3. Impulse Sound Sources
131
4.2 Cavitation-Based Mechanism
Over the years, Kenneth S. Norris and others have casually suggested to me
that if the mechanism of sound production involves air bubble cavitation,
it could result in the release of high-intensity sounds, as have been measured from dolphins. Goodson (1997) espoused a similar suggestion, that
many similarly sized bubbles might collapse simultaneously to produce a
dolphin click. Unfortunately, none of these intriguing ideas have been
described in any detail or subjected to the rigors of experimentation or the
peer review process.
The power contained in a given echolocation click can be great or it can
be modest. Au (1980) describes the highest peak-to-peak sound pressure
level produced by a bottlenose dolphin at 230dB re 1 micropascal at 1m.
Intensities in this neighborhood approach the finite limit of sound intensity
in water, where additional energy begins to be dissipated as heat rather than
greater intensity (Griffin 1980). The acoustic impedance values between
tissue (e.g., odontocete acoustic fats) and sea water suggest nearly transparent propagation of acoustic power. Still, it is reasonable to question
whether the impact area during the sound generation process (a few square
centimeters for the phonic lips of T. truncatus, depending upon the individual) could account for an initial signal of such intensity. Some simple
calculations show that it is possible to generate brief high-intensity clicks
without invoking cavitation.
Clay and Medwin (1977) describe underwater cavitation it this way,
For sound sources near the sea surface, these cavitation nuclei permit rupture to
occur at pressure swings of the order of 1 ATM (10.5 Pa or 220dB re 1 micropascal) depending upon the frequency, duration and repetition rate of the sound pulse.
. . . There are many physical phenomena associated with the cavitating bubble.
When violently oscillating bubbles are close to a solid surface, the stresses associated with the emitted shock waves and acoustic streams result in rapid erosion of
the toughest metals or plastics. The high pressure and high temperatures occurring
during transient bubble collapse cause luminescence of the gas bubble.
It might seem extraordinary to produce such cavitation effects inside the
dolphin airway, but this may not be the case since pressure differentials of
more than one atmosphere are apparently commonplace within the airway
(Ridgway et al.1980;Amundin and Andersen 1983). Of course, the required
conditions are more demanding at depth but it is at least conceivable that
very small bubbles or extremely high velocities could create the phenomenon. Sound production in the human airway may be a case in point (Altes
et al. 1975). Our normal speaking voice produces glottal pulses of bubbles
that in aggregate, may exceed the speed of sound, and hence some of the
sound we produce may be based upon cavitation effects.
If this is also the case for dolphins, it might provide access to high-energy
signals so that even the most intense dolphin sounds could be explained.
This is certainly an interesting possibility, although there is one nagging
131
4.2 Cavitation-Based Mechanism
Over the years, Kenneth S. Norris and others have casually suggested to me
that if the mechanism of sound production involves air bubble cavitation,
it could result in the release of high-intensity sounds, as have been measured from dolphins. Goodson (1997) espoused a similar suggestion, that
many similarly sized bubbles might collapse simultaneously to produce a
dolphin click. Unfortunately, none of these intriguing ideas have been
described in any detail or subjected to the rigors of experimentation or the
peer review process.
The power contained in a given echolocation click can be great or it can
be modest. Au (1980) describes the highest peak-to-peak sound pressure
level produced by a bottlenose dolphin at 230dB re 1 micropascal at 1m.
Intensities in this neighborhood approach the finite limit of sound intensity
in water, where additional energy begins to be dissipated as heat rather than
greater intensity (Griffin 1980). The acoustic impedance values between
tissue (e.g., odontocete acoustic fats) and sea water suggest nearly transparent propagation of acoustic power. Still, it is reasonable to question
whether the impact area during the sound generation process (a few square
centimeters for the phonic lips of T. truncatus, depending upon the individual) could account for an initial signal of such intensity. Some simple
calculations show that it is possible to generate brief high-intensity clicks
without invoking cavitation.
Clay and Medwin (1977) describe underwater cavitation it this way,
For sound sources near the sea surface, these cavitation nuclei permit rupture to
occur at pressure swings of the order of 1 ATM (10.5 Pa or 220dB re 1 micropascal) depending upon the frequency, duration and repetition rate of the sound pulse.
. . . There are many physical phenomena associated with the cavitating bubble.
When violently oscillating bubbles are close to a solid surface, the stresses associated with the emitted shock waves and acoustic streams result in rapid erosion of
the toughest metals or plastics. The high pressure and high temperatures occurring
during transient bubble collapse cause luminescence of the gas bubble.
It might seem extraordinary to produce such cavitation effects inside the
dolphin airway, but this may not be the case since pressure differentials of
more than one atmosphere are apparently commonplace within the airway
(Ridgway et al.1980;Amundin and Andersen 1983). Of course, the required
conditions are more demanding at depth but it is at least conceivable that
very small bubbles or extremely high velocities could create the phenomenon. Sound production in the human airway may be a case in point (Altes
et al. 1975). Our normal speaking voice produces glottal pulses of bubbles
that in aggregate, may exceed the speed of sound, and hence some of the
sound we produce may be based upon cavitation effects.
If this is also the case for dolphins, it might provide access to high-energy
signals so that even the most intense dolphin sounds could be explained.
This is certainly an interesting possibility, although there is one nagging