Vocal Fold and its Oscillation
The larynx includes several structures such as the subglottic dome, vocal folds, ventricles, vestibular folds, epiglottis, and aryepiglottic folds, as shown in Fig.
4a. The vocal folds run anteroposteriorly from the vocal processes of the arytenoid cartilages to the internal surface of the thyroid cartilage. The vocal fold tissue consists of the thyroarytenoid
muscle, vocal ligament, lamina propria, and mucous membrane. They form a special layer structure that yields to aerodynamic forces to oscillate, which is often described as the
body-cover structure.
During voiced speech sounds, the vocal folds are set
into vibration by pressurized air passing through the membranous portion of the narrowed glottis. The glottal airflow thus generated induces wave-like motion of the vocal fold membrane, which appears to propagate from the bottom
to the top of the vocal fold edges. When this oscillatory motion builds up, the vocal fold membranes on either side come into contact with each other, resulting in repetitive closing and opening of the glottis. Figure
4b shows that vocal fold vibration repeats four phases within a cycle: the closed phase, opening phase, open phase, and closing phase. The conditions that determine vocal fold vibration are
the stiffness and mass of the vocal folds, the width of the glottis, and the pressure difference across the glottis.
The aerodynamic parameters that regulate vocal fold vibration are the transglottal pressure difference and glottal airflow.
The former coincides with the measure of subglottal pressure during mid and low vowels, which is about 5–10 cm H2O in comfortable loudness and pitch (1 cm H2O = 0.98 hPa). The latter also coincides with the average measure of oral airflow
during vowel production, which is roughly 0.1–0.2 l/s. These values show a large individual variation: the pressure range is 4.2–9.6 cm H2O in males and 4.4–7.6 cm H2O in females, while the airflow
rate ranges between 0.1–0.3 l/s in males and 0.09–0.21 l/s in females.
Figure 4a,b: Vocal folds and their vibration pattern. (a)
Coronal section of the larynx,
showing the tissues of the vocal and vestibular (false)
folds. The cavity of the larynx
includes supraglottic and subglottic regions.
(b)
Vocal-fold vibration pattern and
glottal
shapes in open phases. As the vocal-fold edge deforms in a glottal cycle,
the
glottis follows four phases: closed, opening, open and closing
Figure 5
shows schematically the relationship between the glottal cycle and volumic airflow change in normal and breathy phonation. The airflow varies within each glottal cycle, reflecting the cyclic variation of the glottal area and subglottal
pressure. The glottal area curve roughly shows a triangular pattern, while the airflow curve shows a skew of the peak to the right due to the inertia of the air mass within the glottis. The
closure of the glottis causes a discontinuous decrease of the glottal airflow to zero, which contributes the main source of vocal tract excitation,
as
shown in Fig. 5a. When the glottal closure is more abrupt, the output sounds are more intense with richer harmonic components.
When the glottal closure is incomplete in soft and breathy voices or the cartilaginous portion of the glottis is open to show the
glottal chink, the airflow includes a direct-current (DC) component and exhibits a gradual decrease of airflow, which results in a more sinusoidal waveform and a lower intensity of the output sounds, as shown in Fig.
5b.
Laryngeal
control of the oscillatory patterns of the vocal folds is one of the major factors in voice quality control.
In sharp voice, the open phase of the glottal cycle becomes shorter, while in soft voice, the open phase becomes longer. The ratio of the open phase within a glottal cycle is called the
open quotient (OQ), and the ratio of the closing slope to the opening slope in the glottal cycle is called
the speed quotient (SQ). These two parameters determine the slope of the spectral envelope. When the open
phase is longer (high OQ) with a longer closing phase (low
SQ), the glottal airflow becomes more sinusoidal, with weak harmonic components. Contrarily, when the open phase is shorter (low
OQ), glottal airflow builds up to pulsating waves with rich harmonics. In modal voice, all the vocal fold layers are involved in vibration, and the membranous glottis is completely closed
during the closed phase of each cycle. In falsetto, only the edges of the vocal folds vibrate, glottal closure becomes incomplete, and harmonic components reduce remarkably.
The
oscillation of the vocal folds during natural speech is quasiperiodic, and cycle-to-cycle variation are observed in speech waveforms as two types of measures:
jitter (frequency perturbation) and shimmer (amplitude perturbation). These irregularities appear to arise from combinations of biomechanical (vocal fold asymmetry), neurogenic (involuntary activities of laryngeal muscles), and aerodynamic
(fluctuations of airflow and subglottal pressure) factors. In sustained phonation of normal voice, the jitter is about 1% in frequency, and the shimmer is about 6% in amplitude.
Figure
5a,b: Changes in glottal area and airflow in relation to output sounds during
1.5
glottal cycles from glottal opening, with glottal shapes at peak opening (in the
circles).
(a) In modal phonation with complete glottal closure in the closed phase,
glottal
closure causes abrupt shut-off of glottal airflow and strong excitation of the
air
in the vocal tract during the closed phase. (b)
In breathy phonation, the glottal
closure
is incomplete, and the airflow wave includes a DC component, which results
in
weak excitation of the tract
版权声明：本文为博主原创文章，未经博主允许不得转载。