3. Impulse Sound Sources
121
Andersen 1983; Cranford et al. 1996, 1997) than most other work of
that time.
Norris (1964,1969) provided the first reviews of odontocete sound generation, beam emission, and echo reception, outlining four proposals to be
tested in subsequent decades: (1) a novel acoustic reception pathway to the
bony ear complex through the fat-filled mandible; (2) the role of cranial
asymmetry in the formation of a sonar beam; (3) the suggestion that the
fatty melon of the forehead might function as an acoustic lens; and (4) localization of the mechanism(s) of sound generation and emission in the forehead region. With the single exception of the role of cranial asymmetry
in biosonar beam formation, a great deal of evidence has been gathered
to provide plausible explanations for the other three proposals. After
the Norris reviews, investigations often addressed both, the question of
sound generation origin along with the sound transmission and beam
formation issue.
In 1966, Purves actuated a "Galton whistle" in the epiglottic spout of the
larynx in a post-mortem porpoise (P. phocoena) on a workbench. He tested
the idea that the larynx was the source of sound by measuring resultant
vibrations at different places in the head and inserted a suture needle at
different locations to transduce the vibrations by means of a piezoelectric
cartridge. The X-ray photograph (in his figure 1) shows the whistle inserted
into the tip of the epiglottic spout of the larynx. It is perhaps not surprising then that, when he produced sounds near the epiglottic spout, vibrations would be transmitted to other parts of the post-mortem head, since
some transmission into adjacent tissue would be expected.
This does not mean that a large proportion of the acoustic power would
be effectively transmitted across the various tissue interfaces required by
his proposed propagation pathway. Purves' proposed pathway passed from
the cartilaginous epiglottic spout, to the palatopharyngeal muscle complex
(against which the cartilaginous parts rested), then into bones of the skull
and rostrum, and finally into the cartilaginous mesorostral canal. Differences in the acoustic impedance values for these various tissues should
cause some acoustic transmission losses. Nevertheless, Purves concluded in
favor of the laryngeal phonation hypothesis and stated emphatically that
nasal structures "are not directly involved in echolocation."
The 1970s brought a flurry of research into the search for a sound generation mechanism. At that time, the dolphin's exquisite ability to use
echolocation had been suspected for almost twenty years, and increasingly
more was known about the accuracy and performance of the system.
Answers to questions about the sound generation site and applicable
mechanism, however, remained intractable and equivocal.
Two papers published in 1971 provided strong evidence in favor of the
nasal phonation hypothesis. Diercks et al. (1971) calculated the position of
a sound source from arrival-time differences at contact hydrophones placed
about the head of a live dolphin. They concluded, "By assuming straightline transmission through soft tissue, the time differences between the
121
Andersen 1983; Cranford et al. 1996, 1997) than most other work of
that time.
Norris (1964,1969) provided the first reviews of odontocete sound generation, beam emission, and echo reception, outlining four proposals to be
tested in subsequent decades: (1) a novel acoustic reception pathway to the
bony ear complex through the fat-filled mandible; (2) the role of cranial
asymmetry in the formation of a sonar beam; (3) the suggestion that the
fatty melon of the forehead might function as an acoustic lens; and (4) localization of the mechanism(s) of sound generation and emission in the forehead region. With the single exception of the role of cranial asymmetry
in biosonar beam formation, a great deal of evidence has been gathered
to provide plausible explanations for the other three proposals. After
the Norris reviews, investigations often addressed both, the question of
sound generation origin along with the sound transmission and beam
formation issue.
In 1966, Purves actuated a "Galton whistle" in the epiglottic spout of the
larynx in a post-mortem porpoise (P. phocoena) on a workbench. He tested
the idea that the larynx was the source of sound by measuring resultant
vibrations at different places in the head and inserted a suture needle at
different locations to transduce the vibrations by means of a piezoelectric
cartridge. The X-ray photograph (in his figure 1) shows the whistle inserted
into the tip of the epiglottic spout of the larynx. It is perhaps not surprising then that, when he produced sounds near the epiglottic spout, vibrations would be transmitted to other parts of the post-mortem head, since
some transmission into adjacent tissue would be expected.
This does not mean that a large proportion of the acoustic power would
be effectively transmitted across the various tissue interfaces required by
his proposed propagation pathway. Purves' proposed pathway passed from
the cartilaginous epiglottic spout, to the palatopharyngeal muscle complex
(against which the cartilaginous parts rested), then into bones of the skull
and rostrum, and finally into the cartilaginous mesorostral canal. Differences in the acoustic impedance values for these various tissues should
cause some acoustic transmission losses. Nevertheless, Purves concluded in
favor of the laryngeal phonation hypothesis and stated emphatically that
nasal structures "are not directly involved in echolocation."
The 1970s brought a flurry of research into the search for a sound generation mechanism. At that time, the dolphin's exquisite ability to use
echolocation had been suspected for almost twenty years, and increasingly
more was known about the accuracy and performance of the system.
Answers to questions about the sound generation site and applicable
mechanism, however, remained intractable and equivocal.
Two papers published in 1971 provided strong evidence in favor of the
nasal phonation hypothesis. Diercks et al. (1971) calculated the position of
a sound source from arrival-time differences at contact hydrophones placed
about the head of a live dolphin. They concluded, "By assuming straightline transmission through soft tissue, the time differences between the