At the beginning of the previous chapter we saw that from the acoustic point of view the complete speech mechanism can be seen as a sound source coupled to a resonant system. The system is the air-way which leads from the larynx outwards through the pharynx and the mouth to the outer air, together with the path through the naso-pharynx and out through the nostrils when this branch is opened by the lowering of the soft palate. This is the system which is driven into forced vibrations by the pulse wave generated in the larynx. The sound waves radiated at the lips and the nostrils are the result of modifications imposed on the larynx wave by this resonating system. The first question therefore is: what are the properties of the vocal tract which determine these changes?

Forced vibrations, as we saw in Chapter 5, are the result of reflections and standing waves in the system, and these in turn are dependent on the natural frequencies and the damping of the system. It is these characteristics of the vocal tract that we need to examine.

Acoustic properties of the vocal tract

The example of the musical wind instruments showed that the dimensions of the air column involved were all-important in determining the frequencies at which resonance would occur. This must be so since the relation between the wavelength of sounds and these dimensions is the key to the phenomenon of resonance.