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Vocal folds

The vocal folds, also known as vocal cords, are found at the base of the human larynx, they are composed of twin infoldings of mucous membrane which vibrate modulating the flow of air being expelled from the lungs during phonation.

Open during breathing, the folds are controlled via the arytenoid cartilages for speech or singing.

The folds vibrate when they are closed to obstruct the airflow through the glottis (see below): 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.

The space between the vocal cords is called the glottis. As the vocal cords vibrate, the resulting vibration produces a "buzzing" quality to the speech, called voice or voicing.

Sounds production involving only the glottis is called glottal. English has a glottal approximant spelt "h". In many accents of English the glottal stop (made by pressing the folds together) is used as a variant allophone of the phoneme /t/.

The vibration produced is an essential component of voiced consonants as well as vowels. If the vocal folds are drawn apart, air flows between them causing no vibration, as in the production of voiceless consonants.

In English, they tend to go in pairs, for example t and d, b and p.