Spectral Shaping 
Source Filter Theory
The cavities of the supra-glottal vocal tract resonate when
excited by sound energy. The exact resonance characteristics of the
vocal tract are a function of many factors, but the two factors of
primary importance are the length of the vocal tract and its shape,
that is, the cross sectional area profile from glottis to
lips. Resonances in the vocal tract reinforce the sound energy of the
source function at particular frequencies. These frequencies are
variously called resonant frequencies, poles, or formant frequencies.
In the following figure, we see the effects of passing a model source
function with its dense harmonic structure through a model vocal tract
producing an /ae/-like vowel. 
The top panel in
this figure shows the spectrum of the source function. The middle panel
in this figure shows the response characteristics of the vocal
tract. This is a relatively smooth and continuous function of
frequency with peaks at frequencies corresponding to the resonant
frequencies of the vocal tract. The bottom panel in this figure shows
the acoustic output from this model. It retains both the fine harmonic
structure of the source and the structure introduced by the resonance
characteristics of the vocal tract.
Example Synthetic Vowels
Here are three examples of vowels produced using the LF source
model.
References
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