Formants and Articulatory Information

Formants, so-called resonances, represent the source of articulatory information and are used as primary features in vowel quality. They are the dark bands observed in the spectrogram window of Figure 3.3. Formants are also identified by number in order of increasing frequency: , , etc. In acoustic phonetics, usually only and are considered, but, for recognition, at least three are meaningful and, for synthesis, five are recommended. The notion of formants is particularly useful in connection with vowel sounds. The arrangement of formants, what is called the formant structure, is the basis for the characteristics of most vowel differences. The peculiar property of the vocal tract is that its acoustic performance can be changed so as to bring about readily perceptible differences in formant structure. These changes are of course the result of differences in articulation which affect the shape and the dimensions of the vocal tract. The most important modifications are due to alterations in the configuration of the tongue. The tongue moves up and down and wherever it is highest in the mouth it forms a short tube which couples together the vocal tract. The length and the cross-section of this tube have an influence on the formant structure of the vowel sound and a further modification is introduced by the shape and the extent of the lip opening[30].