A computational study of the effect of false vocal folds on glottal flow and vocal fold vibration during phonation

Abstract

The false vocal folds are believed to be components of the acoustic filter that is responsible for shaping the voice. However, the effects of false vocal folds on the vocal fold vibration and the glottal aerodynamic during phonation remain unclear. This effect has implications for computational modeling of phonation as well as for understanding laryngeal pathologies such as glottal incompetence resulting from unilateral vocal fold paralysis. In this study, a high fidelity, two-dimensional computational model, which combines an immersed boundary method for the airflow and a continuum, finite-element method for the vocal folds, is used to examine the effect of the false vocal folds on flow-induced vibration (FIV) of the true vocal folds and the dynamics of the glottal jet. The model is notionally based on a laryngeal CT scan and employs realistic flow conditions and tissue properties. Results show that the false vocal folds potentially have a significant impact on phonation. The false vocal folds reduce the glottal flow impedance and increase the amplitude as well as the mean glottal jet velocity. The false vocal folds also enhance the intensity of the monopole acoustic sources in the glottis. A mechanism for reduction in flow impedance due to the false vocal folds is proposed.

Figure 1

Coronal view of the CT scan of a normal human larynx showing key features along the approximate anterior posterior midline of the glottis.

Figure 2

Schematic of the immersed boundary representation in the current method. IP: image point, GC: ghost point, BI: Boundary Intercept point.

Figure 3

A close-up coronal view of the CT at the anterior-posterior midline of the glottis and the current geometric model that attempts to match the key geometrical features in the CT scan.

Figure 4

Grid used in the vicinity of vocal folds and false vocal folds in the current simulations.

Figure 5

Three layer vocal fold inner structure and triangular elements used in the current solver.

Figure 6

Comparison of time variation of glottal gap width for original grid and finer grid.

Figure 7

Comparison of flow volume flux rate for original grid and finer grid.

Figure 8

Comparison of the time-variation of the glottal gap width for the two cases.

Figure 9

Vocal fold shapes at different instantaneous time instant within a vibration cycle: (a) 0.3384s (b) 0.3392s, (c)0.3400s, (d) 0.3409s, (e) 0.3416s, (f) 0.3427s

Figure 10

Comparison of the time variation of the volume flow rate for the two cases.

Figure 11

Comparison of the time-rate of change of the flow rate,Q˙, for the two cases. This quantity is related to the monopole source strength of sound.

Figure 12

Contours of spanwise vorticity for the two cases over one vocal fold vibration cycle. The eight plots correspond to eight equispaced time intervals over the vibration cycle.

Figure 13

(a) Streamwise velocity profile immediately downstream of the glottis at one time instance for both cases. (b) Temporal variation in jet deflection for both cases.

Figure 14

Contours of fluctuation shear stress in the glottal jet. (a) (−)FVF case (b) (+)FVF case.

Figure15

Points selected on TVF surface for analysis of vibration symmetry.

Figure 16

Phase plane plots of the displacement of the points on the superior side of the two vocal folds. (a) x displacement (inferior-superior) (b) y displacement (abduction-adduction) from centerline.