Biomedical Engineering Reference
In-Depth Information
3 Anatomy of the Vocal Organ
In Chap. 2, we discussed sources and filters. We stated that the establishment
of a time-varying airflow was the basic mechanism by which density fluctu-
ations are created, giving rise to sound. We also described the phenomena
due to the existence of a filter between the source and the environment. In
that framework, we invited ourselves to touch our neck, at the approximate
location of the larynx, while pronouncing a vowel, and we said that as the
result of that simple experiment we would perceive a vibration. That percep-
tion is due to the motion that is established in a pair of tissues called vocal
folds. Why should this discussion be of any interest when we are discussing
birdsong? After all, if we were to try to imitate a bird, we would produce
some sort of whistle, the physical mechanism of which is certainly well differ-
entiated from the mechanism described above. The whole point of beginning
our description by discussing the vibrations of vocal folds in humans dur-
ing voiced sounds is that, for many species, there are important analogies
between this phenomenon and birdsong. And of course differences as well.
3.1 Morphology and Function
3.1.1 General Mechanism of Sound Production
The mechanism of sound production in birds resembles that in humans in
that an airflow driven by air sac pressure is modulated by some kind of
vibrating valve [Greenwalt 1968]. The acoustic output of this valve excites the
air column in the vocal tract. In many cases (both in birds and in humans)
this occurs in much the same way as in a source-filter system, and the output
of the valve is modified by the resonances of the tract. This mechanism is
not the only one to have been proposed to explain the nature of birdsong. A
second one, based on an aerodynamic whistle, has been proposed to account
for some tonal sounds [Nottebohm 1976, Casey and Gaunt 1985], although
direct endoscopic observation [Goller and Larsen 1997b], experiments with
light atmospheres [Nowicki 1987] and other studies [Suthers and Zuo 1991]
have failed to build confidence in the “whistle” picture.
The mechanism that is emerging as the most adequate one to account
for the generation of birdsong can be therefore summarized as one in which
