3. Impulse Sound Sources
145
video endoscopy to observe the sound generation event could help resolve
this question. The distinction to be made here is one that ascertains whether
the pulse is generated when the phonic lips come together, as is my current
bias, or when the lips part, as is required for cavitation to be in effect.
Fortunately, these two options appear to be mutually exclusive, so, with the
appropriate observations, it should be possible to settle this issue with
respect to the production of sonar clicks.
6.2 Acoustic Prey Debilitation Hypothesis
Another hypothesis related to sound generation that has languished in the
literature for many years without serious investigation is the acoustic debilitation of prey by odontocetes (Norris and Ml1lhl 1983). Norris and Ml1lhl
marshaled a good deal of evidence to support this notion. One key here is
that there is excellent impedance matching between the animal's tissues and
the water, so that much of the energy transduced into acoustic vibrations
in tissue can be transmitted into the water with minimal loss. If some odontocetes have this ability, it is probably correlated with muscle mass, since
muscles ultimately power all mammalian motor activity. This implies that
larger odontocetes are more likely to have this ability but that there should
be a lower (muscle mass) limit beyond which the effect cannot be sustained.
It is possible that one class of sound we did not address, bangs, are used for
this purpose and perhaps others. It seems to me that the future of this work
is ripe with possibilities.
6.3 Sonar Beam Formation
The general shape and direction of the transmitted sonar beam is known
from far-field measurements, for a couple of odontocete species, in narrowly
defined circumstances. We do not know much about the detailed structure
of the beam, or the potential for control of variation in beam structure or
direction. Unfortunately, we are forced to be tentative about where the
beam emanates from the head and we are left with unsatisfactory speculation about the effects that the rich morphological variety we see in
odontocete foreheads has on beam formation.
A few studies have begun to address the problems inherent in the difficult approaches to some of these issues (Flint et al. 1997; Sigurdson 1997a;
Aroyan et al., Chapter 10). There will no doubt be other clever approaches
and application of new technology in the work on these issues. For example,
actual pathways and the definition of beam emanation patterns from the
animal's head might be addressed with the use of Schlieren optical techniques (Mackay 1966), laser interferometry, or piezopolymer hoods. No
doubt, computer modeling tools will help us understand the effects of
variation in morphology on beam formation and the development of
interference patterns.
145
video endoscopy to observe the sound generation event could help resolve
this question. The distinction to be made here is one that ascertains whether
the pulse is generated when the phonic lips come together, as is my current
bias, or when the lips part, as is required for cavitation to be in effect.
Fortunately, these two options appear to be mutually exclusive, so, with the
appropriate observations, it should be possible to settle this issue with
respect to the production of sonar clicks.
6.2 Acoustic Prey Debilitation Hypothesis
Another hypothesis related to sound generation that has languished in the
literature for many years without serious investigation is the acoustic debilitation of prey by odontocetes (Norris and Ml1lhl 1983). Norris and Ml1lhl
marshaled a good deal of evidence to support this notion. One key here is
that there is excellent impedance matching between the animal's tissues and
the water, so that much of the energy transduced into acoustic vibrations
in tissue can be transmitted into the water with minimal loss. If some odontocetes have this ability, it is probably correlated with muscle mass, since
muscles ultimately power all mammalian motor activity. This implies that
larger odontocetes are more likely to have this ability but that there should
be a lower (muscle mass) limit beyond which the effect cannot be sustained.
It is possible that one class of sound we did not address, bangs, are used for
this purpose and perhaps others. It seems to me that the future of this work
is ripe with possibilities.
6.3 Sonar Beam Formation
The general shape and direction of the transmitted sonar beam is known
from far-field measurements, for a couple of odontocete species, in narrowly
defined circumstances. We do not know much about the detailed structure
of the beam, or the potential for control of variation in beam structure or
direction. Unfortunately, we are forced to be tentative about where the
beam emanates from the head and we are left with unsatisfactory speculation about the effects that the rich morphological variety we see in
odontocete foreheads has on beam formation.
A few studies have begun to address the problems inherent in the difficult approaches to some of these issues (Flint et al. 1997; Sigurdson 1997a;
Aroyan et al., Chapter 10). There will no doubt be other clever approaches
and application of new technology in the work on these issues. For example,
actual pathways and the definition of beam emanation patterns from the
animal's head might be addressed with the use of Schlieren optical techniques (Mackay 1966), laser interferometry, or piezopolymer hoods. No
doubt, computer modeling tools will help us understand the effects of
variation in morphology on beam formation and the development of
interference patterns.