How the Vocal Tract Filters Sound
How are vowels formed?
As we phonate, our vocal folds produce a complex
sound spectrum, made up of a wide range of frequencies and overtones.
As this spectrum travels through the various differently-sized
areas in the vocal tract, some of these frequencies will resonate
more than others, depending on the sizes of the resonant areas
in the tract. Larger spaces in the vocal tract will resonate at
lower frequencies, while smaller spaces
resonate at higher frequencies.
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 following vowel chart, adapted from the work of G.E. Peterson and H.L.
Barney in 1952, shows the frequency regions for F1 and F2 which
result in the 10 English 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:
- For the , or "ah" sound
as in father, the tongue is low and back;
- For the [i], or "ee" sound as in keep, the tongue is high
and front; and
- For the [u], or "oo" sound as in loot, the tongue is high
and back.
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
As a concluding note, a rich area of research involves the study of differences
among individuals in regard to formant frequencies. These differences are attributable
to differences in size, age, gender and speech habits. Differences can also
be observed between speaking and singing, and between singers due to variability
of training techniques. Often, the singer must balance vowel intelligibility
with a beautiful quality of sound.
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~ National Center for Voice and Speech, www.ncvs.org
