444
IL. Aroyan et al.
6. Blue Whale Sound Production Model
6.1 Water, Solid, or Air
All possible mechanisms for sound production in the blue whale must
involve radiating sound into seawater, but the oscillating media could,
theoretically, be made of water, bone, or air within the whale. Sound can
be produced underwater by fluctuations in water pressure (water), as in a
hydrodynamic oscillator (Albers 1965, pp. 176-177); by vibration of rigid
materials (solid), as in the sounds produced by snapping shrimp (Albers
1965, pp. 221-227) and possibly the jaws of whales (Brodie 1993); or by
oscillations of an air-filled chamber (air), as is the case in most biological
sound production systems. A water-filled source may be possible in a whale
using a hydrodynamic oscillator based on the large water-filled pleated
ventral cavity of a blue whale, perhaps using the flexible walls as a waveguide to shorten the dimensions required to achieve resonance at 17 Hz
(Fletcher 1992). Such a water-based sound source would appear to be very
inefficient due to losses associated with moving the whale's ventral tissues.
Another possibility is to design a resonant 17-Hz oscillator based on shear
mode vibrations of the lower jaw bone of a blue whale, but again this would
be an inefficient sound radiator, the bone dimensions being significantly less
than the 22-m optimum length for an efficient dipole radiator. Systems able
to compress water or bone are considered less efficient than air-filled
systems and require much higher differential pressures. The system that is
the least complex, most versatile, and most efficient is an air-filled cavity
oscillation.
6.2 Volume Displacement Requirements
Just as a human singer can hold a note only until all the air in the lungs is
expired, we believe the blue whale can only produce a continuous call of
given intensity and frequency for some maximum duration dependent on
the volume of air available. We assume the whale has no system equivalent
to a series of pumps and valves that would allow continuous recirculation
of air. The volume of air required to produce an underwater sound is then
dependent on the displacement required to produce each cycle of a sound
of given intensity, given frequency, and the resonance of the system, which
is a direct measure of the efficiency of producing that displacement. The
resonance quality factor is proportional to the efficiency and is discussed
later, but first we calculate the displacement requirements.
The intensity of an underwater sound produced by oscillation of an airfilled cavity can be directly related to the associated volume displacement
required to produce each cycle of that sound. The whale is compact in an
acoustical sense, so that the displacement is the total net fluctuation in the
volume of the whale each cycle, requiring air to be alternately compressed
and released. Assuming this compression occurs as air is moved past a fluc-
IL. Aroyan et al.
6. Blue Whale Sound Production Model
6.1 Water, Solid, or Air
All possible mechanisms for sound production in the blue whale must
involve radiating sound into seawater, but the oscillating media could,
theoretically, be made of water, bone, or air within the whale. Sound can
be produced underwater by fluctuations in water pressure (water), as in a
hydrodynamic oscillator (Albers 1965, pp. 176-177); by vibration of rigid
materials (solid), as in the sounds produced by snapping shrimp (Albers
1965, pp. 221-227) and possibly the jaws of whales (Brodie 1993); or by
oscillations of an air-filled chamber (air), as is the case in most biological
sound production systems. A water-filled source may be possible in a whale
using a hydrodynamic oscillator based on the large water-filled pleated
ventral cavity of a blue whale, perhaps using the flexible walls as a waveguide to shorten the dimensions required to achieve resonance at 17 Hz
(Fletcher 1992). Such a water-based sound source would appear to be very
inefficient due to losses associated with moving the whale's ventral tissues.
Another possibility is to design a resonant 17-Hz oscillator based on shear
mode vibrations of the lower jaw bone of a blue whale, but again this would
be an inefficient sound radiator, the bone dimensions being significantly less
than the 22-m optimum length for an efficient dipole radiator. Systems able
to compress water or bone are considered less efficient than air-filled
systems and require much higher differential pressures. The system that is
the least complex, most versatile, and most efficient is an air-filled cavity
oscillation.
6.2 Volume Displacement Requirements
Just as a human singer can hold a note only until all the air in the lungs is
expired, we believe the blue whale can only produce a continuous call of
given intensity and frequency for some maximum duration dependent on
the volume of air available. We assume the whale has no system equivalent
to a series of pumps and valves that would allow continuous recirculation
of air. The volume of air required to produce an underwater sound is then
dependent on the displacement required to produce each cycle of a sound
of given intensity, given frequency, and the resonance of the system, which
is a direct measure of the efficiency of producing that displacement. The
resonance quality factor is proportional to the efficiency and is discussed
later, but first we calculate the displacement requirements.
The intensity of an underwater sound produced by oscillation of an airfilled cavity can be directly related to the associated volume displacement
required to produce each cycle of that sound. The whale is compact in an
acoustical sense, so that the displacement is the total net fluctuation in the
volume of the whale each cycle, requiring air to be alternately compressed
and released. Assuming this compression occurs as air is moved past a fluc-