The vocal folds, also known commonly as vocal cords, are composed of twin infoldings of mucous membrane stretched horizontally across the larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation.  
Open during inhalation, closed when holding one's breath, and vibrating for speech or singing (oscillating 440 times per second when singing A above middle C); the folds are controlled via the vagus nerve. They are white because of scant blood circulation.
Vocal fold oscillation
The larynx is a major (but not the only) source of sound in speech, generating sound through the rhythmic opening and closing of the vocal folds. To oscillate, the vocal folds are brought near enough together such that air pressure builds up beneath the larynx. The folds are pushed apart by this increased subglottal pressure, with the inferior part of each fold leading the superior part. Under the correct conditions, this oscillation pattern will sustain itself. In essence, sound is generated in the larynx by chopping up a steady flow of air into little puffs.
The perceived pitch of a person's voice is determined by a number of different factors, not least of which is the fundamental frequency of the sound generated by the larynx. A person's natural fundamental frequency is influenced by many factors, including the length, size, and tension of the vocal folds. In an adult male, this frequency averages about 125 Hz, adult females around 210 Hz, in children the frequency is over 300 Hz. Depth-Kymography is an imaging method to visualize the complex horizontal and vertical movements of vocal folds.
The vocal folds generate a sound rich in harmonics. Some singers can isolate some of those harmonics in a way that is perceived as singing in more than one pitch at the same time -- a technique called overtone singing.
The folds are located below the epiglottis, the lid-like flap that separates the windpipe from the esophagus.
The folds are located just above the trachea or the windpipe which travels from the lungs. Food and drink does not pass through the folds but is instead taken through the esophagus, an unlinked tube. Both tubes are separated by the tongue and an automatic gag reflex. When food goes down through the folds and trachea it causes choking.
Folds in both sexes are within the larynx. They are attached at the back (side nearest the spinal cord) to the arytenoid cartilages, and at the front (side under the chin) to the thyroid cartilage. Their outer edges (as shown in the illustration) are attached to muscle in the larynx while their inner edges, or margins are free (the hole). They are constructed from epithelium, but they have a few muscle fibres in them, namely the vocalis muscle which tightens the front part of the ligament near to the thyroid cartilage. They are flat triangular bands and are pearly white in color. Above both sides of the vocal cord (the hole and the ligament itself) is the vestibular fold or false vocal fold, which has a small sac between its two folds (not illustrated).
Men and women have different vocal fold sizes. Adult male voices are usually lower pitched and have larger folds. The male vocal folds (which would be measured vertically in the opposite diagram), are between 17 mm and 25 mm in length.
The female vocal folds are between 12.5 mm and 17.5 mm in length.
Folds are pearly white in color - whiter in females than they are in males.
The difference in vocal fold size between men and women means that they have differently pitched voices. Additionally, genetics also causes variances amongst the same sex, with men's and women's voices being categorised into types.
Vocal cords, a term commonly used to refer to the vocal folds, is also spelled 'vocal chords', possibly due to the musical connotations or to confusion with the geometrical definition of the word "chord". While both spellings are historically correct, standard American spelling is 'vocal cords'.  According to the Oxford English corpus, contemporary writers opt for vocal chords instead of vocal cords 49% of the time.  The 'vocal chords' variant has long been accepted in the United Kingdom (along with other anatomical uses like 'spinal chord'). Even in the United States, both variants can be found from early on, and it was only later on that American writers settled on 'vocal cords' as the standard version.
False vocal folds
The vocal folds discussed above are sometimes called 'true vocal folds' to distinguish them from the false vocal folds. These are a pair of thick folds of mucous membrane that protect and sit slightly superior to the more delicate true folds. They have minimal role in normal phonation, but are often used in musical screaming and the death grunt singing style. They are also used in Tuvan throat singing.
The false folds are also called vestibular folds and ventricular folds. They can be seen on the diagram above as ventricular folds.
False vocal folds, when surgically removed, can regenerate completely.
Published - November 2008
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