated in several odontocete species, including the short-beaked common dolphin ( Delphinus delphis ) [ 10 ], bottlenose dolphin ( Tursiops truncatus ) [ 8 , 11 , 12 , 13 ], harbor porpoise ( Phocoena phocoena ) [ 7 , 14 ], and baiji ( Lipotes vexillifer ) [ 15 ]. With a gradient in acoustic impedance formed by the fatty melon (acoustic impedance increases from inner core to outer layer) and connective tissues [ 16 ], the forehead structures act as an acoustic waveguide and a collimator to channel the sound.…”

“…With a gradient in acoustic impedance formed by the fatty melon (acoustic impedance increases from inner core to outer layer) and connective tissues [ 16 ], the forehead structures act as an acoustic waveguide and a collimator to channel the sound. They also provide impedance matching to transfer the acoustic energy between the animal forehead tissues and surrounding water [ 6 , 11 , 13 , 14 ]. However, all those odontocete species, whose biosonar click production and propagation have been studied, have a rounded and smooth forehead without a cleft.…”