Vocal mechanism

In Chapter 4 we introduced the physiology of the human vocal mechanism from a physical modelling point of view. Here we will study the vocal tract from a subtractive synthesis standpoint. 

An 18-year-old man was found collapsed outside the nightclub. He had taken five ecstasy tablets and some powder that was later confirmed as ecstasy. He was vomiting and agitated, had a tachycardia and hyperthermia, and needed mechanical ventilation. He later developed rhabdomyolysis and renal impairment with raised liver enzymes. He went on to develop pneumonia and a urinary tract infection. He was discharged after 32 days with a mildly ataxic gait and dysphonia secondary to vocal cord damage. 

This model, whereby we see speech as being generated by a basic sound source, and then further modified by the vocal tract is known as the source/filter model of speech. The separation into source and filter not only adequately represents the mechanics of production, it also represents a reasonable model of perception in that it is known that listeners separate their perception of the source in terms of its fundamental frequency from the modified pattern of its harmonics. Furthermore, we know that the main acoustic dimension of prosody is fundamental frequency, whereas the main dimensions of verbal distinction are made from a combination of the type of sound source (but not its frequency), and the modification by the vocal tract. The mathematics of both the source and the filter will be fully described in Chapter 10. 

An amplification caused by a filter is called a resonance, and in speech these resonances are known as formants. The frequencies at which resonances occur are determined solely by the position of the vocal tract they are independent of the glottis. So no matter how the harmonics are spaced, for a certain vocal tract position the resonances will always occur at the same frequencies. Different mouth shapes give rise to different patterns of formants, and in this way, the production mechanisms of height and loudness give rise to different characteristic acoustic patterns. As each vowel has a different vocal tract shape, it will have different formant pattern, and it is these that the listener uses as the main cue to vowel identity. The relationship between mouth shapes and formant patterns is complicated, and is fully examined in Chapter 11. 

While these comphcations were attributed by the authors to high-dose intralesional injections of ddofovir, it is possible that similar morphological changes could occur in the absence of the medication, owing to mechanical damage to the vocal folds from the devices used. Carefiil foUow-up observation of future patients wiU be required to assess whether these complications occur in other cases. 

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