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Formants are defined by Fant [1] as 'the spectral peaks of the sound spectrum |P(f)|' of the voice. Formant is also used to mean an acoustic resonance,[2] and, in speech science and phonetics, a resonance of the human vocal tract. It is often measured as an amplitude peak in the frequency spectrum of the sound, using a spectrogram (in the figure) or a spectrum analyzer, though in vowels spoken with a high fundamental frequency, as in a female or child voice, the frequency of the resonance may lie between the widely-spread harmonics and hence no peak is visible. In acoustics, it refers to a peak in the sound envelope and/or to a resonance in sound sources, notably musical instruments, as well as that of sound chambers. However, it is equally valid to talk about the formant frequencies of a room, as exploited, for example, by Alvin Lucier in his piece I Am Sitting in a Room.


Formants and phonetics

Formants are the distinguishing or meaningful frequency components of human speech and of singing. By definition, the information that humans require to distinguish between vowels can be represented purely quantitatively by the frequency content of the vowel sounds. In speech, these 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 lowest frequency 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 determine the quality of vowels in terms of the open/close and front/back dimensions (which have traditionally, though not entirely accurately, been associated with the position of the tongue). Thus the first formant f1 has a higher frequency for an open vowel (such as [a]) and a lower frequency for a close vowel (such as [i] or [u]); and the second formant f2 has a higher frequency for a front vowel (such as [i]) and a lower frequency for a back vowel (such as [u]).[3][4] Vowels will almost always have four or more distinguishable formants; sometimes there are more than six. However, the first two formants are most important in determining vowel quality, and this is often displayed in terms of a plot of the first formant against the second formant,[5] though this is not sufficient to capture some aspects of vowel quality, such as rounding.[6]

Nasals usually have an additional formant around 2500 Hz. The liquid [l] usually has an extra formant at 1500 Hz, while the English "r" sound ([ɹ]) is distinguished by virtue of a very low third formant (well below 2000 Hz).

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