Formant
In speech science and phonetics, a formant is the broad spectral maximum that results from an acoustic resonance of the human vocal tract. In acoustics, a formant is usually defined as a broad peak, or local maximum, in the spectrum. For harmonic sounds, with this definition, the formant frequency is sometimes taken as that of the harmonic partial that is most augmented by a resonance. The difference between these two definitions resides in whether "formants" characterise the production mechanisms of a sound or the produced sound itself. In practice, the frequency of a spectral peak differs from the associated resonance frequency, except when, by luck, harmonics are aligned with the resonance frequency.
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.
History
From an acoustic point of view, phonetics had a serious problem with the idea that the effective length of the 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 frequency of the “formant” may vary a little without altering the character of the vowel. For a, for example, the “formant” may vary from 350 to 440 Hz even in the same person.Phonetics
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 or 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 sufficient to identify 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 through a pair of bandpass filters.
Nasal consonants usually have an additional formant around 2500 Hz. The liquid usually has an extra formant at 1500 Hz, whereas the English "r" sound is distinguished by a very low third formant.
Plosives modify the placement of formants in the surrounding vowels. Bilabial sounds cause a lowering of the formants; velar sounds 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 cause fewer systematic changes in neighbouring 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 a resonance frequency of the system, then the formant usually imparted by that resonance 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 resonances 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 may be used to visualise formants. In spectrograms, it can be hard to distinguish formants from naturally occurring harmonics when one sings. However, one can hear the natural formants in a vowel shape through atonal techniques such as vocal fry.
Formant estimation
Formants, whether they are seen as acoustic resonances of the vocal tract, or as local maxima in the speech spectrum, like band-pass filters, are defined by their frequency and by their spectral width.Different methods exist to obtain these informations. Formant frequencies, in their acoustic definition, can be estimated from the frequency spectrum of the sound, using a spectrogram or a spectrum analyzer. However, to estimate the acoustic resonances of the vocal tract from a speech recording, one can use linear predictive coding. An intermediate approach consists in extracting the spectral envelope by neutralizing the fundamental frequency, and only then looking for local maxima in the spectral envelope.
Formant plots
The first two formants are important in determining the quality of vowels, and are frequently said to correspond to the open/close and front/back dimensions. Thus the first formant F1 has a higher frequency for an open vowel and a lower frequency for a close vowel ; and the second formant F2 has a higher frequency for a front vowel and a lower frequency for a back vowel. as can be seen in Fig. 1. 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, though this is not sufficient to capture some aspects of vowel quality, such as rounding. An example of how the vowels of a language or dialect may be plotted on a traditional auditory vowel chart and also on a formant plot may be seen in the case of Norwegian.Many writers have addressed the problem of finding an optimal alignment of the positions of vowels on formant plots with those on the conventional vowel quadrilateral. The pioneering work of Ladefoged used the Mel scale because this scale was claimed to correspond more closely to the auditory scale of pitch than to the acoustic measure of fundamental frequency expressed in Hertz as in Fig. 1. Two alternatives to the Mel scale are the Bark scale and the ERB-rate scale. A comparison of these three scales is shown by Hayward, p. 141, and formant plots based on the Hertz scale and on the Bark scale are compared on p. 153. Another strategy for improving formant plots that has been widely adopted is to plot on the horizontal axis not the value of F2 but the difference between F1 and F2 for a given vowel.