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The vocal folds, also known popularly as vocal chords, are composed of twin infoldings of mucous membrane stretched horizontally across the larynx (like elastic bands). They vibrate, modulating (changing) the flow of air being expelled from the lungs during phonation/singing. One can change the thickness of the vocal chords by making different sounds. This also determines the loudness of your voice. When speaking or singing the vocal chords come together and when taking a breath in, they open. In singing, when the vocal chords are not closing completely, the voice tone will be breathy. On the other hand, if the vocal chords are closing shut all the way along their length, the singing voice sound will be clear because the sound wave will be strong.
Another name for the airway at the level of the vocal folds is the glottis, and the opening between the folds is called the glottic chink. The size of the glottic chink is important in respiration and phonation. Open during inhalation, closed when holding one's breath, and held apart just a tiny bit for speech or singing; the folds are controlled via the vagus nerve. They are white because of scant blood circulation.
The vocal folds vibrate when they are closed to obstruct the airflow through the glottis, the space between the folds: they are forced open by increased air pressure in the lungs, and closed again as the air rushes past the folds, lowering the pressure (Bernoulli's principle). A person's voice pitch is determined by the resonant frequency of the vocal folds. In an adult male this frequency averages about 125 Hz, adult females around 210, in children the frequency is over 300 Hz.
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.
Matching the female body, which on the whole has less muscle than the male, females have smaller folds. The female vocal folds are between 12.5 mm and 17.5 mm in length.
As seen in the illustration, 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 ligaments 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 on 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 colour - whiter in females then they are in males. 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).
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.
The term vocal cords is occasionally misspelled 'vocal chords', possibly due to the musical connotations or to confusion with the geometrical definition of the word "chord".
The vocal folds discussed above are sometimes called 'true vocal folds' to distinguish them from the false vocal folds (false vocal cords). These are a pair of thick folds of mucous membrane that sit just above, and protect, the more delicate true folds. They have minimal role in normal phonation, but are often used in screaming and the death grunt singing style.
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.