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Validation of a vibroacoustic finite element model using bottlenose dolphin simulations: The dolphin biosonar beam is focused in stages Vanessa Trijoulet 1,3 , Ted Cranford 2 and Petr Krysl 3

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Materials and methods

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Materials and methods

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

Vanessa Trijoulet1,3, Ted Cranford2 and Petr Krysl3

1Department of Mathematics and Statistics, University of Strathclyde, Glasgow, Scotland (work undertaken at the Centre d’Océanologiede Marseille) – [email protected]

2Department of Biology, San Diego State University, USA

3Department of Structural Engineering, Jacobs School of Engineering, University of California San Diego, USA

bursae

melon

brain

melon

  • Objectives

  • Validate the model created as representative of the echolocation in bottlenose dolphins (Tursiops truncatus) by comparing our results to the live dolphin results of Au et al. (1986).

  • The transmitting beam of -3 dB is directed forward and upward by 5° with a beam width between 5° and 8° in the vertical and horizontal planes respectively and with a Directivity Index (DI) of 26.5 dB.

  • Study the function of the organs such as the melon, the skull and the air sacs in the beam formation and compare these results with the literature (Evan et al., 1964; Norris, 1964, 1969; Karol et al., 1978; Aroyan et al., 1992; Au, 2000).

  • The melon is able to focus the beam

  • The skull and the air sacs act as reflectors

  • Study the effect of small changes in the position of the sound sources.

Introduction

Hearing in toothed whales is divided in 3 components: the sound generation, the sound reception and the central nervous system.

The sound source in odontocetes was identified as the Monkey Lip/Dorsal bursae complex (Cranford, 1988, 1992, 1996, Cranford et al., 2000, 2011). The biosonar click is created as pressurized air is pushed between the labia of the phonic lips and makes them slap together.

Echolocation clicks have been recorded with captive dolphins (Au et al., 1986) and the organs in the dolphin’s head play a role in the beam formation (Norris, 1964, 1969, Karol et al., 1978, Aroyan et al., 1992, Au, 2000).

In the current context of the possible impact of anthropogenic sound on cetaceans it is necessary to better understand the biosonar apparatus of odontocetes. Since experiments with live specimens may be difficult and require considerable resources, modelling becomes a good way to counter these problems.

Introduction

Hearing in toothed whales is divided in 3 components: the sound generation, the sound reception and the central nervous system.

The sound source in odontocetes was identified as the Monkey Lip/Dorsal bursae complex (Cranford, 1988, 1992, 1996, Cranford et al., 2000, 2011). The biosonar click is created as pressurized air is pushed between the labia of the phonic lips and makes them slap together.

Echolocation clicks have been recorded with captive dolphins (Au et al., 1986) and the organs in the dolphin’s head play a role in the beam formation (Norris, 1964, 1969, Karol et al., 1978, Aroyan et al., 1992, Au, 2000).

In the current context of the possible impact of anthropogenic sound on cetaceans it is necessary to better understand the biosonar apparatus of odontocetes. Since experiments with live specimens may be difficult and require considerable resources, modelling becomes a good way to counter these problems.

Materials and methods

Vibroacoustic finite element model (FEM) + CT scans of a post-mortem and a live dolphin’s head

(Krysl et al., 2008)

Make the pair of right bursae slap together

Creation of a sound beam

Hydrophone screen to record sound pressure levels

Different simulations moving the position of the bursae and looking at the function of the different organs in the beam formation


Materials and methods

Results 1/2

The melon helps focusing the beam

  • The organs help to focus the beam

The skull acts as reflector

Melon filled with sea water

Smoothed melon

(heterogeneous in density)

Homogeneous melon

Skull-only

Skull + all the tissues

  • Formation of a consistent beam with the skull alone

  • “telescoping process” (Miller, 1923)

  • Organs play a significant role focusing the beam

  • The heterogeneity in density in the melon plays a significant role in the beam formation

The air sacs act as reflector

  • Our results

  • Results of Au et al. (1986)

Post-mortem dolphin

  • Results slightly wider in both planes but consistent with live dolphin experiments

Air sacs added in the CT scan

Original CT scan

  • Addition of air spaces by hands in the CT scan is responsible for collection and redirection of the acoustic energy forward

Live dolphin


Materials and methods

Results 2/2

  • Conclusion

  • The model is able to create a beam

  • +

  • Results close to the live dolphin experiment results

  • +

  • Confirm the conclusions of previous studies about the role of the different organs in the beam formation

  • Model validated as representative of the biosonar beam generation in Tursiops truncatus

  • The acoustic transmission beam seems manufactured in a series of stages: the beam is largely determined by the overall geometry of the skull and soft tissue structures.

  • These results attest the value of FEM as an investigative technique for understanding the functional morphology of sound propagation in a dolphin’s head.

  • Future directions

  • Study of the mechanisms that produce frequency structure in the transmitting beam.

  • Validation of the model for sound reception in the bottlenose dolphin already underway.

  • For further details

  • Cranford, T., Trijoulet, V., Smith, C., Krysl, P., Validation of a vibroacoustic finite element model using bottlenose dolphin simulations: The dolphin biosonar beam is focused in stages, Bioacoustics, in review.

The position of the sound sources is important to the beam formation

  • A small change (≈5mm) in the position of the bursae induces significant changes in the shape of the transmitting beam

  • Acknowledgements

  • This project was supported by a grant to Dr. Ted Cranford, by Dr. Frank Stone and Capt. Ernie Young at the Chief of Naval Operations, Environmental Readiness Division; and Dr. Curtis Collins and Cmdr. John Joseph from the Naval Postgraduate School. Assistance with specimens and CT scans were provided by SeaWorld, San Diego and the Navy Marine Mammal Program at SPAWAR Pacific in San Diego.


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