3. Impulse Sound Sources
133
A testable hypothesis for a pneumatic sound production mechanism was
described by Cranford (1992a) and Cranford et al. (1996). The hypothesis
states that odontocetes generate sonar signals by pushing air past pairs of
the internal nasal lips or, more accurately, the phonic lips. These phonic lips
function in much the same way as those of a musician playing a brass instrument (Martin 1942; Copley and Strong 1996). The difference is that in odontocetes the functional vibrations are born from the lips and adjacent tissues
themselves, rather than directly from the air passing between them as they
are in the playing of a brass instrument. This is an important distinction
because it highlights the primary difference between airborne vibrations
magnified by, say, a trumpet and the tissue-borne vibrations that are transmitted into the aquatic environment from a dolphin's head. In the dolphin,
any airborne vibrations are "trapped" within the air spaces, by the impedance mismatch with surrounding tissues (absent any air cavity resonancebased mechanism). These tissue vibrations may be channeled, focused, and
matched to the aqueous environment by various structures and surfaces
within the forehead.
4.3.1 Evidence from High-Speed Video Endoscopy
High-speed video endoscopy was used recently to observe the process of
sound generation in a phonating dolphin (Cranford et al. 1997). It provided
the first direct confirmation of the proposed sound generation site as proposed by Cranford et al. (1996). This work substantially answered the perplexing question of locating the site of sound generation. A complete report
of the endoscope study is currently in preparation, but a short summary of
it is provided here.
Activities were recorded within the pharyngeal and nasal cavities of two
bottlenose dolphins (T. truncatus) while eliciting sound production bouts,
both at the surface and during simple echolocation discrimination tasks
(Cranford et al. 1997). A dual-camera, high-speed (frame rate = 400Hz)
video system provided synchronized observational windows for recording
two concomitant events. One camera recorded tissue movements visible
through an endoscope while the other camera focused upon oscilloscope
traces of acoustic pressure at a hydrophone placed in the water near the
animal's head.
Bottlenose dolphins apparently produce acoustic pulses by pushing air
across the phonic lips, setting the associated tissue complex into vibration.
An acoustic pulse occurs coincident with one oscillatory cycle of the lips.
The cycle begins with the lips parting, followed by an explosion of air and
fluid erupting from the gap between the lips, and concludes with closure of
the lips.
Most significantly, changes in the acoustic pulse repetition rate and the
vibration cycles of the phonic lips are simultaneous, indicating that their
rates and periods are synchronous (within our 2.5ms time frame). There-
133
A testable hypothesis for a pneumatic sound production mechanism was
described by Cranford (1992a) and Cranford et al. (1996). The hypothesis
states that odontocetes generate sonar signals by pushing air past pairs of
the internal nasal lips or, more accurately, the phonic lips. These phonic lips
function in much the same way as those of a musician playing a brass instrument (Martin 1942; Copley and Strong 1996). The difference is that in odontocetes the functional vibrations are born from the lips and adjacent tissues
themselves, rather than directly from the air passing between them as they
are in the playing of a brass instrument. This is an important distinction
because it highlights the primary difference between airborne vibrations
magnified by, say, a trumpet and the tissue-borne vibrations that are transmitted into the aquatic environment from a dolphin's head. In the dolphin,
any airborne vibrations are "trapped" within the air spaces, by the impedance mismatch with surrounding tissues (absent any air cavity resonancebased mechanism). These tissue vibrations may be channeled, focused, and
matched to the aqueous environment by various structures and surfaces
within the forehead.
4.3.1 Evidence from High-Speed Video Endoscopy
High-speed video endoscopy was used recently to observe the process of
sound generation in a phonating dolphin (Cranford et al. 1997). It provided
the first direct confirmation of the proposed sound generation site as proposed by Cranford et al. (1996). This work substantially answered the perplexing question of locating the site of sound generation. A complete report
of the endoscope study is currently in preparation, but a short summary of
it is provided here.
Activities were recorded within the pharyngeal and nasal cavities of two
bottlenose dolphins (T. truncatus) while eliciting sound production bouts,
both at the surface and during simple echolocation discrimination tasks
(Cranford et al. 1997). A dual-camera, high-speed (frame rate = 400Hz)
video system provided synchronized observational windows for recording
two concomitant events. One camera recorded tissue movements visible
through an endoscope while the other camera focused upon oscilloscope
traces of acoustic pressure at a hydrophone placed in the water near the
animal's head.
Bottlenose dolphins apparently produce acoustic pulses by pushing air
across the phonic lips, setting the associated tissue complex into vibration.
An acoustic pulse occurs coincident with one oscillatory cycle of the lips.
The cycle begins with the lips parting, followed by an explosion of air and
fluid erupting from the gap between the lips, and concludes with closure of
the lips.
Most significantly, changes in the acoustic pulse repetition rate and the
vibration cycles of the phonic lips are simultaneous, indicating that their
rates and periods are synchronous (within our 2.5ms time frame). There-