Influence of level difference due to vocal folds angular asymmetry on auto-oscillating replicas.

Dysphonia is often caused by level difference between left and right vocal folds, which are positioned on different angles with respect to the transverse plane, resulting in angular asymmetry. Unilateral vocal fold paralysis may cause such angular asymmetry. In this case, the normal vocal fold is located on the transverse plane, whereas the paralyzed vocal fold is rotated in the sagittal plane as its posterior edge is moved up to the superior direction. The effect of such angular asymmetry (up to 25°) between the left and right vocal fold on the auto-oscillation is experimentally studied using mechanical replicas. For all replicas, it is observed that, as full contact between vocal folds is lost, increase of angular asymmetry results in a decrease of the signal-to-noise ratio, an increase of the total harmonic distortion rate, and an increase of the oscillation threshold pressure. These general tendencies are in agreement with clinical findings reported for vertical level difference during phonation. In analogy to the preceding experimental study in which vocal folds are spaced in parallel with a vertical trade-off, a formula is proposed to describe the oscillation threshold as a function of angular asymmetry.