Editing Phonation (section)

==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),<ref name=":0" /><ref name=":1" /> and for humans with normal vocal folds, it is approximately 2–3&nbsp;cm.  The motion of the vocal folds during oscillation is mostly lateral, though there is also some superior component as well.  However, there is almost no motion along the length of the vocal folds.  The oscillation of the vocal folds serves to modulate the pressure and flow of the air through the larynx, and this modulated airflow is the main component of the sound of most [[Voice (phonetics)|voiced]] [[Phone (phonetics)|phones]].

The sound that the larynx produces is a [[Harmonic series (music)|harmonic series]]. In other words, it consists of a fundamental tone (called the fundamental frequency, the main acoustic cue for the percept [[Pitch (psychophysics)|pitch]]) accompanied by harmonic overtones, which are multiples of the fundamental frequency.<ref>The human instrument. Principles of Voice Production, Prentice Hall (currently published by NCVS.org)</ref> According to the [[Source–filter model of speech production|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.<ref name=":2" /> In linguistics, a [[Phone (phonetics)|phone]] is called [[Voice (phonetics)|voiceless]] if there is no phonation during its occurrence.<ref name="Greene">{{cite book
| title = The Voice and its Disorders
| last = Greene
| first = Margaret
|author2=Lesley Mathieson
| year = 2001
| publisher = John Wiley & Sons; 6th Edition
| isbn =  978-1-86156-196-1}}</ref>  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.<ref name="Zemlin">{{cite book
| title = Speech and hearing science : anatomy and physiology
| last = Zemlin
| first = Willard
| year = 1998
| publisher = Allyn and Bacon; 4th edition
| isbn = 0-13-827437-1}}</ref>

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 cartilage]]s, 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.<ref name="Zemlin"/>  In addition to tension changes, fundamental frequency is also affected by the pressure drop across the larynx, which is mostly affected by the pressure in the lungs, and will also vary with the distance between the vocal folds. Variation in fundamental frequency is used linguistically to produce [[Intonation (linguistics)|intonation]] and [[tone (linguistics)|tone]].

There are currently two main theories as to how vibration of the [[vocal folds]] is initiated: the '''myoelastic theory''' and the '''aerodynamic theory'''.<ref name="Titze, I. R. 2006">Titze, I. R. (2006). The Myoelastic Aerodynamic Theory of Phonation, Iowa City:National Center for Voice and Speech, 2006.</ref> 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]] are brought together and breath pressure is applied to them, the cords 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 cords are pushed apart, and the whole cycle keeps repeating itself. The rate at which the cords open and close, the number of cycles per second, determines the pitch of the phonation.<ref name="McKinney">{{cite book
| title = The Diagnosis and Correction of Vocal Faults
| last = McKinney
| first = James
| year = 1994
| publisher = Genovex Music Group
| isbn =  978-1-56593-940-0}}</ref>

The aerodynamic theory is based on the [[Bernoulli's principle|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.<ref name=":0">{{Cite journal|title = The physics of small-amplitude oscillation of the vocal folds|last = Titze|first = I.R.|date = 1988|journal = Journal of the Acoustical Society of America|doi = 10.1121/1.395910|pmid = 3372869|volume = 83|issue = 4|pages = 1536–1552|bibcode = 1988ASAJ...83.1536T}}</ref> 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.<ref name=":1">{{Cite journal|title = The minimum lung pressure to sustain vocal fold oscillation|last = Lucero|first = J. C.|date = 1995|journal = Journal of the Acoustical Society of America|doi = 10.1121/1.414354|pmid = 7642816|volume = 98|issue = 2|pages = 779–784|bibcode = 1995ASAJ...98..779L|s2cid = 24053484}}</ref> The amount of lung pressure needed to begin phonation is defined by Titze as the oscillation threshold pressure.<ref name=":0" /> During glottal closure, the air flow is cut off until breath pressure pushes the folds apart and the flow starts up again, causing the cycles to repeat.<ref name="McKinney"/>
The textbook entitled [[Myoelastic Aerodynamic Theory of Phonation]]<ref name="Titze, I. R. 2006"/> by [[Ingo Titze]] credits Janwillem van den Berg as the originator of the theory and provides detailed mathematical development of the theory.

===Neurochronaxic theory===
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.  Advocates of this theory thought that every single vibration of the vocal folds was due to an impulse from the recurrent laryngeal nerves and that the acoustic center in the brain regulated the speed of vocal fold vibration.<ref name="McKinney"/>  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.  In addition, persons with paralyzed vocal folds can produce phonation, which would not be possible according to this theory.  Phonation occurring in excised larynges would also not be possible according to this theory.