Formant frequency
A formant, as defined by James Jeans, is a harmonic of a note that is augmented by a resonance. Speech researcher Gunnar Fant defined 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 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.
From an acoustic point of view, phonetics had a serious problem with the idea that length of vocal tract changed vowels. It was unclear how they could depend on frequencies when everyone from bass to soprano can make the same vowels. There had to be some way to normalize the frequencies. Hermann suggested a solution to this problem in 1894, coining the term "formant". "A vowel, according to him, is a special acoustic phenomenon, depending on the intermittent production of a special partial, or "formant", or "characteristique". The pitch of the "formant" may vary a little without altering the character of the vowel. For a, for example, the "formant" may vary from fa4 to la4 even in the same person."
...
Wikipedia