Although the vocal folds can produce an amazing variety of sounds, it is the vocal tract that molds the raw sounds into language and music. The tract imposes a pattern on the folds¿ composite sound by picking out a certain combination of tones: namely, those that match the natural resonant frequencies of the air within the tract. As people speak or sing, they raise and lower the resonant frequencies--also known as formant frequencies--by moving their tongue, lips and so on.

These movements are normally perceived as shifts in vowel articulation. The frequency of the first formant, F1, is inversely related to tongue height (F1 falls as the tongue rises, as during the change from /a/ in "hot" to /i/ in "heed"). The frequency of the second formant, F2, is related to tongue advancement (F2 rises as the tongue moves forward, as when /o/ in "hoe" moves toward /i/ in "heed"). Theoretically, the vocal tract has an infinite number of formants, but the arrangement of the first two or three accounts for most of the difference among vowel sounds (below, right).

   
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