5 - The imitating voice

Summary

Like any other musical instrument, our voice consists of a vibration generator and resonators. The vocal chords in the larynx generate the sound vibration to which the large number of interconnected and oddly shaped air-filled cavities resonate; the size and shape of some of the cavities and therefore their resonances can be varied. The harmonicity of a voice depends on the form of vibration of the vocal chords. Descriptions of the process often make the same erroneous assumption that has been used in describing the vibration generators of instruments, that the larynx produces a sawtooth wave form (Fig. 5B). That vibration has an ideal set of harmonics, and hence all voices would have perfect harmonicity. In reality, the behaviour of the larynx varies enormously from one person to another, and since no mechanical system can produce an ideal sawtooth wave anyway, you can draw your own conclusion about what a larynx can generate.

Whatever harmonics are produced by the larynx, their sizes in the vocal sound depend upon which ones excite the resonances of the various cavities. In particular, we learn to vary the resonances of the mouth cavity by changing its shape and size, and select which harmonics to emphasise in the sounds; that produces characteristic tone sensations which we identify as vowels. In principle, an ‘eee’ sound has emphasised higher harmonics from a small mouth cavity and an ‘ooo’ sound has emphasised lower harmonics from a large cavity. The mouth cavity is not that different in size in males and females, but the fundamentals of their larynx vibrations are very different in frequency. The human voice is therefore an extreme example of the way in which harmonics combine to produce the pitch sensation, even if the fundamental is comparatively small, but the majority of the sound energy is in the harmonics which provide the vowel sensation. With a bass voice the pitch is far below the harmonics creating ‘eee’.

The voice mechanism is an example of the confusion that arises from the vague way in which the word tone is used. Using the definition of tone we have adopted, the vowel sounds demonstrate how the tone of a sound changes with the change in balance between the higher and lower harmonics. The harmonicity of the larynx vibrations determines whether the voice is ‘sweet’, has an ‘edge’ or makes a crude noise.