Formant
|
Spectrogram of American English vowels [i, u, ɑ] showing the formants f1 and f2 |
A
formant is a peak in an acoustic
frequency spectrum which results from the
resonant frequencies of any acoustical system. It is most commonly invoked in
phonetics or
acoustics involving the resonant frequencies of
vocal tracts or
musical instruments. However, it is equally valid to talk about the formant frequencies 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 components 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 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 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 1000 Hz for a male adult, with 1000 Hz per formant. Vowels will almost always have four or more distinguishable formants; sometimes there are more than six.
Nasals usually have an additional formant around 2500 Hz. The liquid usually has an extra formant at 1500 Hz, while the
English "r" sound (
IPA ) is distinguished by virtue of a very low third formant (well below 2000 Hz).
Plosives (and, to some degree, fricatives) modify the placement of formants in the surrounding vowels.
Bilabial sounds (such as 'b' and 'p' as in "ball" or "sap") cause a lowering of the formants;
velar sounds ('k' and 'g' in English) almost always show
f2 and
f3 coming together in a 'velar pinch' before the velar and separating from the same 'pinch' as the velar is released;
alveolar sounds (English 't' and 'd') cause less systematic changes in neighboring vowel formants, depending partially on exactly which vowel is present. The time-course of these changes in vowel formant frequencies are referred to as 'formant transitions'.
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
overtone 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.
Vowel formant centers| Vowel | IPA | Formant f1 | Formant f2 |
|---|
| u | 320 Hz | 800 Hz |
| o | 500 Hz | 1000 Hz |
| aː | 700 Hz | 1150 Hz |
| a | 1000 Hz | 1400 Hz |
| ø | 500 Hz | 1500 Hz |
| y | 320 Hz | 1650 Hz |
| ɛ | 700 Hz | 1800 Hz |
| e | 500 Hz | 2300 Hz |
| i | 320 Hz | 3200 Hz |
Vowel formants| Vowel | Main formant region |
|---|
| 200 to 400 Hz |
| 400 to 600 Hz |
| 800 to 1200 Hz |
| 400 to 600 and 2200 to 2600 Hz |
| 200 to 400 and 3000 to 3500 Hz |
Studies of the frequency spectrum of trained
singers, especially male singers, indicate a clear formant around 3000 Hz (between 2800 and 3400) that is absent in speech or in the spectra of untrained singers. It is this formant which allows singers to be heard and understood over an
orchestra. This formant is actively developed through
vocal training, for instance through so-called
voce di strega exercises.
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