Compensatory Vocal Folds for Source Voice Generation: Computational Modeling of Vocal Folds Function

Práce se zabývá výpočtovým modelováním funkce lidských hlasivek a vokálního traktu s využitím metody konečných prvků (MKP). Hlas hraje klíčovou roli v lidské komunikaci. Proto je jedním z důležitých cílů současné medicíny vytvořit umělé hlasivky, které by mohly být implantovány pacientům, kterým musely být odstraněny jejich hlasivky původní. Pro pochopení principů tvorby hlasu, určení parametrů, které musí umělé hlasivky splňovat a ověření jejich funkčnosti je možno využít výpočtového modelování. První část práce se zabývá výpočtovým modelováním pro tvorbu lidského hlasu šeptem. V této kapitole byl na MKP modelu vokálního traktu a průdušnice zkoumán vliv velikosti mezihlasivkové mezery na rozložení vlastních frekvencí pro jednotlivé samohlásky. Dále je v práci prezentován rovinný (2D) konečnoprvkový model samobuzeného kmitání lidských hlasivek v interakci s akustickými prostory vokálního traktu. Rovinný model vokálního traktu byl vytvořen na základě snímků z magnetické rezonance (MRI). Pro řešení interakce mezi strukturou a tekutinou je použito explicitní výpočtové schéma s oddělenými řešiči pro strukturu a pro proudění. Vytvořený výpočtový model zahrnuje: velké deformace tkáně hlasivek, kontakt mezi hlasivkami, interakci mezi strukturou a tekutinou, morfování sítě vzduchu podle pohybu hlasivek (metoda Arbitrary Lagrangian-Eulerian), neustálené viskózní a stlačitelné nebo nestlačitelné proudění popsané pomocí Navier-Stokesových rovnic a přerušování proudu vzduchu během uzavření hlasivek. Na tomto modelu jsou zkoumány projevy změn tuhosti a tlumení jednotlivých vrstev (zejména pak laminy proprii). Součástí této výpočtové analýzy je také porovnání chování hlasivek pro stlačitelný a nestlačitelný model proudění. Ze získaných výsledků výpočtu MKP modelu jsou následně vytvářeny videokymogramy (VKG), které umožňují porovnat pohyb mezi jednotlivými variantami modelu a se skutečnými lidskými hlasivkami. V další části práce je potom prezentován prostorový (3D) MKP model samobuzeného kmitání lidských hlasivek. Tento prostorový model vznikl z předchozího rovinného modelu vytažením do třetího rozměru. Na tomto modelu byl opět porovnáván vliv použití stlačitelného a nestlačitelného modelu proudění na pohyb hlasivek a vytvářený zvuk s využitím videokymogramů a zvukových spekter. Poslední část práce se zabývá jednou z možností náhrady přirozeného zdrojového hlasu v podobě plátkového elementu. Chování plátkového elementu bylo zkoumáno na výpočtovém a experimentálním modelu. Experimentální model umožňuje změny v nastavení vzájemné polohy plátku vůči dorazu a provádění akustických a optických měření.This doctoral thesis focuses on computational modelling of human vocal folds and vocal tract functions using finite element method (FEM). Human voice is crucial in human communication. Therefore one of the main targets of current medicine is creation of artificial vocal folds, which would substitute the original vocal folds. The computational modelling can be used to understand principles of voice production, determination of parameters that the artificial vocal folds have to meet and verification of their functionality. First part of this thesis focuses on modelling of human voice creation by whisper. Influence of intraglottal gap on eigenvalues distribution for individual vowels was analysed using FEM vocal tract and trachea model. Further there is presented two-dimensional (2D) finite element model of the flow-induced self-oscillation of the human vocal folds in interaction with acoustic spaces of the vocal tract. The 2D vocal tract model was created on the basis of converting the data from magnetic resonance images (MRI). Explicit coupling scheme with separated solvers for structure and fluid domain was used for modelling of the fluid-structure interaction. Created computational model comprises: large deformations of the vocal folds tissue, contact between vocal folds, fluid-structure interaction, morphing the fluid mesh according to the vocal-fold motion (Arbitrary Lagrangian-Eulerian approach), unsteady viscous compressible or incompressible airflow described by the Navier-Stokes equations and airflow separation during glottis closure. This model is used to analyse the influence of stiffness and damping changes in individual vocal fold tissue layers (in particular in superficial lamina propria). Part of this computational analysis is also comparison of vocal folds behaviour for compressible and incompressible flow model. Videokymograms (VKG) are subsequently created from obtained results of FEM calculations which enable to compare individual variants between themselves and with motion of real human vocal folds. In next part of this thesis is presented three-dimensional (3D) finite element model of the flow-induced self-oscillation of the human vocal folds. This 3D model was created from a previous 2D model by extrude to the third direction. Using this model was again compared influence of compressible and incompressible flow model on vocal folds motion and generated sound by using videokymograms and acoustic spectra. The last part of this thesis focuses on the possibility to replace missing natural source voice in form reed-based element. Behaviour of reed-based element was analysed using computational modelling and using measurements on experimental physical model. The physical model enables changes in setting gap between reed and reed stop and performing acoustical and optical measurements.

Modal Locking Between Vocal Fold Oscillations and Vocal Tract Acoustics

During voiced speech, vocal folds interact with the vocal tract acoustics. The resulting glottal source-resonator coupling has been observed using mathematical and physical models as well as in in vivo phonation. We propose a computational time-domain model of the full speech apparatus that contains a feedback mechanism from the vocal tract acoustics to the vocal fold oscillations. It is based on numerical solution of ordinary and partial differential equations defined on vocal tract geometries that have been obtained by magnetic resonance imaging. The model is used to simulate rising and falling pitch glides of [alpha, i] in the fundamental frequency (f(o)) interval [145 Hz, 315 Hz]. The interval contains the first vocal tract resonance f(R1) and the first formant F-1 of [i] as well as the fractions of the first resonance f(R1)/5, f(R1)/4, and f(R1)/3 of [alpha]. The glide simulations reveal a locking pattern in the f(o) trajectory approximately at f(R1) of [i]. The resonance fractions of [alpha] produce perturbations in the pressure signal at the lips but no locking.Peer reviewe

Mechanics of airflow in human inhalation

The mechanics of airflow in the large airways during inspiration affects important physiological functions such as ventilation, olfaction, heat exchange and mass transfer. The behaviour of the airflow is important not only for healthcare applications including diagnosis, intervention planning and assessment, but for inhalation toxicology. This research aims to further the understanding of human nasal physiology through computational modelling. Specifically, the effects of transient inhalation conditions on flow dynamics and transport were characterised and the changes in flow behaviour in response to certain pathologies quantified. The key findings can be summarised as follows: Firstly, the time scales for airflow in the large airways have been identified and the initial flow patterns revealed. Three phases in the temporal behaviour of the flow were identified (flow initiation, quasi-equilibrium and decay). The duration of each phase differs depending on the quantity of interest. Flow in the nose was characterised as transitional, whilst in parts of the descending airways it is turbulent, particularly in the faster moving regions around the jets which may occur in the pharynx, larynx and at the superior end of the trachea. The bulk of the flow is biased to fill only certain regions of the airways, whilst other regions carry little flow, due to features upstream. Analysis of cross-sectional images provided by medical imaging does not necessarily provide a representative view of the area available to the flow. Various scalar species were employed to represent the fate of nanoparticles and gaseous species within the airways. Only species with high diffusion rates exhibited significant absorption at the airway walls. Airway pathologies often cause changes to the geometry of the airway. One such pathology, the goitre, was found to curve the trachea and in some cases cause constriction. Both these geometric changes were found to increase the pressure loss and energy required to drive flow through the trachea. Furthermore, the flow in pathological cases was more disturbed. High resolution simulations have been used to address these topics and the scales simulated have been analysed in terms of the smallest features possible in the flow to determine their fidelity.Open Acces

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

Extension of the single-matrix formulation of the vocal tract: consideration of bilateral channels and connection of self-oscillating models of the vocal folds with a glottal chink

International audienceThe paper presents extensions of the single-matrix formulation (Mokhtari et al., 2008, Speech Comm. 50(3) 179 – 190) that enable self-oscillation models of vocal folds, including glottal chink, to be connected to the vocal tract. They also integrate the case of a local division of the main air path into two lateral channels, as it may occur during the production of lateral consonants. Provided extensions are detailed by a reformulation of the acoustic conditions at the glottis, and at the upstream and downstream connections of bilateral channels. The simulation framework is validated through numerical simulations. The introduction of an antiresonance in the transfer function due to the presence of asymmetric bilateral channels is confirmed by the simulations. The frequency of the antiresonance agrees with the theoretical predictions. Simulations of static vowels reveal that the behavior of the vocal folds is qualitatively similar whether they are connected to the single-matrix formulation or to the classic reflection-type line analog model. Finally, the acoustic effect of the glottal chink on the production of vowels is highlighted by the simulations: the shortening of the vibrating part of the vocal folds lowers the amplitude of the glottal flow, and therefore lowers the global acoustic level radiated at the lips. It also introduces an offset in the glottal flow waveform

Direct numerical simulation of human phonation

The generation and propagation of the human voice is studied using direct numerical simulation. A full body domain is employed for the purpose of directly computing the sound in the region past the speaker's mouth. The air in the vocal tract is modeled as a compressible and viscous fluid interacting with the vocal folds (VFs). The vocal fold tissue material properties are multi-layered, with varying stiffness, and a finite-strain model is utilized and implemented in a quadratic finite element code. The fluid-solid domains are coupled through a boundary-fitted interface and utilize a Poisson equation-based mesh deformation method. The domain includes an anatomically relevant vocal tract geometry, either in two dimensions or in three dimensions. Adult and two-year-old child anatomy inspired simulations are performed. Phonation simulations using a non-linear hyper elastic, linear elastic and viscoelastic models of the VFs are performed and compared. The sensitivity of phonation to the VF Poisson's ratio is also evaluated. Simulations are employed to investigate voice disorders related to vocal fold stiffness asymmetry and unilateral vocal fold paralysis (UVFP). Additionally, an analysis is performed for medialization laryngoplasty, a well known surgical treatment for UVFP. Phonation onset is determined from all the simulations as a measure of degree of voice disorder with phonation threshold pressure (PTP) as a key parameter for the quantification. The computational model developed is demonstrated to be consistent with prior measurements and sufficiently sensitive to be used in future studies involving VF pathologies, surgical procedures to restore voice, and/or closed loop models of voice, speech and perception.Ope

Experimental Mechanical and Fluid Mechanical Investigations of the Brass Instrument Lip-reed and the Human Vocal Folds

The mechanical properties of the lips are of crucial importance to the function of a brass instrument. The natural resonance modes must be able to usefully interact with the instrument air column in order to sustain oscillations. Mechanical frequency responses of human and arti cial lips used to play a brass instrument were measured using a high-speed digital video technique in an attempt to classify the true nature of the lipreed. The results revealed the presence of at least two lip modes that exhibited the characteristic outward-inward striking behaviour seen in many in vitro replica lip-reed measurements. The Q-values of the human lip resonances were considerably lower than those seen for the replica lips. Transverse mechanical response measurements were also performed on an in vitro lip-reed to investigate the coupling between the outward and inward striking modes. The two dimensional motion of the lips during full oscillations was investigated. It is shown that a computational four degree-of-freedom model would be required to fully simulate the observed mechanical motion. The uid behaviour downstream from an in vitro vocal fold model was investigated using particle image velocimetry (PIV). A `free jet' con guration with no downstream acoustical coupling was rst investigated. The measurements revealed an unsteady glottal jet ow, consisting of a high velocity jet core, a transitional region of high jet deceleration and a turbulent mixing region. The jet was consistently skewed at angles to the glottal centreline, and appeared to oscillate back and forth across the centreline during the glottal cycle. The behaviour of the jet core was investigated in detail. A temporal asymmetry was observed in the mean velocity across the jet core such that the highest jet velocities were encountered during the closing phase of the vocal folds. The overall jet behaviour also showed a strong turbulent asymmetry between the opening and closing phases. High levels of vorticity and turbulent motion encountered during the closing phase were associated with the deceleration of the jet. Three vocal fold con gurations that included static replicas of the ventricular bands were nally investigated with the aim of characterising the aerodynamic interaction between the ventricular bands and the vocal folds. A marked e ect on the glottal jet was observed for all con gurations. The most physically realistic con guration appeared to stabilise the glottal jet, leading to a reattachment of the jet to the ventricular bands and a subsequent secondary ow separation from the downstream end. The implications of the aerodynamic interaction is discussed, with particular note to its possible relevance to the lip-reed and mouthpiece interaction in brass playing

Models and Analysis of Vocal Emissions for Biomedical Applications

The MAVEBA Workshop proceedings, held on a biannual basis, collect the scientific papers presented both as oral and poster contributions, during the conference. The main subjects are: development of theoretical and mechanical models as an aid to the study of main phonatory dysfunctions, as well as the biomedical engineering methods for the analysis of voice signals and images, as a support to clinical diagnosis and classification of vocal pathologies