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 figure on the left, 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. To hear what the vowel sounds like, click on the vowel phonetic symbol. To see the spectrum, click on the word Spectrum and to see a graph of the vowel waveform, click on the word Waveform.