High Fidelity Computational Modeling and Analysis of Voice Production

This research aims to improve the fundamental understanding of the multiphysics nature of voice production, particularly, the dynamic couplings among glottal flow, vocal fold vibration and airway acoustics through high-fidelity computational modeling and simulations. Built upon in-house numerical solvers, including an immersed-boundary-method based incompressible flow solver, a finite element method based solid mechanics solver and a hydrodynamic/aerodynamic splitting method based acoustics solver, a fully coupled, continuum mechanics based fluid-structure-acoustics interaction model was developed to simulate the flow-induced vocal fold vibrations and sound production in birds and mammals. Extensive validations of the model were conducted by comparing to excised syringeal and laryngeal experiments. The results showed that, driven by realistic representations of physiology and experimental conditions, including the geometries, material properties and boundary conditions, the model had an excellent agreement with the experiments on the vocal fold vibration patterns, acoustics and intraglottal flow dynamics, demonstrating that the model is able to reproduce realistic phonatory dynamics during voice production. The model was then utilized to investigate the effect of vocal fold inner structures on voice production. Assuming the human vocal fold to be a three-layer structure, this research focused on the effect of longitudinal variation of layer thickness as well as the cover-body thickness ratio on vocal fold vibrations. The results showed that the longitudinal variation of the cover and ligament layers thicknesses had little effect on the flow rate, vocal fold vibration amplitude and pattern but affected the glottal angle in different coronal planes, which also influenced the energy transfer between glottal flow and the vocal fold. The cover-body thickness ratio had a complex nonlinear effect on the vocal fold vibration and voice production. Increasing the cover-body thickness ratio promoted the excitation of the wave-type modes of the vocal fold, which were also higher-eigenfrequency modes, driving the vibrations to higher frequencies. This has created complex nonlinear bifurcations. The results from the research has important clinical implications on voice disorder diagnosis and treatment as voice disorders are often associated with mechanical status changes of the vocal fold tissues and their treatment often focus on restoring the mechanical status of the vocal folds

A computational study of systemic hydration in vocal fold collision

Mechanical stresses develop within vocal fold (VF) soft tissues due to phonation-associated vibration and collision. These stresses in turn affect the hydration of VF tissue and thus influence voice health. In this paper, high-fidelity numerical computations are described, taking into account fully 3D geometry, realistic tissue and air properties, and high-amplitude vibration and collision. A segregated solver approach is employed, using sophisticated commercial solvers for both the VF tissue and glottal airflow domains. The tissue viscoelastic properties were derived from a biphasic formulation. Two cases were considered, whereby the tissue viscoelastic properties corresponded to two different volume fractions of the fluid phase of the VF tissue. For each case, hydrostatic stresses occurring as a result of vibration and collision were investigated. Assuming the VF tissue to be poroelastic, interstitial fluid movement within VF tissue was estimated from the hydrostatic stress gradient. Computed measures of overall VF dynamics (peak airflow velocity, magnitude of VF deformation, frequency of vibration and contact pressure) were well within the range of experimentally observed values. The VF motion leading to mechanical stresses within the VFs and their effect on the interstitial fluid flux is detailed. It is found that average deformation and vibration of VFs tend to increase the state of hydration of the VF tissue, whereas VF collision works to reduce hydration

Numerical Study of Laryngeal Control of Phonation using Realistic Finite Element Models of a Canine Larynx

While many may take it for granted, the human voice is an incredible feat. An average person can produce a great variety of voices and change voice characteristics agilely even without formal training. Last several decades of research has established that the production of voice is largely a mechanical process: i.e., the sustained vibration of the vocal folds driven by the glottal air flow. Since one only has a single pair of vocal folds, the versatility comes with the ability to change the mechanical status of the vocal folds, including vocal fold length and thickness, tension, and level of adduction, through activation of the laryngeal muscles. However, the relationship between laryngeal muscle activity and the characteristics of voice is not well understood due to limitations in experimental observation and simplifications in modelling and simulations. The science is still far behind the art. The current research aims to investigate first the relationship between laryngeal muscle activation and the posture of the vocal folds and second the relationship between voice source characteristics and vocal fold mechanical status using more comprehensive numerical models and simulations, thus improving the understanding of the roles of each laryngeal muscle in voice control. To do so, (1) the mechanics involved in vocal fold posturing and vibration, especially muscle contraction; (2) the realistic anatomical structure of the larynx must be considered properly. To achieve this goal, a numerical model of the larynx as realistic as possible was built. The geometry of the laryngeal components was reconstructed from high resolution MRI (Magnetic Resonance Imaging) data of an excised canine larynx, which makes more accurate the representation of the muscles and their sub-compartments, cartilages, and other important anatomical features of the larynx. A previously proposed muscle activation model was implemented in a 3D finite element package and applied to the larynx model to simulate the action of laryngeal muscles. After validation of the numerical model against experimental data, extensive parametric studies involving different combination of muscle activations were conducted to investigate how the voice source is controlled with laryngeal muscles. In the course of this study, some work was done to couple the same finite element tool with a Genetic Algorithm program to inversely determine model parameters in biomechanical models. The method was applied in a collaborated study on shape changes of a fish fin during swimming. This study is presented as a separate chapter at the end of this thesis. The method has potential application in determining parameters in vocal fold models and optimizing clinical vocal fold procedures. This thesis is essentially an assembly of the papers published by the author during the doctoral study, with the addition of an introductory chapter. Chapter 1 reviews the overall principles of voice production, the biomechanical basis of voice control, and past studies on voice control with a focus on the fundamental frequency. Chapter 2 describes the major numerical methods employed in this research with an emphasis on the finite element method. The muscle activation model is also described in this chapter. Chapter 3 describes the building of the larynx model from MRI data and its partial validation. Chapter 4 presents the application of the larynx model to posturing studies, including parametric activation of muscle groups and specific topics related to vocal fold posturing. Chapter 5 describes the change of vocal fold vibration dynamics under the influence of the interaction of the cricothyroid muscle and the thyroarytenoid muscle. The Flow-structure interaction simulations was realized by coupling the larynx model to a simple Bernoulli flow model and a two-stage simulation technique. Chapter 6 concludes the current thesis study. Suggestions for future studies are proposed. Chapter 7 is an independent study that is not related to voice control. It describes a numerical framework that inversely determines and validates model parameters of biomechanical models. The application of the proposed framework to a finite element model of a fish fin is presented

Computational Investigations of the Fluid-Structure Interaction During Phonation: The Role of Vocal Fold Elasticity and Glottal Flow Unsteadiness

Human voice production arises from the biomechanical interaction between vocal fold vibrations and airflow dynamics. Changes in vocal fold stiffness can lead to changes in vocal fold vibration patterns and further changes in voice outcomes. A good knowledge of the cause-and-effect relationship between vocal fold stiffness and voice production can not only deepen the understanding of voice production mechanisms but also benefit the treatment of voice disorders associated with vocal fold stiffness changes. This constitutes the first objective of this dissertation. The second objective of this dissertation is to further examine the range of validity of the quasi-steady assumption of glottal flow during phonation. The assumption is of vital importance for phonation modeling since it enables to eliminate the unsteady aspects of glottal flow, which greatly simplifies the flow modeling. A three-dimensional flow-structure interaction model of voice production is employed to investigate the effects of vocal fold stiffness parameters on voice production. The vocal fold is modeled as the cover-ligament-body structure with a transversely isotropic constitutive relation. Stiffness parameters in both the transverse plane and the longitudinal direction of each layer of the vocal fold are systematically varied. The results show that varying the stiffness parameters has obvious monotonic effects on the fundamental frequency, glottal flow rate and glottal opening, but has non-monotonic effects on the glottal divergent angle, open quotient and closing velocity. Compared to the transverse stiffness parameters, the longitudinal stiffness parameters generally have more significant impacts on glottal flows and vocal fold vibrations. Additionally, the sensitivity analysis reveals that the stiffness parameters of the ligament layer have the largest effect on most output measures. Next, flow-structure interaction simulations are carried out to study the effect of fiber orientation in the conus elasticus on voice production. Two continuum vocal fold models with different fiber orientations in the conus elasticus are constructed. The more realistic fiber orientation (caudal-cranial) in the conus elasticus is found to yield smaller structural stiffness and larger deflection at the junction of the conus elasticus and ligament than the anterior-posterior fiber orientation, which facilitates vocal fold vibrations and eventually causes a larger peak flow rate and higher speed quotient. The generated voice is also found to have a lower fundamental frequency and smaller spectral slope. Finally, the validity of the quasi-steady assumption for glottal flow is systematically examined by considering the voice frequency range, complexity of glottal shapes and air inertia in the vocal tract. The results show that at the normal speech frequency (~ 100 Hz), the dynamics of the quasi-steady flow greatly resembles that of a dynamic flow, and the glottal flow and glottal pressure predicted by the quasi-steady approximation have very small errors. However, the assumption produces huge errors at high frequencies (~ 500 Hz). In addition, air inertia in the vocal tract can undermine the validity of the assumption via the nonlinear interaction with the unsteady glottal flow. The role of glottal shapes in the validation is found to be insignificant

Numerical Modeling of Vocal Control and Patient-specific Surgical Planning of Type 1 Thyroplasty

This study aims to develop knowledge about the roles of intrinsic laryngeal muscles on voice control in both healthy and disordered conditions through comprehensive computational models. The phonation simulator was built by combining a three-dimensional high-fidelity MRI-based model of the larynx, active muscle mechanics, and fluid-structure-acoustic interaction model, which enabled the exploration of the underlayer mechanisms of the link between individual and/or group muscles contractions under both symmetric and asymmetric activations, vocal fold posture, vocal fold vibration, and voice outcomes during voice production. The first part of this research extensively investigated the effects of cricothyroid and thyroarytenoid muscle activations on voice characteristics through a parametric study. The role of these intrinsic muscles in the adjustment of geometrical and mechanical properties of vocal fold pre-phonatory posture, glottic flow aerodynamics, and acoustic and how all these components interact were explored. Results were comprehensively validated, and the link between elements of phonation was described in detail. In the next step, due to the model\u27s ability in the individual muscle activations, unilateral vocal fold paralysis was simulated, and the characteristics of disordered voice were analyzed. The voice simulator was then combined with the implant insertion model and genetic algorithm method to build a computational framework for patient-specific surgical planning of type 1 thyroplasty. This surgery is a standard procedure for treating unilateral vocal fold paralysis; however, it is subject to challenges mainly due to the small size of the implant and the high sensitivity of the voice outcome to the implant shape and position. Therefore, although the patient\u27s voice could be improved, the results might not be as satisfying as expected. Despite actual surgery, with very little room for try and error, the ideal implant could be achieved by optimizing the implant based on the patient\u27s desired voice using the presented computational framework. Both healthy and diseased cases and the corrected case using the optimized implant were simulated. Results revealed that the optimized implant could restore the aerodynamic and acoustic features of the disordered voice in producing a sustained vowel utterance. Furthermore, the performance of the implant in the pitch gliding test, which was simulated using temporal activation of the cricothyroid and thyroarytenoid muscles based on the first part of the study, was evaluated. In the final step, a physics-informed neural network-based algorithm was presented to reconstruct the three-dimensional cyclic vibration of vocal fold using two-dimensional sparse experimental data and laws of physics. Key acoustic parameters and vibratory dynamics of vocal folds and other parameters, such as flow rate, pressure distribution, and contact force, which are difficult to measure experimentally, were successfully predicted

Modelling Physical Mechanisms of Nodule Development in Phonotraumatic Vocal Hyperfunction using Computational Vocal Fold Models

Vocal hyperfunction is a prevalent voice disorder with significant impacts on the daily lives of patients, but has poorly understood causes. At its root, vocal hyperfunction is neurological, involving excessive muscular activation due to compensation for some underlying issue. In order to improve understanding of the causes of this disorder and ultimately improve its treatment, this thesis uses computational models to investigate mechanical aspects in the development of vocal fold nodules in phonotraumatic vocal hyperfunction (a specific class of vocal hyperfunction), specifically: whether biomechanical differences in stiffness of the vocal folds can lead to inefficient speech production that predisposes one to developing these nodule, and whether swelling can establish an amplifying feedback loop, a so-called "vicious cycle", wherein swelling leads to compensatory adjustments that incur further swelling and ultimately lead to nodule. To address these questions a two-dimensional finite-element vocal fold model coupled with a simplified one-dimensional flow model was developed with modifications to this basic model made to study the phenomena of interest. Towards modelling swelling, a computationally efficient approach to model the epithelium layer of the vocal folds is also developed and validated. To investigate the first research question, the aforementioned model was adapted to study phonation onset pressure, a measure of effort required to produce speech, as a function of vocal fold stiffness. The results show that onset pressure is primarily dependent on just three stiffness distributions: smooth distributions with body-cover stiffness differences and smooth distributions with inferior-superior stiffness differences minimize onset pressure while a uniform stiffness increase increases onset pressure. Since a uniform stiffness increase increases the natural frequency of the vocal folds, this increase in onset pressure is roughly associated with increases in frequency. This suggests that for a given average stiffness (onset frequency) deviations from an optimal body-cover and inferior-superior-like distribution lead to increases in phonatory effort that could increase susceptibility to vocal hyperfunction. To investigate the second research question, the finite element model was augmented with a model of swelling, as well as an epithelium using a membrane model. Results showed that swelling has negligible impact on loudness of speech but significantly influences frequency, and that furthermore, swelling increases measures of phonotrauma. These results suggest that swelling could incur a vicious cycle. Specifically, a decrease in fundamental frequency initiates compensatory adjustments through increased muscle tension and subglottal pressure, which tends to increase phonotrauma in the folds, and increased swelling with phonotrauma does not tend to limit further swelling. This result demonstrates how swelling can potentially lead to the formation of nodule

Pan European Voice Conference - PEVOC 11

The Pan European VOice Conference (PEVOC) was born in 1995 and therefore in 2015 it celebrates the 20th anniversary of its establishment: an important milestone that clearly expresses the strength and interest of the scientific community for the topics of this conference. The most significant themes of PEVOC are singing pedagogy and art, but also occupational voice disorders, neurology, rehabilitation, image and video analysis. PEVOC takes place in different European cities every two years (www.pevoc.org). The PEVOC 11 conference includes a symposium of the Collegium Medicorum Theatri (www.comet collegium.com