The term phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation or phonating is the process by which the vocal cords, or vocal folds, produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology and speech production in general. Phoneticians in other subfields, such as linguistic phonetics, call this process voicing, and use the term phonation to refer to any oscillatory state of any part of the larynx that modifies the airstream, of which voicing is just one example. Voiceless and supra-glottal phonations are included under this definition.
Voicing
The phonatory process, or voicing, occurs when air is expelled from the lungs through the glottis, creating a pressure drop across the larynx. When this drop becomes sufficiently large, the vocal folds start to oscillate. The minimum pressure drop required to achieve phonation is called the phonation threshold pressure (PTP), According to the source–filter theory, the resulting sound excites the resonance chamber that is the vocal tract to produce the individual speech sounds.
The vocal folds will not oscillate if they are not sufficiently close to one another, are not under sufficient tension or under too much tension, or if the pressure drop across the larynx is not sufficiently large. In speech, voiceless phones are associated with vocal folds that are elongated, highly tensed, and placed laterally (abducted) when compared to vocal folds during phonation.
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. Smaller changes in tension can be effected by contraction of the thyroarytenoid muscle or changes in the relative position of the thyroid and cricoid cartilages, as may occur when the larynx is lowered or raised, either volitionally or through movement of the tongue to which the larynx is attached via the hyoid bone. These two theories are not in contention with one another and it is quite possible that both theories are true and operating simultaneously to initiate and maintain vibration. A third theory, the neurochronaxic theory, was in considerable vogue in the 1950s, but has since been largely discredited.
Myoelastic and aerodynamic theory
The myoelastic theory states that when the vocal cords, or vocal folds, are brought together and breath pressure is applied to them, the folds remain closed until the pressure beneath them, the subglottic pressure, is sufficient to push them apart, allowing air to escape and reducing the pressure enough for the muscle tension recoil to pull the folds back together again. The pressure builds up once again until the folds are pushed apart, and the whole cycle keeps repeating itself. The rate at which the folds open and close, the number of cycles per second, determines the pitch of the phonation.
The aerodynamic theory is based on the Bernoulli energy law in fluids. 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. The push occurs during glottal opening, when the glottis is convergent, and the pull occurs during glottal closing, when the glottis is divergent. 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. The amount of lung pressure needed to begin phonation is defined by Titze as the oscillation threshold pressure.
{| class="wikitable"
|+ style="text-align: left;"|Types of Laryngeal Setting
! Type
! Definition
|-
! Modal voice
| Regular vibrations of the vocal folds
|-
! Voiceless
| Lack of vibration of the vocal folds; arytenoid cartilages usually apart
|-
! Aspirated
| Having greater airflow than in modal voice before or after a stricture; arytenoid cartilages may be further apart than in voiceless
|-
! Breathy voice
| Vocal folds vibrating but without appreciable contact; arytenoid cartilages further apart than in modal voice
|-
! Slack voice
| Vocal folds vibrating but more loosely than in modal voice
|-
! Creaky voice
| Vocal folds vibrating anteriorly, but with the arytenoid cartilages pressed together; lower airflow than in modal voice
|-
! Stiff voice
| Vocal folds vibrating but more stiffly than in modal voice
|}
class=skin-invert-image|framed|center|alt=Diagram of glottis positions|
If the vocal folds are completely relaxed, with the arytenoid cartilages apart for maximum airflow, the folds do not vibrate. This is voiceless phonation, and is extremely common with obstruents. If the arytenoids are pressed together for glottal closure, the vocal folds block the airstream, producing stop sounds such as the glottal stop. In between there is a sweet spot of maximum vibration. 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. This is modal voice, and is the normal state for vowels and sonorants in all the world's languages. However, the aperture of the arytenoid cartilages, and therefore the tension in the vocal folds, 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.
Glottal consonants
It has long been noted that in many languages, both phonologically and historically, the glottal consonants do not behave like other consonants. Phonetically, they have no manner or place of articulation other than the state of the glottis: glottal closure for , breathy voice for , and open airstream for . Some phoneticians have described these sounds as neither glottal nor consonantal, but instead as instances of pure phonation, at least in many European languages. However, in Semitic languages they do appear to be true glottal consonants.
- glottal (the vocal cords), producing the distinctions described above
- ventricular (the 'false vocal cords', partially covering and damping the glottis)
- arytenoid (sphincteric compression forwards and upwards)
- epiglotto-pharyngeal (retraction of the tongue and epiglottis, potentially closing onto the pharyngeal wall)
- raising or lowering of the entire larynx
- narrowing of the pharynx
Until the development of fiber-optic laryngoscopy, the full involvement of the larynx during speech production was not observable, and the interactions among the six laryngeal articulators is still poorly understood. However, at least two supra-glottal phonations appear to be widespread in the world's languages. These are harsh voice ('ventricular' or 'pressed' voice), which involves overall constriction of the larynx, and faucalized voice ('hollow' or 'yawny' voice), which involves overall expansion of the larynx. The term "register" may be used for several distinct aspects of the human voice: