Page 61 - Fall2021

Figure 1. A: computed tomography image of the head showing the airway and the larynx. B: top view of the larynx. The vocal folds are far apart at rest. C: vocal folds are brought together to close the glottis during phonation.
during the glottal closing phase is the main mechanism for harmonic sound production, by which voices of dif- ferent quality are produced and differentiated. An abrupt cessation of the glottal flow produces a voice with strong harmonic excitation at high frequencies and a bright voice quality that often carries well in a room or open space. On the other hand, a sinusoidal-like shape of the glottal flow with a gradual flow decline, often in the pres- ence of an incomplete glottal closure, produces a voice with a limited number of higher order harmonics in the voice spectrum and a weak voice quality.
The glottal closure pattern during voice production is controlled by adductory laryngeal muscles that bring the two folds together (vocal fold approximation) to reduce the glottal gap. Indeed, phonation is impossible if the glottal gap is too large. Vocal folds that are insufficiently approximated tend to vibrate without complete glottal closure. This produces a breathy voice quality with weak excitation of harmonics and strong noise in the voice spectrum. Increasing approximation of the vocal folds leads to increased vocal fold contact and glottal closure, reducing air leakage through the glottis and increasing harmonic sound generation.
Activation of the adductory laryngeal muscles also modifies vocal fold shape and, particularly, the vertical thickness of the vocal fold medial surface. The medial surface vertical thickness plays an important role in regulating the duration of glottal closure and the produced voice quality. Increasing the vertical thickness allows the vocal folds to better main- tain their position against the subglottal pressure. This is
essential to achieve complete glottal closure at high lung pressure while producing a loud voice where vocal fold approximation alone is insufficient to ensure glottal clo- sure during phonation (Zhang, 2016).
In general, thicker vocal folds tend to close the glottis for a longer duration during phonation than thinner vocal folds. Thus, changes in vertical thickness are essential to producing voice qualities ranging from breathy (see Mul- timedia 2 at acousticstoday.org/zzhangmedia) to normal (see Multimedia 3 at acousticstoday.org/zzhangmedia) to pressed (see Multimedia 4 at acousticstoday.org/zzhangmedia). In the extreme case of very large vocal fold thickness due to strong vocal fold adduction, the folds often exhibit sub- harmonic or irregular vibration, producing a rough voice quality (Zhang, 2018), known as creak in the linguistic lit- erature and more colloquially as vocal fry (see Multimedia 5 at acousticstoday.org/zzhangmedia).
Pitch is controlled by elongating and shortening the vocal folds, which regulates the tension and stiffness of the vocal folds. This is possible because the cover layer of each vocal fold consists of collagen and elastin fibers aligned along the anterior-posterior (front-back) direction. These fibers are in a wavy, crimped state at rest but are gradually straightened with elongation and thus become load bearing. As more fibers are gradually straightened with vocal fold elongation, the vocal folds become increasingly stiff, thus increasing pitch.
Because the laryngeal muscles that control the vocal fold length also regulate the vocal fold vertical thickness,
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