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 of sound waves.
You can see a movie clip showing human vocal folds in action here.
The perceived pitch of a person's voice is determined by a number of different factors, most importantly 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.
Read more about this topic: Vocal Folds
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