usually stem from the difficulty of obtaining or working with human cadaveric larynges rather than from an interest in comparative physiology
(Brown and Cannitto 1995 and Mergell et al. 1999 are exceptions).
Nonetheless, over the years, the larynges of a wide variety of mammalian
species have been investigated, and all available data are consistent with
the hypothesis that the vibratory mechanics of the larynx are fundamentally similar among all mammals, including humans. Nonhuman mammals
whose larynges have been experimentally studied include baboons, sheep,
dogs, horses, cattle, and Syke’s monkeys (Slavitt et al. 1990; Hirano 1991;
Brown and Cannitto 1995; Bless et al. unpublished). In all cases, the vocal
folds act as vibrators, and the myoelastic–aerodynamic theory applies.
Although current evidence suggests that similar considerations apply to the
avian syrinx, it is only very recently that in situ vibrations of the syrinx have
been directly observed (Larsen and Goller 1999), and the basic mechanisms
underlying avian sound production are still the subject of active investigation. Because the vocal folds are solid masses of tissue, whereas the
syringeal membranes are relatively thin and light, it is quite likely that the
details of their vibratory patterns will differ in some ways. Nonetheless,
current data for all tetrapods are consistent with the idea that the voice
source involves movement of a set of vibrators (vocal folds or syringeal
membranes) that modulate air flow and thus generate acoustic energy.
Often the oscillation of the vibrators is periodic, with their opening and
closing occurring regularly. The time it takes for one open/close cycle is
the period, and the rate at which these cycles occur is the fundamental
frequency (abbreviated f 0 ). A fundamental insight of the myoelastic–
aerodynamic theory is the realization that this rate of opening and closing
is determined passively by the setting of muscle tensions, effective mass of
the vibrators, and lung pressure. It is unnecessary for any muscles in the
vocal folds to twitch, or the motor neurons to fire, at the fundamental frequency. Indeed, for the sounds of many vertebrates, this would be impossible because the fundamental frequencies are much higher than the
maximum rate of muscular tetany or even neural firing. Only relatively low
vibration rates can typically be generated by rapid muscle twitching in
tetrapods (e.g., 25-Hz purring in cats; Remmers and Gautier 1972). The neurally passive feature of the larynx or syrinx is best demonstrated by the fact
that a larynx or syrinx can be removed from the body and deprived of all
nervous input and still be induced to produce sound by blowing air through
the approximated vocal folds or syringeal membranes (Rüppell 1933;
Schmidt 1965; van den Berg 1968; see also Yamaguchi and Kelley, Chapter
6). All that is necessary is to place the vibrators in an appropriate state of
tension and approximation, with the proper air flow, and the system will
vibrate with a motion and at rates closely approximating those seen in vivo.
The passive frequency control of the voice source of terrestrial vertebrates
contrasts sharply with frequency control of the swim bladder production
system found in most fish, where each pulse of acoustic energy is produced
76
W.T. Fitch and M.D. Hauser
(Brown and Cannitto 1995 and Mergell et al. 1999 are exceptions).
Nonetheless, over the years, the larynges of a wide variety of mammalian
species have been investigated, and all available data are consistent with
the hypothesis that the vibratory mechanics of the larynx are fundamentally similar among all mammals, including humans. Nonhuman mammals
whose larynges have been experimentally studied include baboons, sheep,
dogs, horses, cattle, and Syke’s monkeys (Slavitt et al. 1990; Hirano 1991;
Brown and Cannitto 1995; Bless et al. unpublished). In all cases, the vocal
folds act as vibrators, and the myoelastic–aerodynamic theory applies.
Although current evidence suggests that similar considerations apply to the
avian syrinx, it is only very recently that in situ vibrations of the syrinx have
been directly observed (Larsen and Goller 1999), and the basic mechanisms
underlying avian sound production are still the subject of active investigation. Because the vocal folds are solid masses of tissue, whereas the
syringeal membranes are relatively thin and light, it is quite likely that the
details of their vibratory patterns will differ in some ways. Nonetheless,
current data for all tetrapods are consistent with the idea that the voice
source involves movement of a set of vibrators (vocal folds or syringeal
membranes) that modulate air flow and thus generate acoustic energy.
Often the oscillation of the vibrators is periodic, with their opening and
closing occurring regularly. The time it takes for one open/close cycle is
the period, and the rate at which these cycles occur is the fundamental
frequency (abbreviated f 0 ). A fundamental insight of the myoelastic–
aerodynamic theory is the realization that this rate of opening and closing
is determined passively by the setting of muscle tensions, effective mass of
the vibrators, and lung pressure. It is unnecessary for any muscles in the
vocal folds to twitch, or the motor neurons to fire, at the fundamental frequency. Indeed, for the sounds of many vertebrates, this would be impossible because the fundamental frequencies are much higher than the
maximum rate of muscular tetany or even neural firing. Only relatively low
vibration rates can typically be generated by rapid muscle twitching in
tetrapods (e.g., 25-Hz purring in cats; Remmers and Gautier 1972). The neurally passive feature of the larynx or syrinx is best demonstrated by the fact
that a larynx or syrinx can be removed from the body and deprived of all
nervous input and still be induced to produce sound by blowing air through
the approximated vocal folds or syringeal membranes (Rüppell 1933;
Schmidt 1965; van den Berg 1968; see also Yamaguchi and Kelley, Chapter
6). All that is necessary is to place the vibrators in an appropriate state of
tension and approximation, with the proper air flow, and the system will
vibrate with a motion and at rates closely approximating those seen in vivo.
The passive frequency control of the voice source of terrestrial vertebrates
contrasts sharply with frequency control of the swim bladder production
system found in most fish, where each pulse of acoustic energy is produced
76
W.T. Fitch and M.D. Hauser