410
J.L. Aroyan et al.
2. Three-Dimensional Numerical Simulation of
Biosonar Emission in the Common Dolphin
Measurements of the acoustic field of echolocating dolphins have demonstrated that dolphins emit a rapid series of intense pulses in a narrowly
focused beam that emanates from the forehead and rostrum. Despite application of a variety of experimental techniques, the exact mechanisms
involved in the generation, emission, and reception of delphinid biosonar
signals have remained conjectural. Advances in the methodology of bioacoustic simulations have led to powerful combinations of techniques
capable of addressing questions that have proven difficult to resolve experimentally. Aroyan (1996) combined methods for three-dimensional (3-D)
acoustic simulation and far-field extrapolation with a novel approach to the
mapping of acoustic tissue parameters from X-ray computed tomographic
(CT) data. These techniques, applied to models of the forehead and lower
jaw tissues of the common dolphin, Delphinus delphis, enabled a detailed
study of the patterns of sound propagation within the modeled tissues and,
hence, of the acoustic principles operating in the biosonar emission and
reception processes. The following sections discuss the methods used in this
investigation and present a series of results concerning the location of the
biosonar signal source tissues, the roles of the skull, air sacs, and soft tissues
(including the melon) in beam formation, and the acoustical consequences
of forehead asymmetry in Delphinus delphis.
2.1 Computational Methods
The following approach was used to investigate the D. delphis emission
system. First, computer models of the tissues of the dolphin's head were
constructed using a novel mapping of both tissue density and acoustic velocity from X-ray CT attenuation data to a simulation grid. Second, 3-D finite
difference programs were used to simulate acoustic propagation into the
tissue models to locate the source region of the dolphin's biosonar emissions (analogous to the use of inverse seismologic simulations to pinpoint
the underground source point or "hypocenter" of an earthquake, or to
reversing the emission/collection role of an optical lens system). Third,
sources were placed at these inverse simulation "hypocenters" or at conjectured anatomical source locations, and finite difference programs were
again used to propagate the acoustic field of the source and tissue models
out to a surface surrounding the tissue region of the grid. Fourth, boundary extrapolation programs were used to compute the emitted acoustic farfield from the pressure and its normal derivative over this surface. Biosonar
mechanisms were investigated by visualizing the acoustic energy density
within a variety of tissue models and by comparing the fields emitted by
these models to the experimentally measured sonar emissions of live
animals. Conceptually, this approach involves: (1) contructing the best pos-
J.L. Aroyan et al.
2. Three-Dimensional Numerical Simulation of
Biosonar Emission in the Common Dolphin
Measurements of the acoustic field of echolocating dolphins have demonstrated that dolphins emit a rapid series of intense pulses in a narrowly
focused beam that emanates from the forehead and rostrum. Despite application of a variety of experimental techniques, the exact mechanisms
involved in the generation, emission, and reception of delphinid biosonar
signals have remained conjectural. Advances in the methodology of bioacoustic simulations have led to powerful combinations of techniques
capable of addressing questions that have proven difficult to resolve experimentally. Aroyan (1996) combined methods for three-dimensional (3-D)
acoustic simulation and far-field extrapolation with a novel approach to the
mapping of acoustic tissue parameters from X-ray computed tomographic
(CT) data. These techniques, applied to models of the forehead and lower
jaw tissues of the common dolphin, Delphinus delphis, enabled a detailed
study of the patterns of sound propagation within the modeled tissues and,
hence, of the acoustic principles operating in the biosonar emission and
reception processes. The following sections discuss the methods used in this
investigation and present a series of results concerning the location of the
biosonar signal source tissues, the roles of the skull, air sacs, and soft tissues
(including the melon) in beam formation, and the acoustical consequences
of forehead asymmetry in Delphinus delphis.
2.1 Computational Methods
The following approach was used to investigate the D. delphis emission
system. First, computer models of the tissues of the dolphin's head were
constructed using a novel mapping of both tissue density and acoustic velocity from X-ray CT attenuation data to a simulation grid. Second, 3-D finite
difference programs were used to simulate acoustic propagation into the
tissue models to locate the source region of the dolphin's biosonar emissions (analogous to the use of inverse seismologic simulations to pinpoint
the underground source point or "hypocenter" of an earthquake, or to
reversing the emission/collection role of an optical lens system). Third,
sources were placed at these inverse simulation "hypocenters" or at conjectured anatomical source locations, and finite difference programs were
again used to propagate the acoustic field of the source and tissue models
out to a surface surrounding the tissue region of the grid. Fourth, boundary extrapolation programs were used to compute the emitted acoustic farfield from the pressure and its normal derivative over this surface. Biosonar
mechanisms were investigated by visualizing the acoustic energy density
within a variety of tissue models and by comparing the fields emitted by
these models to the experimentally measured sonar emissions of live
animals. Conceptually, this approach involves: (1) contructing the best pos-