How the Vocal Tract Filters Sound
The two largest spaces in the vocal tract, the throat and mouth, therefore, produce the two lowest resonant frequencies, or formants. These formants are designated as F1 (the throat/pharynx) and F2 (the mouth). In singing or speaking, it is these two lowest formants that are controlled by shaping the resonant areas with lip and tongue movements to produce vowels. [Vocalists can also change the length of the vocal tract to modify formant frequencies; for details on this, see the tutorial on Rules for Modifying Vowels.]
Which formant frequencies result in which vowels?
The vowels , [i], and [u] represent the three extremes of F1-F2 locations in the vowel chart and tongue placement. The other seven vowels are placed within these extremes. The three "corner vowels" are easy to remember by tongue placement:
Note, too, that the vocal tract shape (as in the word "us") is located at the center of the vowel chart. It is oftened referred to as the neutral vowel because the tongue is neither high nor low, forward nor back. Make this sound. Do you see why scientists can use a tube shape to mimic the vocal tract for speech simulation research? The vocal tract is roughly uniform in cross-sectional shape from bottom (just above the larynx) to top (lips).
Individual Differences in Vowel Production