Rules for Modifying
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
- 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 .)
- 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.
In addition to these front-end and back-end modifications,
jaw lowering can be used to modify vowels. In particular, F1,
the first formant, can be raised substantially by lowering the jaw.
Two other rules are applicable to vowel formation
in which either the front half (the mouth) or the back half (the
pharynx) is narrowed.
- A mouth constriction lowers the first formant
and raises the second formant. This creates a more diffuse
vowel spectrum. The acoustic energy is spread out over both low
and high frequencies, as in the vowels [i] and [e].
- A pharyngeal constriction raises the first
formant and lowers the second formant. This makes the
vowel spectrum more compact, as in the case of or