A numerical and experimental investigation of the effect of false vocal fold geometry on glottal flow

Acoustic, aerodynamic, and vibrational effects of ventricular folds adduction in an ex vivo experiment

Objectives

The excessive adduction of ventricular folds has been observed in patients with dysphonia and professional singers. Whether these changes in the ventricular folds are the cause or just a result of disease progression remains unclear, and their potential pathological and physiological implications are yet to be determined. This study aimed to examine the impact of different degrees of ventricular adduction on acoustics, aerodynamics, and vocal fold vibration.

Methods

The excised models of mild and severe ventricular adduction were established. We recorded the vibration pattern of vocal folds and ventricular folds and measured acoustic metrics, including fundamental frequency (F0), Jitter, Shimmer, harmonic-to-noise ratio (HNR), and sound pressure level (SPL). Furthermore, we evaluated the aerodynamics index through phonation threshold pressure (PTP), phonation instability pressure (PIP), mean flow rate (MFR), phonation threshold flow (PTF), and phonation instability flow (PIF).

Results

Irregular vibrations of the ventricular fold were observed during ventricular adduction. Notably, mild and severe ventricular adduction conditions showed a significant increase in PTP, Shimmer, and Jitter, whereas MFR, PIF, and HNR decreased compared with the control condition.

Conclusions

Ventricular adduction leads to the deterioration of acoustic and aerodynamic parameters. The aperiodic and irregular vibration of the ventricular folds may be responsible for this phenomenon, although further experiments are warranted. Understanding the functioning of ventricular folds can be beneficial in directing the treatment of muscle tension dysphonia and improving voice training techniques.Level of evidence: level 4.

Effects of False Vocal Folds on Intraglottal Velocity Fields

Previous models have theorized that, during phonation, skewing of the glottal waveform (which is correlated with acoustic intensity) occurred because of inertance of the vocal tract. Later, we reported that skewing of the flow rate waveform can occur without the presence of a vocal tract in an excised canine larynx. We hypothesized that in the absence of a vocal tract, the skewing formed when dynamic pressures acted on the glottal wall during the closing phase; such pressures were greatly affected by formation of intraglottal vortices. In this study, we aim to identify how changes in false vocal folds constriction can affect the acoustics and intraglottal flow dynamics. The intraglottal flow measurements were made using particle image velocimetry in an excised canine larynx where a vocal tract model was placed above the larynx and the constriction between the false vocal folds was varied. Our results show that for similar values of subglottal pressures, the skewing of the glottal waveform, strength of the intraglottal vortices, and acoustic energy increased as the constriction between the false vocal folds was increased. These preliminary findings suggest that acoustic intensity during phonation can be increased by the addition of a vocal tract with false fold constriction.

Aerodynamic impact of the ventricular folds in computational larynx models

Ventricular folds (VeFs) act as passive, non-moving structures during normal phonation. According to the literature, VeFs potentially aid the flow-driven oscillations of the vocal folds (VFs) that produce the primary sound of human phonation. In this study, large eddy simulations were performed to analyze this influence in a numerical model with imposed VF motion as measured experimentally from a synthetic silicone vocal fold model. Model configurations with and without VeFs were considered. Furthermore, configurations with rectangular and elliptical glottis shapes were simulated to investigate the effects of three-dimensional glottal jet evolutions. Results showed that VeFs increased flow rate and transglottal pressure difference by a decrease in the pressure level in the ventricles immediately downstream of the VFs. This led to an increase in the glottal flow resistance, increased energy transfer rate between the flow and VFs, and a simultaneous decrease in the laryngeal flow resistance, which shows a higher amount of kinetic energy in the glottal flow. This enhancement was more pronounced in the rectangular glottis and varied with the subglottal pressure and VeF gap size.

Computational Models of Laryngeal Aerodynamics: Potentials and Numerical Costs

Human phonation is based on the interaction between tracheal airflow and laryngeal dynamics. This fluid-structure interaction is based on the energy exchange between airflow and vocal folds. Major challenges in analyzing the phonatory process in-vivo are the small dimensions and the poor accessibility of the region of interest. For improved analysis of the phonatory process, numerical simulations of the airflow and the vocal fold dynamics have been suggested. Even though most of the models reproduced the phonatory process fairly well, development of comprehensive larynx models is still a subject of research. In the context of clinical application, physiological accuracy and computational model efficiency are of great interest. In this study, a simple numerical larynx model is introduced that incorporates the laryngeal fluid flow. It is based on a synthetic experimental model with silicone vocal folds. The degree of realism was successively increased in separate computational models and each model was simulated for 10 oscillation cycles. Results show that relevant features of the laryngeal flow field, such as glottal jet deflection, develop even when applying rather simple static models with oscillating flow rates. Including further phonatory components such as vocal fold motion, mucosal wave propagation, and ventricular folds, the simulations show phonatory key features like intraglottal flow separation and increased flow rate in presence of ventricular folds. The simulation time on 100 CPU cores ranged between 25 and 290 hours, currently restricting clinical application of these models. Nevertheless, results show high potential of numerical simulations for better understanding of phonatory process.

Effect of the ventricular folds in a synthetic larynx model

Within the human larynx, the ventricular folds serve primarily as a protecting valve during swallowing. They are located directly above the sound-generating vocal folds. During normal phonation, the ventricular folds are passive structures that are not excited to periodical oscillations. However, the impact of the ventricular folds on the phonation process has not yet been finally clarified. An experimental synthetic human larynx model was used to investigate the effect of the ventricular folds on the phonation process. The model includes self-oscillating vocal fold models and allows the comparison of the pressure distribution at multiple locations in the larynx for configurations with and without ventricular folds. The results indicate that the ventricular folds increase the efficiency of the phonation process by reducing the phonation threshold level of the pressure below the vocal folds. Two effects caused by the ventricular folds could be identified as reasons: (1) a decrease in the mean pressure level in the region between vocal and ventricular folds (ventricles) and (2) an increase in the glottal flow resistance. The reason for the first effect is a reduction of the pressure level in the ventricles due to the jet entrainment and the low static pressure in the glottal jet. The second effect results from an increase in the glottal flow resistance that enhances the aerodynamic energy transfer into the vocal folds. This effect reduces the onset threshold of the pressure difference across the glottis.

Computational Modeling of Fluid-Structure-Acoustics Interaction during Voice Production

The paper presented a three-dimensional, first-principle based fluid-structure-acoustics interaction computer model of voice production, which employed a more realistic human laryngeal and vocal tract geometries. Self-sustained vibrations, important convergent-divergent vibration pattern of the vocal folds, and entrainment of the two dominant vibratory modes were captured. Voice quality-associated parameters including the frequency, open quotient, skewness quotient, and flow rate of the glottal flow waveform were found to be well within the normal physiological ranges. The analogy between the vocal tract and a quarter-wave resonator was demonstrated. The acoustic perturbed flux and pressure inside the glottis were found to be at the same order with their incompressible counterparts, suggesting strong source-filter interactions during voice production. Such high fidelity computational model will be useful for investigating a variety of pathological conditions that involve complex vibrations, such as vocal fold paralysis, vocal nodules, and vocal polyps. The model is also an important step toward a patient-specific surgical planning tool that can serve as a no-risk trial and error platform for different procedures, such as injection of biomaterials and thyroplastic medialization.

The Effect of False Vocal Folds on Laryngeal Flow Resistance in a Tubular Three-dimensional Computational Laryngeal Model

OBJECTIVE

The current study used a three-dimensional (3D) computational laryngeal model to investigate the effect of false vocal folds (FVFs) on laryngeal flow resistance.

METHOD

A 3D, tubular shaped computational laryngeal model was designed with a high level of realism with respect to the human laryngeal anatomy. Two cases, one with the FVFs and the other without the FVFs, were created in the numerical simulation to compare the laryngeal flow behaviors.

RESULTS AND CONCLUSION

The results were discussed in a comparative manner with the previous two-dimensional (2D) computational model. On the one hand, the results demonstrated the similar mechanism as observed in the 2D model that the presence of the FVFs suppressed the deflection of the glottal jet and in doing so, reduced the mixing-related minor loss in the supraglottal region. On the other hand, the 3D flow was more stable and straighter, so the effect of FVFs on suppressing the jet deflection in the 3D model was not as prominent as in the 2D model. Furthermore, the presence of the FVFs also increased the friction-related major loss due to the increased velocity gradient in the restricted flow channel. Therefore, it was hypothesized that the final effect of the FVFs on flow resistance is the combined effect of the reduced mixing-related minor loss and increased friction-related major loss, both of which are highly related to the gap between the FVFs.

Time-Dependent Pressure and Flow Behavior of a Self-oscillating Laryngeal Model With Ventricular Folds

OBJECTIVE

The purpose of the study was to better understand the pressure-flow behavior of a self-oscillating vocal fold model at various stages of the glottal cycle.

METHODS

An established self-oscillating vocal fold model was extended to include the false vocal folds (FVFs) and was used to study time-dependent pressure and velocity distributions through the larynx (including the true vocal folds [TVFs] and FVFs). Vocal fold vibration was modeled with a finite element method, laryngeal flow was simulated with the solution of unsteady Navier-Stokes equations, and the acoustics of the vocal tract was modeled with a wave reflection method.

RESULTS

The results demonstrate realistic phonatory behaviors and therefore may be considered as a pedagogical tool for showing detailed aerodynamic, kinematic, and acoustic characteristics. The TVFs self-oscillated regularly with reasonable amplitude and mucosal waves. There were large pressure gradients in the glottal region. The centerline velocity was highest during glottal closing and sharply dropped near the center of the flow vortex. The average centerline velocity was about 25 m/second in the glottal region. The transglottal pressure was higher during glottal closing when the glottal shape was divergent and pressure recovery was present within the glottis. The centerline velocity increased as expected throughout the convergent glottis, tended to decrease throughout the divergent glottis, and decreased past the TVFs within the ventricle-FVF region.

CONCLUSIONS

This model produces realistic results and demonstrates interactions among phonation variables of a highly instructive nature, including the influence of the FVFs.

Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx

Compressible large eddy simulation is employed to numerically investigate the laryngeal flow. Symmetric static models of the human larynx with a divergent glottis are considered, with the presence of false vocal folds (FVFs). The compressible study agrees well with that of the incompressible study. Due to the high enough Reynolds number, the flow is unsteady and develops asymmetric states downstream of the glottis. The glottal jet curvature decreases with the presence of FVFs or the ventricular folds. The gap between the FVFs stretches the flow structure and reduces the jet curvature. The presence of FVFs has a significant effect on the laryngeal flow resistance. The intra-glottal vortex structures are formed on the divergent wall of the glottis, immediately downstream of the separation point. The vortices are then convected downstream and characterized by a significant negative static pressure. The FVFs are a main factor in the generation of stronger vortices, and thus on the closure of the TVFs. The direct link between the FVFs geometry and the motion of the TVFs, and by extension to the voice production, is of interest for medical applications as well as future research works. The presence of the FVFs also changes the dominant frequencies in the velocity and pressure spectra.