12
w.w.L. Au
the fatty melon of the dolphin forehead may couple sounds from within the
animal's head into the water. The tissue of the melon actually invades the
muscles of the nasal plug and other areas of the nasal passage, making it
possible for a direct link between a source associated with the monkey lips
and the rest of the melon. The melon and jaw fat are composed of a translucent lipid very rich in oil, which is completely different from other tissue.
The oil-rich lipid has been referred to as an "acoustic tissue," and the velocity gradient of the lipid has been speculated to focus outgoing acoustic emissions (Varanasi and Malin 1971). Sonar transducers are often constructed
with a rho-c rubber housing to contain a low viscosity mineral oil (a lossless acoustic medium) to couple the piezoelectric sensing elements into the
water and vice versa (Wilson 1985).
Norris and Harvey (1974) found a low-velocity core surrounded by a
higher velocity shell in the melon of T. truncatus (Figure 1.5), a finding that
has been substantiated for other dolphins and porpoises by computed
tomography (CT) and magnetic resonance imaging (MRI) scans performed
by Cranford (Chapter 3). The low-velocity core consisted of pellucid oily
fat surrounded by denser blubber that is heavily invaded with connective
tissue fibers near the surface. Such a velocity profile could act like a sound
channel, causing sounds propagating in the core to be refracted inward. This
type of structure will also provide a gradual impedance transformation from
the nasal plug region to the water.
The roles of the melon and the different structures in the head of a
dolphin are addressed by Aroyan et al. (Chapter 10), who numerically solve
the three-dimensional wave equation to show that the bony structure of the
head, the air-sacs, and the melon all work toward focusing the outgoing
acoustic energy in a beam that is directed forward and elevated slightly,
similar to the measured beam patterns of echolocating dolphins (Au 1993).
They also demonstrate that if the source was at the larynx, the outgoing
acoustic energy would not form into a beam that is pointed directly forward.
4.3 Sound Production by Sperm Whales
The sound production mechanism of sperm whales is discussed in detail by
Cranford (Chapter 3) and their use of sounds is discussed by Tyack and
Clark (Chapter 4). The geometry and physical properties of the forehead
of a sperm whale (which makes up about one-third of the body length)
make it a very unusual structure for sounds to propagate through. A
schematic of a planar cross-sectional view of the internal structure of a
sperm whale forehead from a dorsal-ventral perspective, along with a threedimensional anterior-lateral cut-away view are shown in Figure 1.6. The
museau de singe is situated at the anterior portion of the forehead, adjacent
to the distal and parabolic frontal air sac. The spermaceti organ is an elongated bag containing a complex mass of oil-filled connective tissue enclosed
by a membrane case that is surrounded by muscle and blubber. Norris and
w.w.L. Au
the fatty melon of the dolphin forehead may couple sounds from within the
animal's head into the water. The tissue of the melon actually invades the
muscles of the nasal plug and other areas of the nasal passage, making it
possible for a direct link between a source associated with the monkey lips
and the rest of the melon. The melon and jaw fat are composed of a translucent lipid very rich in oil, which is completely different from other tissue.
The oil-rich lipid has been referred to as an "acoustic tissue," and the velocity gradient of the lipid has been speculated to focus outgoing acoustic emissions (Varanasi and Malin 1971). Sonar transducers are often constructed
with a rho-c rubber housing to contain a low viscosity mineral oil (a lossless acoustic medium) to couple the piezoelectric sensing elements into the
water and vice versa (Wilson 1985).
Norris and Harvey (1974) found a low-velocity core surrounded by a
higher velocity shell in the melon of T. truncatus (Figure 1.5), a finding that
has been substantiated for other dolphins and porpoises by computed
tomography (CT) and magnetic resonance imaging (MRI) scans performed
by Cranford (Chapter 3). The low-velocity core consisted of pellucid oily
fat surrounded by denser blubber that is heavily invaded with connective
tissue fibers near the surface. Such a velocity profile could act like a sound
channel, causing sounds propagating in the core to be refracted inward. This
type of structure will also provide a gradual impedance transformation from
the nasal plug region to the water.
The roles of the melon and the different structures in the head of a
dolphin are addressed by Aroyan et al. (Chapter 10), who numerically solve
the three-dimensional wave equation to show that the bony structure of the
head, the air-sacs, and the melon all work toward focusing the outgoing
acoustic energy in a beam that is directed forward and elevated slightly,
similar to the measured beam patterns of echolocating dolphins (Au 1993).
They also demonstrate that if the source was at the larynx, the outgoing
acoustic energy would not form into a beam that is pointed directly forward.
4.3 Sound Production by Sperm Whales
The sound production mechanism of sperm whales is discussed in detail by
Cranford (Chapter 3) and their use of sounds is discussed by Tyack and
Clark (Chapter 4). The geometry and physical properties of the forehead
of a sperm whale (which makes up about one-third of the body length)
make it a very unusual structure for sounds to propagate through. A
schematic of a planar cross-sectional view of the internal structure of a
sperm whale forehead from a dorsal-ventral perspective, along with a threedimensional anterior-lateral cut-away view are shown in Figure 1.6. The
museau de singe is situated at the anterior portion of the forehead, adjacent
to the distal and parabolic frontal air sac. The spermaceti organ is an elongated bag containing a complex mass of oil-filled connective tissue enclosed
by a membrane case that is surrounded by muscle and blubber. Norris and