The 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.
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 of sound waves, called triygergonus.
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. The fundamental frequency is influenced by 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. The harmonics are produced by collisions of the vocal folds with themselves, by recirculation of some of the air back through the trachea, or both.
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.
Vocal folds are located within the larynx at the top of the trachea. They are attached posteriorly to the arytenoid cartilages, and anteriorly 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 folds (the hole and the ligament itself) are the ''vestibular folds'' or ''false vocal folds'' which have a small sac between the two folds (not illustrated).
Situated above the larynx, the epiglottis acts as a flap which closes off the trachea during the act of swallowing to direct food into a separate tube behind the trachea called the esophagus. If food or liquid does enter the trachea and contacts the vocal folds because of a failure of this safeguard ("going down the wrong pipe"), it causes a cough reflex to expel the matter in order to prevent choking.
Men and women have different vocal fold sizes. Adult male voices are usually lower pitched and have larger folds. The male vocal folds are between 17 mm and 25 mm (approx 0.75" to 1.0") in length.
The female vocal folds are between 12.5 mm and 17.5 mm (approx 0.5" to 0.75") 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.
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