438
IL. Aroyan et al.
geometry demonstrated here apparently also shapes and supports the
highly reflective flesh-air sac boundaries. In the common dolphin, skull
acoustic asymmetry parallels the asymmetry of the refractive soft tissues of
the forehead, as discussed below.
Much speculation has surrounded the acoustic role of the fatty melon
tissues of the delphinid forehead. Wood (1964) suggested that the melon
may both focus and acoustically couple internally generated sound to seawater. Sound speed measurements on the melon of a Pacific bottlenose
dolphin (Norris and Harvey 1974) revealed a low-velocity core and a
graded outer shell of high-velocity tissue. A number of biomolecular studies
have confirmed that the lipids of thedelphinid melon are chemically distinct from other body fats and blubber. Evans et al. (1964) found that the
presence of the soft tissue of the bottlenose dolphin's forehead caused narrower beams to be emitted than were produced by the skull alone. Romanenko (1974) measured horizontal beam widths with the whole head that
were roughly half as wide as the widths obtained using the skull alone for
frequencies of 80 kHz and higher. Previous two dimensional simulations
(Aroyan 1990; Aroyan et al. 1992) suggested that a melon velocity profile
of the magnitude measured by Norris and Harvey (1974) is capable of mild
focusing.
The current project substantiates the results of these earlier investigations of the acoustic function of the delphinid melon. The 3D skull and soft
tissue simulations have demonstrated that the melon and other soft tissues
of the D. de/phis forehead significantly narrow the main forward beam, corroborating the results of Evans et al. (1964) and Romanenko (1974), and
confirming the conjectures of Wood (1964) and Norris (1968). The melon's
efficiency in narrowing the horizontal beam width generally appears greater
than its effect on the vertical width. In addition, the inverse skull and soft
tissue simulations demonstrated a pronounced tendency towards collimation or channeling of acoustic energy within the posterior throat of the
melon. The posterior throat of the melon appears effective as a partial
wave-guide, while the larger forward lobe of the melon has been confirmed
to operate as a lens in the acoustic emission process. * Perhaps the magnitude of these effects explains the extraordinary metabolic investment that
the specialized melon tissues represent in the evolution of odontocete
cetaceans, as suggested by Morris (1986).
*See the discussion of fundamental acoustic mechanisms in Aroyan (1996, pp.
28-34). A three-component model of the delphinid biosonar emission system is proposed consisting of an impulsive source mechanism (the MLDB source hypothesis
of Cranford et al. 1987), a damped resonant chamber surrounding this source
(formed by the partially air-bounded soft tissue of the delphinid nasal passages),
and a projector of the ensuing signal consisting of the skull, the melon and other
soft tissues of the forehead. Simplified calculations with this model correctly predict
the frequency ranges and modal/bimodal signal structures of several odontocetes as
a function of signal source level and body size. The surprising consequences of this
simple model will be discussed in later reports..
IL. Aroyan et al.
geometry demonstrated here apparently also shapes and supports the
highly reflective flesh-air sac boundaries. In the common dolphin, skull
acoustic asymmetry parallels the asymmetry of the refractive soft tissues of
the forehead, as discussed below.
Much speculation has surrounded the acoustic role of the fatty melon
tissues of the delphinid forehead. Wood (1964) suggested that the melon
may both focus and acoustically couple internally generated sound to seawater. Sound speed measurements on the melon of a Pacific bottlenose
dolphin (Norris and Harvey 1974) revealed a low-velocity core and a
graded outer shell of high-velocity tissue. A number of biomolecular studies
have confirmed that the lipids of thedelphinid melon are chemically distinct from other body fats and blubber. Evans et al. (1964) found that the
presence of the soft tissue of the bottlenose dolphin's forehead caused narrower beams to be emitted than were produced by the skull alone. Romanenko (1974) measured horizontal beam widths with the whole head that
were roughly half as wide as the widths obtained using the skull alone for
frequencies of 80 kHz and higher. Previous two dimensional simulations
(Aroyan 1990; Aroyan et al. 1992) suggested that a melon velocity profile
of the magnitude measured by Norris and Harvey (1974) is capable of mild
focusing.
The current project substantiates the results of these earlier investigations of the acoustic function of the delphinid melon. The 3D skull and soft
tissue simulations have demonstrated that the melon and other soft tissues
of the D. de/phis forehead significantly narrow the main forward beam, corroborating the results of Evans et al. (1964) and Romanenko (1974), and
confirming the conjectures of Wood (1964) and Norris (1968). The melon's
efficiency in narrowing the horizontal beam width generally appears greater
than its effect on the vertical width. In addition, the inverse skull and soft
tissue simulations demonstrated a pronounced tendency towards collimation or channeling of acoustic energy within the posterior throat of the
melon. The posterior throat of the melon appears effective as a partial
wave-guide, while the larger forward lobe of the melon has been confirmed
to operate as a lens in the acoustic emission process. * Perhaps the magnitude of these effects explains the extraordinary metabolic investment that
the specialized melon tissues represent in the evolution of odontocete
cetaceans, as suggested by Morris (1986).
*See the discussion of fundamental acoustic mechanisms in Aroyan (1996, pp.
28-34). A three-component model of the delphinid biosonar emission system is proposed consisting of an impulsive source mechanism (the MLDB source hypothesis
of Cranford et al. 1987), a damped resonant chamber surrounding this source
(formed by the partially air-bounded soft tissue of the delphinid nasal passages),
and a projector of the ensuing signal consisting of the skull, the melon and other
soft tissues of the forehead. Simplified calculations with this model correctly predict
the frequency ranges and modal/bimodal signal structures of several odontocetes as
a function of signal source level and body size. The surprising consequences of this
simple model will be discussed in later reports..