A formant is a preferred resonating frequency of any acoustical system. It is most commonly invoked in phonetics or acoustics as the preferred vibrations of vocal tracts or musical instruments. However, it is equally valid to talk about the formant frequency of the air in a room, as exploited, for example, by Alvin Lucier in his piece I am sitting in a room.

Formants are the distinguishing or meaningful frequency component of human speech and of singing. By definition, the information that humans require to distinguish between vowels can be representated purely quantitatively by the frequency content of the vowel sounds. Formants are the characteristic partials that identify vowels to the listener. Most of these formants are produced by tube and chamber resonance, but a few whistle tones derive from periodic collapse of Venturi effect low-pressure zones. The formant with the highest sound pressure is called f1, the second f2, and the third f3. Most often the two first formants, f1 and f2, are enough to disambiguate the vowel. These two formants are primarily determined by the position of the tongue--F1 has a higher frequency when the tongue is lowered, and F2 has a higher frequency when the tongue is forward. Generally, formants move about in a range of approximately 1000Hz for a male adult, with 1000Hz per formant. Vowels will almost always have four or more distinguishable formants; sometimes there are more than six.

Not all sounds used in human language are composed of formants. Formants are restricted to a subset of pulmonic sounds including vowels, approximants, and nasals. Nasals usually have a formant around 2500Hz in addition to two lower formants (and, where applicable, voicing). The liquid [l] usually has a formant at 1500Hz, while the English "r" sound (X-SAMPA [r`]) is distinguished by virtue of the third formant, which dips below 2000Hz.

Plosives (and, to some degree, fricatives) modify the placement of formants on the surrounding vowels. The distinguishing formant drop for [r`] is characteristic of retroflexes, for instance. Bilabial sounds (such as 'b' and 'p' as in "ball" or "sap") sometimes feature a dip in the first two formants. Velar sounds ('k' and 'g' in English) almost always show F2 and F3 coming together before the velar and separating from a point once the velar sound is completed. Alveolar and dental sounds (English 't' and 'd') show little change from the ordinary formant positions.

Note that fricatives always lack formant structure and are distinguished by the frequency range with the most noise, as well as overall strength of frication.

If the fundamental frequency of the underlying vibration is higher than the formant frequency of the system, then the character of the sound imparted by the formant frequencies will be mostly lost. This is most apparent in the example of soprano opera singers, who sing high enough that their vowels become very hard to distinguish.

Control of formants is an essential component of the vocal technique known as throat singing, in which the performer sings a low fundamental tone, and creates sharp resonances to select upper harmonics, giving the impression of several tones being sung at once.

Spectrograms are used to visualise formants.

See also

• vocoder
• linear predictive coding

External links

• Formants for fun and profit (http://ccms.ntu.edu.tw/~karchung/Phonetics%20II%20page%20nineteen.htm)
• Formants and wah-wah pedals (http://www.geofex.com/Article_Folders/wahpedl/voicewah.htm)