Purpose The present study was a methodological study designed to reveal the dynamic mechanisms of phonation instability pressure (PIP) using bifurcation analysis. Phonation pressure range (PPR) was also proposed for assessing the pressure range of normal vocal fold vibrations.

Method The authors first introduced the concept of bifurcation on the basis of a symmetric vocal fold model and then applied the bifurcation analysis to data from excised larynges. By recording acoustic signals from 10 excised larynges, the authors measured phonation threshold pressure (PTP), PIP, and PPR at the bifurcation pressure points from the spectrograms as subglottal pressure was progressively increased. Furthermore, to investigate the effects of vocal fold elongation on PTP, PIP, and PPR, the authors manipulated the elongation of the vocal folds at 0%, 5%, 10%, and 15% of the resting vocal fold length.

Results The authors found that PTP, PIP, and PPR were effectively determined using the bifurcation analysis. When vocal fold elongation was increased from 0% to 15%, PTP was significantly increased (p < .001), PIP was not significantly changed (p = .54), and PPR was significantly decreased (p = .003).

Conclusion PIP and PPR represent important parameters to assess phonation instability. Bifurcation analysis represents a valuable procedure for revealing the mechanisms behind PTP, PIP, and PPR and for investigating the effects of vocal fold biomechanical parameters on these 3 pressure parameters.