Nonlinear source–filter coupling has been demonstrated in computer simulations, in excised larynx experiments, and in physical models, but not in a consistent and unequivocal way in natural human phonations. Eighteen subjects (nine adult males and nine adult females) performed three vocal exercises that represented a combination of various fundamental frequency and formant glides. The goal of this study was to pinpoint the proportion of source instabilities that are due to nonlinear source–tract coupling. It was hypothesized that vocal fold vibration is maximally destabilized when F0 crosses F1, where the acoustic load changes dramatically. A companion paper provides the theoretical underpinnings. Expected manifestations of a source–filter interaction were sudden frequency jumps, subharmonic generation, or chaotic vocal fold vibrations that coincide with F0F1 crossovers. Results indicated that the bifurcations occur more often in phonations with F0F1 crossovers, suggesting that nonlinear source–filter coupling is partly responsible for source instabilities. Furthermore it was observed that male subjects show more bifurcations in phonations with F0F1 crossovers, presumably because in normal speech they are less likely to encounter these crossovers as much as females and hence have less practice in suppressing unwanted instabilities.

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