1. INTRODUCTION
Vowel production is typically discussed in terms of
vocal tract shape and corresponding formant (resonance)
frequencies. However,the manifestation of a particular
vowel as a sound depends on the acoustic excitation of the
vocal tract cavities by a sound source. For vowels,the
sound source is most often generated by the vibrating vocal
folds that convert the steady (DC) airflow from the lower
respiratory system into a periodic train of flow pulses.
These pulses,referred to as the glottal flow,are then
acoustically filtered by the vocal tract resonances. This
process effectively redistributes the amplitudes of the
frequency components of the source signal,resulting in a
vowel sound. Figure 1 illustrates the ‘‘source-filter’’
representation [1] in both the time and frequency domains
where the source is shown on the left side,the vocal tract
filter in the center,and the resulting output (at the lips) on
the right. In the time domain (top row of figures) the filter
is shown as a vocal tract area function suggesting that the
glottal flow propagates through it and is transformed into
the output pressure (in a strict mathematical sense the
source would be convolved with the impulse response of
the area function). The frequency domain representation
suggests that the output spectrum is the product of the
glottal flow spectrum and the frequency response of the
vocal tract. In either case,it is apparent that the vibration of
the vocal folds provides an harmonically-rich acoustic
excitation that the vocal tract resonances can ‘‘mold’’ into
the respective vowel sounds.
While the sound source (glottal flow) produced by
vocal fold vibration may appear to be relatively simple,the
mechanisms responsible for inducing and sustaining the
vibration involve complexities of anatomy,muscular
control,nonlinear flow-induced oscillation,and the interaction
of vocal tract pressures with both the glottal flow
and the movement of the vocal folds themselves. To show
how some of these complexities are integrated with vowel
production this paper has three aims: (1) to review the
anatomical structure of vocal folds and their vibratory
kinematics,(2) to present a brief overview of vocal fold
modeling,and (3) to use a low-dimensional model to
illustrate some basic mechanisms of vocal fold oscillation,
including the acoustic interaction with the vocal tract.