Production of child-like vowels with nonlinear interaction of glottal flow and vocal tract resonances

Acoustically, the mechanisms of vocal sound production may be considered to exist along a continuum. At one end, the glottal flow wave is weakly coupled to the resonances of the vocal tract such that the output is a linear combination of their respective acoustic characteristics, whereas at the other end there is strong nonlinear coupling of the flow source to the vocal tract resonances. To express phonetic properties in the output, such as formants, the linear case requires that the source produce sound that is rich in harmonic or broadband energy. In contrast, the nonlinear case allows for the possibility of an harmonically-rich source signal to be generated even when the glottal area variation is so simple that it may contain only one harmonic (i.e., a sinusoid) [Titze, J. Acoust. Soc. Am., 123, 2008]. The latter case is most likely to occur when the fundamental frequency is relatively high, such as in children's speech. The purpose of this study was to investigate the nonlinear end of the continuum with respect to the harmonic content of the glottal flow and pressure waveforms for vowels generated with a model of a child-like speech production system.