The acoustic differences that allow us to distinguish between the various vowel productions are usually explained by a source-filter theory. The ‘source’ is the sound spectrum created by airflow through the glottis, which varies as the vocal folds vibrate. The ‘filter’ is the vocal tract itself, the shape of which can be controlled by the vocalist. We perceive vowels on the basis of the two lowest formant frequencies of the vocal tract. If you have not yet done so, it may be helpful to read the previous tutorial describing how vowels are formed.

Four Rules for Modifying Vowels

1. All formant frequencies decrease uniformly as the length of the vocal tract increases. This rule is good common sense; larger objects resonate sound at lower frequencies. For this reason, one uses long organ pipes, long strings, large drums, and big loudspeakers to produce low notes. We cannot change our vocal tracts’ lengths greatly, however. A 10% increase or decrease (roughly speaking) is possible by lowering or raising the larynx and by protruding or retracting the lips. This produces a comparable percentage shift in the formant frequencies. The result is a darker (or brighter) coloring of the vowels. To convince yourself of this, try phonating an [a] vowel and gradually lower your larynx by attempting to yawn; then try extending your lips forward as far as possible during an . Similar changes should be heard, i.e., the vowel sounds darker as it is modified toward .)
2. All formant frequencies decrease uniformly with lip rounding and increase with lip spreading. In the above exercise, you probably had to round your lips in order to protrude them, because there is only so much tissue to work with. Lip rounding is similar to partially covering the mouth. In both cases, the effective tube length increases (acoustically). This lowers all the resonant frequencies. (This time try phonating an and gradually cover half of your mouth with your hand. The vowel again changes toward .) It is likely that the term “covered sound” originated as a result of musicians covering the mouth ends of their instruments, particularly brass players. In singing, we usually don’t cover our mouths with the hand, but we can cover it by rounding the lips. In combination with larynx height adjustments, lip rounding or spreading can be very effective in darkening or brightening the vowels.
   Here’s how the above exercise sounds