Formants

A formant, as defined by James Jeans, is a harmonic of a note that is augmented by a resonance. The speech researcher Gunnar Fant defines formants as "the spectral peaks of the sound spectrum |P(f)|". In acoustics generally, a very similar definition is widely used: the Acoustical Society of America defines a formant as: "a range of frequencies [of a complex sound] in which there is an absolute or relative maximum in the sound spectrum". In speech science and phonetics, however, a formant is also sometimes used to mean an acoustic resonance of the human vocal tract. Thus, in phonetics, formant can mean either a resonance or the spectral maximum that the resonance produces. Formants are often measured as amplitude peaks in the frequency spectrum of the sound, using a spectrogram (in the figure) or a spectrum analyzer and, in the case of the voice, this gives an estimate of the vocal tract resonances. In vowels spoken with a high fundamental frequency, as in a female or child voice, however, the frequency of the resonance may lie between the widely spaced harmonics and hence no corresponding peak is visible.

A room can be said to have formants characteristic of that particular room, due to the way sound reflects from its walls and objects. Room formants of this nature reinforce themselves by emphasizing specific frequencies and absorbing others, as exploited, for example, by Alvin Lucier in his piece I Am Sitting in a Room.

Formants are distinctive frequency components of the acoustic signal produced by speech or singing. The information that humans require to distinguish between speech sounds can be represented purely quantitatively by specifying peaks in the amplitude/frequency spectrum. 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. The relationship between the perceived vowel quality and the first two formant frequencies can be appreciated by listening to "artificial vowels" that are generated by passing a click train (to simulate the glottal pulse train) through a pair of bandpass filters (to simulate vocal tract resonances). An interactive demonstration of this can be found here.

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