Phonation results from the self-sustained vibration of the vocal folds. But how is this self-sustained vibration and phonation initiated? According to the classic myoelastic-aerodynamic theory of vocal fold vibration (Van den Berg, 1958), the vocal folds close due to the action of a negative Bernoulli pressure, which is followed by a buildup of subglottal pressure. When the subglottal pressure is sufficiently high, the vocal folds are pushed open and the intraglottal pressure is lowered. The cycle then repeats, which leads to sustained oscillation of the vocal folds.
While this description may seem to provide an adequate explanation for sustained vocal fold vibration during normal phonation, questions quickly arise for other voicing conditions. For example, for a breathy voice (in which complete glottal closure does not occur), would the negative Bernoulli pressure be sufficient to move the vocal folds inward? Or similarly, without complete glottal closure, would the build-up of subglottal pressure be sufficient to move the folds apart? Indeed, under some pathological conditions, phonation fails to occur and the vocal folds are simply blown apart—despite high subglottal pressures and high respiratory effort. Under such conditions, do alternate physical mechanisms exist to better explain the phenomenon of phonation onset?
The myoelastic-aerodynamic theory is correct in identifying the interaction between the vocal folds and the airflow as the underlying mechanism of self-sustained vocal fold vibration. However, as pointed out by Ishizaka (1981) and Titze (1994), the theory is inadequate in explaining how energy is transferred from the airflow to the vocal folds to sustain vibration. According to Bernoulli’s equation, the airflow pressure would always be 90 degrees out of phase with the vocal fold surface velocity, resulting in no net energy transfer from the airflow to the vocal folds over one cycle of vibration. Thus, Bernoulli pressure alone does not provide a mechanism for energy transfer from the airflow to the vocal folds.
The key to a non-zero energy transfer lies in that the vocal folds are not rigid and therefore different portions of the vocal fold surface can vibrate in different phases, i.e., the upper and lower margins of the medial surface do not necessarily move inward and outward together (Titze, 1988). With a vertical phase difference between the upper and lower margins of the medial surface, the vocal fold often exhibits a wave-like motion, also called the mucosal wave, along the medial surface which propagates onto the superior surface. This vertical phase difference is produced in a process called eigenmode synchronization, in which two or more eigenmodes of the vocal folds are synchronized by the glottal flow to the same frequency (Ishizaka, 1981; Zhang et al., 2007). One example of this eigenmode synchronization is given in the figure below, in which case the second and third eigenmodes are synchronized, leading to phonation onset. This synchronization of eigenmodes to the same frequency but different phases produces a net energy transfer from airflow to the vocal fold to initiate and sustain vibration (Zhang et al., 2007; Zhang, 2011). The threshold subglottal pressure at which this eigenmode synchronization occurs is determined by the frequency spacing and coupling strength between the two natural modes that are being synchronized (Zhang, 2010). Note that this mechanism of mode synchronization does not require a complete glottal closure, with corresponding pressure build-up. A wide glottal opening (as for breathy voice) would reduce but not eliminate this mode synchronization effect.
Identification of this primary mechanism (eigenmode-synchronization) of phonation onset provides a theoretical framework based on which phonation characteristics under different laryngeal conditions can be systematically investigated and understood. The resulting vocal fold vibration pattern depends criticially upon the characteristics of the two modes that are synchronized. This means that vocal fold vibration and therefore voice quality may be varied by changing the vibrational characteristics of the underlying modes which are synchronized. Changes in these modes are usually induced by changes in the geometry or stiffness of the layered vocal fold structure. In a follow up to the Zhang et al. (2007) study, the influence of vocal fold properties (Zhang, 2009) and glottal flow variables (Zhang, 2008) on the eigenmode-synchronization pattern was investigated. These studies showed that a variety of vocal fold vibration patterns and voice types can be produced by different combinations of body and cover layer stiffnesses of the vocal fold structure. These studies also showed that the fluid-structure interaction induced synchronization of more than one group of eigenmodes so that two or more eigenmodes may be simultaneously destabilized towards phonation onset. At certain conditions, a slight change in vocal fold stiffness or geometry may cause phonation onset to occur at a different eigenmode, leading to sudden changes in phonation onset frequency, vocal fold vibration pattern, and sound production efficiency (Zhang, 2008, 2009).
Zhang, Z., Neubauer, J., Berry, D.A. (2007). Physical mechanisms of phonation onset: a linear stability analysis of an aeroelastic continuum model of phonation, J. Acoust. Soc. Am., 122(4), 2279-2295. [pdf] [link]
Zhang, Z. (2008). Influence of flow separation location on phonation onset, J. Acoust. Soc. Am., 124(3), 1689-1694. [pdf] [link]
Zhang, Z. (2009). Characteristics of phonation onset in a two-layer vocal fold model, J. Acoust. Soc. Am., 125(2), 1091-1102. [pdf] [link]
Zhang, Z. (2010). Dependence of phonation threshold pressure and frequency on vocal fold geometry and biomechanics, J. Acoust. Soc. Am., 127(4), 2554-2562. [pdf] [link]
Zhang, Z. (2011). On the difference between negative damping and eigenmode synchronization as two phonation onset mechanisms, J. Acoust. Soc. Am., 129(4), 2163-2167. [pdf] [link]