The phonatory process, or voicing, occurs when air is expelled from the lungs through the glottis, creating a pressure drop across the larynx.
[6] Fundamental frequency, the main acoustic cue for the percept pitch, can be varied through a variety of means.
Large scale changes are accomplished by increasing the tension in the vocal folds through contraction of the cricothyroid muscle.
The theory states that when a stream of breath is flowing through the glottis while the arytenoid cartilages are held together (by the action of the interarytenoid muscles), a push-pull effect is created on the vocal fold tissues that maintains self-sustained oscillation.
[1] Such an effect causes a transfer of energy from the airflow to the vocal fold tissues which overcomes losses by dissipation and sustain the oscillation.
This theory states that the frequency of the vocal fold vibration is determined by the chronaxie of the recurrent nerve, and not by breath pressure or muscular tension.
[8] Speech and voice scientists have long since abandoned this theory as the muscles have been shown to not be able to contract fast enough to accomplish the vibration.
More intricate mechanisms were occasionally described, but they were difficult to investigate, and until recently the state of the glottis and phonation were considered to be nearly synonymous.
Also, the existence of an optimal glottal shape for ease of phonation has been shown, at which the lung pressure required to initiate the vocal cord vibration is minimum.
However, the aperture of the arytenoid cartilages, and therefore the tension in the vocal cords, is one of degree between the end points of open and closed, and there are several intermediate situations utilized by various languages to make contrasting sounds.
Phonetically, they have no manner or place of articulation other than the state of the glottis: glottal closure for [ʔ], breathy voice for [ɦ], and open airstream for [h].
[9][page needed] In the last few decades it has become apparent that phonation may involve the entire larynx, with as many as six valves and muscles working either independently or together.
[11] The Bor dialect of Dinka has contrastive modal, breathy, faucalized, and harsh voice in its vowels, as well as three tones.
[11] Elements of laryngeal articulation or phonation may occur widely in the world's languages as phonetic detail even when not phonemically contrastive.
For example, simultaneous glottal, ventricular, and arytenoid activity (for something other than epiglottal consonants) has been observed in Tibetan, Korean, Nuuchahnulth, Nlaka'pamux, Thai, Sui, Amis, Pame, Arabic, Tigrinya, Cantonese, and Yi.
[11] In languages such as French and Portuguese, all obstruents occur in pairs, one modally voiced and one voiceless: [b] [d] [g] [v] [z] [ʒ] → [p] [t] [k] [f] [s] [ʃ].
These four registers contrast with each other, but no other combination of phonation (modal, breath, creak, closed) and tone (high, low, falling) is found.