Nonlinear dynamics helps explain how vowel influences register stability

Highly trained singers know what a registration shift feels like, where in their ranges it is likely to occur, and how to make small adjustments to make it less abrupt. They also know that the location of register changes differs depending on vowel choice and the style in which they are performing. Changing registers while singing can result in a significant timbral shift and, if approached unexpectedly, an abrupt jump in f o . If the source and filter interact in a purely linear fashion, f o should not be influenced by the shape of the vocal tract (vowel). This paper will demonstrate how nonlinear source-filter coupling may explain the strong relationship between vowel and registration. Eight volunteers performed f o glides while altering the dimensions of their vocal tracts, predictably changing the formant frequencies. It was hypothesized that if the source and filter operated as a purely linear system, f o stability should not be perturbed by formant/harmonic crossings in a linear system. Acoustic analysis revealed, however, that 85% of f o instabilities were aligned with a crossing of one of the first four harmonics with the first three formants, indicating that source-filter coupling likely occurred.