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

Validation of a vibroacoustic finite element model using bottlenose dolphin simulations:
The dolphin biosonar beam is focused in stages
Vanessa
1,3
Trijoulet ,
Ted
2
Cranford
and Petr
3
Krysl
1Department
of Mathematics and Statistics, University of Strathclyde, Glasgow, Scotland (work undertaken at the Centre d’Océanologie de 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
Objectives
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
Lip/Dorsal bursae
bursae complex
complex(Cranford,
(Cranford,1988,
1988,1992,
1992,1996,
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.
• 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.
bursae
melon
brain
melon
Materials and methods
Vibroacoustic finite element model (FEM)
(Krysl et al., 2008)
+
CT scans of a post-mortem and a live dolphin’s head
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
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Results 1/2
The melon helps focusing the beam
The skull acts as reflector
The organs help to focus the beam
Melon filled with sea water
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
Homogeneous melon
Smoothed melon
(heterogeneous in density)
The heterogeneity in density in the melon plays a significant role in the beam formation
The air sacs act as reflector
Post-mortem dolphin
Our results
Results of Au et al. (1986)
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
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Results 2/2
The position of the sound sources is important to the beam formation
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.
A small change (≈5mm) in the position of the bursae induces significant changes in the
shape of the transmitting beam
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.
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.
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.
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