3. Impulse Sound Sources
129
lips or phonic lips. Originally the term museau de singe, or monkey lips, was
retained because of the anatomic precedent established by Pouchet and
Beauregard (1885) and because we could not ascertain their function.
Recent high-speed video endoscopy work clearly implicates these lips in
dolphin sonal signal generation (Cranford et al. 1997). Consequently, it
seems appropriate to refer to them as the 'phonic lips rather than monkey
lips, and I will use that designation from now on. The posterior portion
of each sound generation complex associated with the phonic lips is
supported by a stiff cartilaginous blade and is anchored by a stout ligament,
the blowhole ligament. It should be note that the structure identified as
the blowhole ligament (BL) by Cranford et al. (1996) from their cryosection (Figure 1b) is actually the glandular tissue that provides lubrication
to the phonic lips (Evans and Maderson 1973). The blowhole ligament
is contained in the tissue just posterior to the lubrication gland. An
intricate series of air sacs surrounds much of the sound generation complex,
except in the anterior region where echolocation sounds undoubtedly
emanate.
Recent results from high-speed video endoscopy confirm the unified
hypothesis and pinpoint the phonic lips as the site of sound production in
a dolphin's head (Cranford et al. 1997). Now we can look with renewed
vigor at the most feasible generation mechanism. The previous work of
Mackay and Liaw (1981), Norris et al. (1971), and Ridgway et al. (1980)
have emphasized the pneumatic nature of the mechanism. Let us now
review what has been shown and suggested about the sound generation
mechanism.
4. The Sound Generation Mechanism
Historically, proposals for the physiologic mechanism of sound generation
have fallen into three categories: friction, pneumatic, and cavitation. Friction mechanisms seem the least likely for reasons I will delineate shortly.
Distinguishing between the likelihood of the other two mechanisms is difficult with the current state of our knowledge. Certainly, the cavitation
mechanism calls for extraordinary circumstances, which does not immediately disqualify the proposal. Alternatively, there is some evidence for a
simple pneumatic mechanism, a mechanism in which the sound generation
events are initiated and metered by a flowing stream of air. In the next few
paragraphs, I will attempt to layout and discuss the arguments for and
against each type of mechanism.
4.1 Friction-Based Mechanism
Early in 1973, Evans attempted to account for the fact that contact
hydrophones recorded strong vibrations from the melon, as well as from
129
lips or phonic lips. Originally the term museau de singe, or monkey lips, was
retained because of the anatomic precedent established by Pouchet and
Beauregard (1885) and because we could not ascertain their function.
Recent high-speed video endoscopy work clearly implicates these lips in
dolphin sonal signal generation (Cranford et al. 1997). Consequently, it
seems appropriate to refer to them as the 'phonic lips rather than monkey
lips, and I will use that designation from now on. The posterior portion
of each sound generation complex associated with the phonic lips is
supported by a stiff cartilaginous blade and is anchored by a stout ligament,
the blowhole ligament. It should be note that the structure identified as
the blowhole ligament (BL) by Cranford et al. (1996) from their cryosection (Figure 1b) is actually the glandular tissue that provides lubrication
to the phonic lips (Evans and Maderson 1973). The blowhole ligament
is contained in the tissue just posterior to the lubrication gland. An
intricate series of air sacs surrounds much of the sound generation complex,
except in the anterior region where echolocation sounds undoubtedly
emanate.
Recent results from high-speed video endoscopy confirm the unified
hypothesis and pinpoint the phonic lips as the site of sound production in
a dolphin's head (Cranford et al. 1997). Now we can look with renewed
vigor at the most feasible generation mechanism. The previous work of
Mackay and Liaw (1981), Norris et al. (1971), and Ridgway et al. (1980)
have emphasized the pneumatic nature of the mechanism. Let us now
review what has been shown and suggested about the sound generation
mechanism.
4. The Sound Generation Mechanism
Historically, proposals for the physiologic mechanism of sound generation
have fallen into three categories: friction, pneumatic, and cavitation. Friction mechanisms seem the least likely for reasons I will delineate shortly.
Distinguishing between the likelihood of the other two mechanisms is difficult with the current state of our knowledge. Certainly, the cavitation
mechanism calls for extraordinary circumstances, which does not immediately disqualify the proposal. Alternatively, there is some evidence for a
simple pneumatic mechanism, a mechanism in which the sound generation
events are initiated and metered by a flowing stream of air. In the next few
paragraphs, I will attempt to layout and discuss the arguments for and
against each type of mechanism.
4.1 Friction-Based Mechanism
Early in 1973, Evans attempted to account for the fact that contact
hydrophones recorded strong vibrations from the melon, as well as from