The voice organ can be regarded as a wind instrument consisting of an air pressure supply driving an oscillator, the output signal of which is fed into a resonator from which the sound is radiated to the air outside the instrument.
The air pressure supply is the respiratory system (i.e. the lungs and the respiratory muscles). In the case of voiced sounds, the oscillator is the set of vocal folds (earlier also called cords); they convert the airstream from the lungs into a complex sound built up by harmonic partials. For voiceless sounds the oscillator is a narrow slit through which the airstream is forced; the laminar airstream is then converted into a turbulent airstream which generates noise. The sound generated by the oscillator is called the ‘voice source’. It propagates through the resonator constituted by the cavities separating the oscillator from the free air outside the instrument. In resonators the ability to transmit sound varies considerably with the frequency of the transmitted sound. At certain frequencies (the resonance frequencies), this ability reaches maximum. Thus in the case of the voice, those voice source partials that lie closest to a resonance are radiated with higher amplitudes than other partials. In this way the spectral form of the radiated sound mirrors the properties of the resonator. The resonances and the resonance frequencies of the vocal tract are called ‘formants’ and ‘formant frequencies’ respectively.
In singing, the air pressure is much more carefully regulated than in normal speech, by a skilled control of the inspiratory and expiratory muscles. The air pressure provided by the respiratory system in singing varies with pitch and vocal effort, generally between 5 and 40 cm of water. The resulting air flow depends also on the glottal conditions. Air flow rates of 0.1–0.3 litres per second have been observed in singers. These air pressure and air flow ranges do not appear to deviate appreciably from values observed in untrained speakers.