While physically-based continuum models of voice production have potential applications in clinical intervention of voice disorders and personalized natural speech synthesis, their current use is limited due to the high computational cost associated with resolving the complex fluid-structure interaction during voice production process. The goal of this study is to summarize our recent efforts in developing a physically-based, computationally-efficient continuum model of voice production toward near real-time applications. The model uses an eigenmode-based formulation of the governing equations, in which vocal fold eigenmodes are used as building blocks to reconstruct more complex vocal fold vibration patterns. Simulations show that a reasonable accuracy in the fundamental frequency, vocal intensity, and selected spectral measures can be reached with the use of the first 100 vocal fold eigenmodes, thus significantly reducing the degrees of freedom of the governing equations (as compared to tens of thousands in finite element models) and computational time. It is expected that for applications in which absolute values are not as essential, even a smaller number of eigenmodes would be acceptable. Examples are provided to demonstrate the capability of the model in modeling large range of voice qualities, natural voice quality change over time, and speech production in general.

Objectives

This study evaluates potential compensation strategies under conditions of glottal insufficiency.

Methods

Using a numerical respiratory-laryngeal model of voice production, voice production under conditions of glottal insufficiency is investigated across a large range of voice conditions, and compared with normal voice production.

Results

This study shows that glottal insufficiency leads to increased noise production, reduced fundamental frequency range, and inability to produce very low-intensity voice. Glottal insufficiency also leads to significantly increased respiratory effort of phonation and difficulty in maintaining a normal breath group duration, which restricts high-intensity voice production and falsetto-like voice production. Although compensation strategies exist to alleviate these undesirable voice changes, they often require hyperfunctional laryngeal and respiratory muscle activities and thus are more likely to result in vocal fatigue.

Conclusions

The laryngeal and respiratory subsystems need to be considered as a whole to fully understand the effect of glottal insufficiency on voice production. Strategies that compensate for laryngeal weakness at the cost of compromising the normal function of the respiratory subsystem are undesirable and may impose additional constraints on voice production and the effectiveness of available compensation strategies.

The goal of this study is to identify laryngeal strategies that minimize vocal fold contact pressure while producing a target sound pressure level (SPL) using a three-dimensional voice production model. The results show that while the subglottal pressure and transverse stiffness can be manipulated to reduce the peak contact pressure, such manipulations also reduce the SPL, and are thus less effective in reducing contact pressure in voice tasks targeting a specific SPL level. In contrast, changes in the initial glottal angle and vocal fold vertical thickness that reduce the contact pressure also increase the SPL. Thus, in voice tasks targeting a specific SPL, such changes in the initial glottal angle and vertical thickness also lower the subglottal pressure, which further reduces the peak contact pressure. Overall the results show that for voice tasks with a target SPL level, vocal fold contact pressure can be significantly reduced by adopting a barely abducted glottal configuration or reducing the vocal fold vertical thickness. Aerodynamic measures are effective in identifying voice production with large initial glottal angles, but by themselves alone are not useful in differentiating hyperadducted vocal folds from barely abducted vocal folds, which may be better differentiated by closed quotient and voice type measures.

The anisotropic mechanical properties of the vocal fold lamina propria play an important role in voice production and control. The goal of this study is to develop a constitutive model capable of predicting lamina propria elastic moduli along both the longitudinal and transverse directions under different conditions of vocal fold elongation, which can be used as input to reduced-order phonation models based on linear elasticity. A structurally-based constitutive model that links microstructural characteristics of the lamina propria to its macromechanical properties is proposed. The model prediction has been shown to agree reasonably well with recent biaxial tensile testing results.

This study investigates the peak vocal fold contact pressure at different conditions of epilaryngeal narrowing and laryngeal adjustments. The results show that for a given subglottal pressure, the peak vocal fold contact pressure may increase or decrease with epilaryngeal narrowing, depending on a complex interaction between vocal fold vertical thickness, initial glottal angle, and subglottal pressure. However, epilaryngeal narrowing also significantly increases vocal efficiency so that for a target sound pressure level, the peak vocal fold contact pressure decreases with epilaryngeal narrowing. Overall, the peak vocal fold contact pressure and respiratory effort can be minimized by epilaryngeal narrowing, adopting a small initial glottal angle, and an intermediate vocal fold thickness.

Although vocal folds are known to be anisotropic, the influence of material anisotropy on vocal fold vibration remains largely unknown. Using a linear stability analysis, phonation onset characteristics were investigated in a three-dimensional anisotropic vocal fold model. The results showed that isotropic models had a tendency to vibrate in a swing-like motion, with vibration primarily along the superior-inferior direction. Anterior-posterior (AP) out-of-phase motion was also observed and large vocal fold vibration was confined to the middle third region along the AP length. In contrast, increasing anisotropy or increasing AP-transverse stiffness ratio suppressed this swing-like motion and allowed the vocal fold to vibrate in a more wave-like motion with strong medial-lateral motion over the entire medial surface. Increasing anisotropy also suppressed the AP out-of-phase motion, allowing the vocal fold to vibrate in phase along the entire AP length. Results also showed that such improvement in vibration pattern was the most effective with large anisotropy in the cover layer alone. These numerical predictions were consistent with previous experimental observations using self-oscillating physical models. It was further hypothesized that these differences may facilitate complete glottal closure in finite-amplitude vibration of anisotropic models as observed in recent experiments.

The goal of this study is to identify vocal fold conditions that produce irregular vocal fold vibration and the underlying physical mechanisms. Using a three-dimensional computational model of phonation, parametric simulations are performed with co-variations in vocal fold geometry, stiffness, and vocal tract shape. For each simulation, the cycle-to-cycle variations in the amplitude and period of the glottal area function are calculated, based on which the voice is classified into three types corresponding to regular, quasi-steady or subharmonic, and chaotic phonation. The results show that vocal folds with a large medial surface vertical thickness and low transverse stiffness are more likely to exhibit irregular vocal fold vibration when tightly approximated and subject to high subglottal pressure. Transition from regular vocal fold vibration to vocal instabilities is often accompanied by energy redistribution among the first few vocal fold eigenmodes, presumably due to nonlinear interaction between eigenmodes during vocal fold contact. The presence of a vocal tract may suppress such contact-related vocal instabilities, but also induce new instabilities, particularly for less constricted vocal fold conditions, almost doubling the number of vocal fold conditions producing irregular vibration.

Objective

This study evaluates the need of airflow conservation and the effect of glottal resistance on respiratory effort of phonation under different phonation conditions.

Methods

A computational model of the pressure-volume-flow relationship of the respiratory system is developed.

Results

Simulations show that increasing the glottal resistance reduces the glottal airflow and allows phonation to be sustained for a longer breath group duration. For a given breath group duration, the reduced airflow also allows phonation to be sustained within a narrow range of lung volumes, thus lowering the overall respiratory effort.

Conclusions

This study shows that for breath group durations and subglottal pressures typical of normal conversational speech, airflow conservation or maintaining "effortless" respiratory support does not provide a stricter requirement on the glottal resistance than that required for initiating phonation. However, the need for airflow conservation and respiratory effort reduction becomes relevant when the target subglottal pressure and breath group duration increase as in prolonged speech or singing or in conditions of weakened pulmonary function. In those conditions, the glottal resistance is expected to increase proportionally with increasing subglottal pressure to conserve airflow consumption and reduce respiratory effort.

The goal of this study is to identify vocal fold geometric and mechanical conditions that are likely to produce large contact pressure and thus high risk of vocal fold injury. Using a three-dimensional computational model of phonation, parametric simulations are performed with co-variations in vocal fold geometry and stiffness, with and without a vocal tract. For each simulation, the peak contact pressure is calculated. The results show that the subglottal pressure and the transverse stiffness of the vocal folds in the coronal plane have the largest and most consistent effect on the peak contact pressure, indicating the importance of maintaining a balance between the subglottal pressure and transverse stiffness to avoiding vocal fold injury. The presence of a vocal tract generally increases the peak contact pressure, particularly for an open-mouth vocal tract configuration. While a low degree of vocal fold approximation significantly reduces vocal fold contact pressure, for conditions of moderate and tight vocal fold approximation changes in vocal fold approximation may increase or decrease the peak contact pressure. The effects of the medial surface thickness and vocal fold stiffness along the anterior-posterior direction are similarly inconsistent and vary depending on other control parameters and the vocal tract configuration.

Voice therapy often emphasizes vibratory sensations in the front part of the vocal tract during phonation to improve vocal efficiency. It remains unclear what laryngeal and vocal tract adjustments are elicited in speakers by this emphasis on oral vibratory sensations. Using a three-dimensional phonation model, this study aims to identify laryngeal and epilaryngeal adjustments that might produce maximal oral vibratory sensations during phonation, as quantified by the oral sound pressure level (SPL), and thus are likely to be elicited in voice therapy at different semi-occluded vocal tract configurations. Results show that maximum oral SPL occurs at intermediate vocal fold adduction configurations characterized by a trade-off between glottal gap and vocal fold vertical thickness. Epilaryngeal tube narrowing further increases the oral SPL in an open vocal tract, but has little effect on oral SPL in semi-occluded vocal tracts. Laryngeal and epilaryngeal configurations producing the maximum oral SPL generally have lower peak vocal fold contact pressure when producing a target output SPL. These favorable configurations are more easily identified in open vocal tracts than semi-occluded vocal tracts. However, semi-occlusion increases both the mean and dynamic oral pressure, which may familiarize speakers with oral vibratory sensations and facilitate adoption of favorable laryngeal configurations.