The larynx is used to transform an airstream into audible sounds. This process is called phonation and it is of special importance to perceived voice quality.
Laver (1994:184) defines phonation1 as the use of the laryngeal system to generate an audible source of acoustic energy (the source in the sense of the source-filter model of speech production) which can then be modified by the articulatory actions of the rest of the vocal apparatus (the filter in the source-filter model). The main function of the larynx is to transform the potential energy of the air compressed below the larynx into the kinetic energy of egressive airflow. If the transformation occurs sufficiently fast the air pressure changes generate acoustic waves that expand into the surrounding air (Orlikoff & Kahane, 1996:127). A more detailed description of the phonation process will be given in the next section. For the moment it is sufficient to consider only the basic types of phonation.
In the simplest case the sound is produced by airflow through a narrow constriction. Depending on the volume of the airflow and the degree of constriction either laminar flow or turbulent airflow are effected. Turbulence occurs with higher airflow volumes and higher degrees of constriction. The turbulent flow creates an audible friction which is perceived as a hiss. At the glottis turbulence accordingly results in the glottal fricative [h]. Other sounds are produced by completely blocking off the airstream and then releasing this closure. The sound of this type produced at the glottis is called a glottal stop [?].
As the vocal folds come very close together they will normally vibrate, producing a pulsed, periodic sound (phonation). Shape, duration and amplitude of the pulses depend on muscular and aerodynamic factors. This process can be combined with other ways of generating sounds to create voiced fricatives or voiced stops. Phonation constitutes the fundamental set of voice quality parameters and is used primarily on the para- and extralinguistic layers of human communication.However, it may also have phonetic significance (Lin, 1994:14). The voice source is used to change the sentence melody (intonation) and the tonal form of words (in tonal langueges) by varying the subglottal pressure as well as the tension of the vocal folds. This leads to changes in the rate of vibration of the vocal folds, which are in turn perceived by the listener as modifications in pitch and/or in loudness.
The frequency (fundamental frequency, F0) of the vocal folds vibrations may be approximated by Sonninen's (1956) formula given in (1):
Based on this formula, the frequency of the vibration of the vocal folds (perceived as pitch) is inversely proportional to the vibrating mass and directly proportional to the tension of the folds.
Assuming equal density (tissue density is constant for all phonation conditions (Titze, 1994) and equal width of the folds, F0 depends inversely on the length of the vibrating part of the folds.
Assuming and mass of the vocal fold per unit area m=0.476 [g/cm3] and K1+K2=200 [kdyn/cm3] (Titze, 1988) the natural oscillation frequency is F0=103 [Hz].
1 also defined in: Trask, 1996:263; Ladefoged & Maddieson, 1996:48-94; Hardcastle & Laver, 1997; Fant, 1960.