Flow fields and acoustics in a unilateral scarred vocal fold model

From prior work in an excised canine larynx model, it has been shown that intraglottal vortices form between the vocal folds during the latter part of closing. It has also been shown that the vortices generate a negative pressure between the folds, producing a suction force that causes sudden, rapid closing of the folds. This rapid closing will produce increased loudness and increased higher harmonics. We used a unilateral scarred excised canine larynx model to determine whether the intraglottal vortices and resulting acoustics were changed, compared to those of normal larynges.

Acoustic, flow field, and high-speed imaging measurements from 5 normal and 5 unilaterally scarred canine larynges are presented in this report. Scarring was produced by complete resection of the vocal fold mucosa and superficial layer of the lamina propria on the right vocal fold only. Two months later, these dogs were painlessly sacrificed, and testing was done on the excised larynges during phonation. High-speed video imaging was then used to measure vocal fold displacement during different phases. Particle image velocimetry and acoustic measurements were used to describe possible acoustic effects of the vortices.

A higher phonation threshold was required to excite the motion of the vocal fold in scarred larynges. As the subglottal pressure increased, the strength of the vortices and the higher harmonics both consistently increased. However, it was seen that increasing the maximum displacement of the scarred fold did not consistently increase the higher harmonics. The improvements that result from increasing subglottal pressure may be due to a combination of increasing the strength of the intraglottal vortices and increasing the maximum displacement of the vocal fold; however, the data in this study suggest that the vortices play a much more important role.

The current study indicates that higher subglottal pressures may excite higher harmonics and improve loudness for patients with unilateral vocal fold scarring. This finding implies that therapies that raise the subglottal pressure may be helpful in improving voice quality.