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IT IS WELL KNOWN THAT a spectrum of harmonic frequencies is associated with every vowel and that this spectrum also defines the overall timbre of the voice. The strength of each harmonic can be measured by conducting routine spectrography on the acoustic signal radiated from the mouth and picked up by a microphone. But this mouth output spectrum contains the filtering characteristic of the vocal tract and therefore does not necessarily give a clear picture of what the origin of the harmonic is at the source (the glottis). Furthermore, it does not tell us how we can control the production of the harmonics.
Source harmonics are produced in two primary ways: (1) by collision of the vocal folds, and (2) by acoustic energy from the vocal tract being fed back to the glottis and altering the glottal flow. In both cases, it is a distortion of an otherwise simpler glottal airflow that has only one harmonic, the fundamental. Consider Figure 1. On the left side we see a panel of three glottal flow waveforms (three cycles of vibration for each), and on the right side we see corresponding spectra of harmonics. The top waveform has a little flat portion on the bottom, suggesting a very brief period of collision of the vocal folds. The glottis is open 80% of the time and closed only 20% of the time. The little flat portion is a distortion of an otherwise rounded bottom that the simplest (smoothest) possible waveform would have; but it is this distortion that produces a second harmonic (see second vertical line on the...