Vocal Fold Geometry Research Articles

Principal dimensions of voice production and their role in vocal expression.

How we produce and perceive voice is constrained by laryngeal physiology and biomechanics. Such constraints may present themselves as principal dimensions in the voice outcome space that are shared among speakers. This study attempts to identify such principal dimensions in the voice outcome space and the underlying laryngeal control mechanisms in a three-dimensional computational model of voice production. A large-scale voice simulation was performed with parametric variations in vocal fold geometry and stiffness, glottal gap, vocal tract shape, and subglottal pressure. Principal component analysis was applied to data combining both the physiological control parameters and voice outcome measures. The results showed three dominant dimensions accounting for at least 50% of the total variance. The first two dimensions describe respiratory-laryngeal coordination in controlling the energy balance between low- and high-frequency harmonics in the produced voice, and the third dimension describes control of the fundamental frequency. The dominance of these three dimensions suggests that voice changes along these principal dimensions are likely to be more consistently produced and perceived by most speakers than other voice changes, and thus are more likely to have emerged during evolution and be used to convey important personal information, such as emotion and larynx size.

Principal dimensions of laryngeal vocal control in a computational model of voice production

Voice production is constrained by laryngeal physiology and physics. Such constraints may present themselves as principal dimensions that are shared among speakers in how they produce and perceive voice. In this study, we attempt to identify such principal dimensions in voice outcome measures and the underlying laryngeal control mechanisms in a three-dimensional computational model of voice production. A large-scale voice simulation was performed with parametric variations in vocal fold geometry, stiffness, glottal gap, and subglottal pressure. Principal component analysis was applied to data combining both the laryngeal control parameters and acoustic and aerodynamic voice outcome measures. The results showed two dominant dimensions of vocal control. The first dimension describes interaction between respiration and vocal fold adduction in a way that increases glottal flow amplitude and vocal intensity at the cost of decreasing high-frequency harmonic production. The second dimension mainly describes control of medial surface thickness and glottal closure, which allows simultaneous increase in vocal intensity and high-frequency harmonic production but also increases risk of vocal fold injury. A third dimension with a reduced weight describes control of the fundamental frequency. The importance of these principal control dimensions to vocal expression of emotion and vocal health is discussed.

Contribution of laryngeal size to differences between male and female voice production.

In this study we investigated the effect of sex- and age-related differences in vocal fold length, thickness, and depth on voice production in a three-dimensional vocal fold model. The results showed that the cause-effect relationships between vocal fold physiology and voice production previously identified in an adult male-like vocal fold geometry remained qualitatively the same in vocal folds with geometry representative of adult females and children. We further showed that the often-observed differences in voice production between adult males, adult females, and children can be explained by differences in length and thickness. The lower F0, higher flow rate, larger vocal fold vibration amplitude, and higher sound pressure level (SPL) in adult males as compared to adult females and children can be explained by differences in vocal fold length. In contrast, the thickness effect dominated and contributed to the larger closed quotient of vocal fold vibration, larger normalized maximum flow declination rate, and lower H1-H2 in adult males as compared to adult females and children. The effect of differences in vocal fold depth was generally small. When targeting a specific SPL, adult males experienced a lower peak vocal fold contact pressure during phonation than adult females and children.

Contribution of vocal fold thickness to gender perception

Male vocal folds are often longer and thicker than female vocal folds. The length difference is generally considered responsible for the notable difference in fundamental frequency between men and women, which plays an important role in gender perception. The role of the thickness difference in gender perception is less clear, although our recent studies demonstrated an important role of thickness in the control of voice quality. The goal of this study is to investigate the contribution of changes in vocal fold thickness and stiffness to gender perception. Synthetic voices were generated using a three-dimensional vocal fold model with parametric variations in vocal fold geometry, stiffness, adduction, and subglottal pressure. The vocal tract was kept constant in order to focus on the contribution of the voice source. Listening subjects were asked to judge speaker gender in a multiple choice task. Preliminary results showed a significant effect of vocal fold thickness and transverse stiffness on gender perception, with voices produced by thicker vocal folds and lower transverse stiffnesses more likely to be judged as male. Acoustically, gender perception scores were correlated with the fundamental frequency and spectral shape. [Work supported by NIH.]

Voice Feature Selection to Improve Performance of Machine Learning Models for Voice Production Inversion

Estimation of physiological control parameters of the vocal system from the produced voice outcome has important applications in clinical management of voice disorders . Previously we developed a simulation-based neural network for estimation of vocal fold geometry, mechanical properties, and subglottal pressure from voice outcome features that characterize the acoustics of the produced voice. The goals of this study are to (1) explore the possibility of improving the estimation accuracy of physiological control parameters by including voice outcome features characterizing vocal fold vibration; and (2) identify voice feature sets that optimize both estimation accuracy and robustness to measurement noise. Feedforward neural networks are trained to solve the inversion problem of estimating the physiological control parameters of a three-dimensional body-cover vocal fold model from different sets of voice outcome features that characterize the simulated voice acoustics, glottal flow, and vocal fold vibration. A sensitivity analysis is then performed to evaluate the contribution of individual voice features to the overall performance of the neural networks in estimating the physiologic control parameters. While including voice outcome features characterizing vocal fold vibration increases estimation accuracy, it also reduces the network's robustness to measurement noise, due to high sensitivity of network performance to voice outcome features measuring the absolute amplitudes of the glottal flow and area waveforms, which are also difficult to measure accurately in practical applications. By excluding such glottal flow-based features and replacing glottal area-based features by their normalized counterparts, we are able to significantly improve both estimation accuracy and robustness to noise. We further show that similar estimation accuracy and robustness can be achieved with an even smaller set of voice outcome features by excluding features of small sensitivity.

Impact of the Paraglottic Space on Voice Production in an MRI-Based Vocal Fold Model

While the vocal fold is in direct contact anteriorly with the thyroid cartilage, posteriorly the vocal fold connects to the thyroid cartilage through a soft tissue layer in the paraglottic space. Currently the paraglottic space is often neglected in computational models of phonation, in which a fixed boundary condition is often imposed on the lateral surface of the vocal fold. The goal of this study was to investigate the effect of the paraglottic space on voice production in an MRI-based vocal fold model, and how this effect may be counteracted by vocal fold stiffening due to laryngeal muscle activation. Parametric simulation study using an MRI-based computational vocal fold model. The results showed that the presence of the paraglottic space increased the mean and amplitude of the glottal area waveform, decreased the phonation frequency and closed quotient. For the particular vocal fold geometry used in this study, the presence of the paraglottic space also reduced the occurrence of irregular vocal fold vibration. These effects of the paraglottic space became smaller with increasing paraglottic space stiffness and to a lesser degree with vocal fold stiffening. The results suggest that the paraglottic space may be neglected in qualitative evaluations of normal phonation, but needs to be included in simulations of pathological phonation or vocal fold posturing.

Vocal Tradeoffs in Anterior Glottoplasty for Voice Feminization.

Anterior (Wendler) glottoplasty has become a popular surgery for voice feminization. However, there has been some discrepancy between its theoretical pitch-raising potential and what is actually achievable, and downsides to shortening the glottis have not been fully explored. In addition, descriptions of the surgery are inconsistent in their treatment of the vocal ligament. This study aimed to determine 1) how fundamental frequency (fo ) is expected to vary with length of anterior glottic fixation, 2) the impact of glottic shortening on sound pressure level (SPL), and 3) the effect of including the ligament in fixation. Computational simulation. Voice production was simulated in a fiber-gel finite element computational model using canonical male vocal fold geometry incorporating a three-layer vocal fold composition (superficial lamina propria, vocal ligament, and thyroarytenoid muscle). Progressive anterior glottic fixation (0, 1/8, 2/8, 3/8, etc. up to 7/8 of membranous vocal fold length) was simulated. Outcome measures were fo , SPL, and glottal flow waveforms. fo increased from 110 Hz to 164 Hz when the anterior one-half vocal fold was fixed and continued to progressively rise with further fixation. SPL progressively decreased beyond 1/8 to 1/4 fixation. Inclusion of the vocal ligament in fixation did not further increase fo . Any fixation increased aperiodicity in the acoustic signal. The optimal length of fixation is a compromise between pitch elevation and reduction in output acoustic power. The simulation also provided a potential explanation for vocal roughness that is sometimes noted after anterior glottoplasty. NA Laryngoscope, 131:1081-1087, 2021.

Estimation of vocal fold physiology from voice acoustics using machine learning.

The goal of this study is to estimate vocal fold geometry, stiffness, position, and subglottal pressure from voice acoustics, toward clinical and other voice technology applications. Unlike previous voice inversion research that often uses lumped-element models of phonation, this study explores the feasibility of voice inversion using data generated from a three-dimensional voice production model. Neural networks are trained to estimate vocal fold properties and subglottal pressure from voice features extracted from the simulation data. Results show reasonably good estimation accuracy, particularly for vocal fold properties with a consistent global effect on voice production, and reasonable agreement with excised human larynx experiment.

Effects of thyroplasty implant stiffness on glottal shape and voice acoustics.

ObjectivesVocal fold (VF) stiffness and geometry are determinant variables in voice production. Type 1 medialization thyroplasty (MT), the primary surgical treatment for glottic insufficiency, changes both of these variables. Understanding the cause and effect relationship between these variables and acoustic output might improve voice outcomes after MT. In this study, the effects of thyroplasty implants with variable stiffness on glottal shape and acoustics were investigated.MethodsIn an ex vivo human larynx phonation model, bilateral MT with implants of four stiffness levels (1386, 21.6, 9.3, and 5.5 kPa) were performed. Resulting acoustics and aerodynamics were measured across multiple airflow levels. A vertical partial hemilaryngectomy was performed and stereoscopic images of the VF medial surface taken to reconstruct its three‐dimensional (3D) surface contour. The results were compared across implants.ResultsThe effects of implant stiffness on acoustics varied by airflow. Softer implants resulted in improved acoustics, as measured by cepstral peak prominence (CPP), at lower airflow levels compared to stiffer implants but this relationship reversed at high airflow levels. Stiffer implants generally required less airflow to generate a given subglottal pressure. Stiffer implants resulted in greater medialized surface area and maximal medialization, but all implants had similar effects on overall VF medial surface contour.ConclusionSofter implants result in less medialization but better acoustics at low airflow rates. Stiffer implants provide better acoustics and more stable pressure‐flow relationships at higher airflow rates. This highlights a potential role for patient‐specific customized thyroplasty implants of various stiffness levels.Level of EvidenceNA.

Voice production in a MRI-based subject-specific vocal fold model with parametrically controlled medial surface shape.

The goal of this study was to investigate how realistic changes in medial surface shape, as occur in human phonation, affect voice production. In a parametric magnetic resonance imaging-based three-dimensional vocal fold model, the superior and inferior portions of the medial surface were systematically manipulated to produce different medial surface contours similar to those observed in previous excised larynx and in vivo canine larynx experiments. Voice simulations were performed to investigate the differences in the resulting voice production. The results showed that both superior-medial bulging and inferior-medial bulging of the medial surface, which led to an increased vertical thickness and a more rectangular glottal configuration, increased the closed quotient of vocal fold vibration. Changes in medial surface shape also had significant effects on the phonation threshold pressure. The degree of these effects of changes in medial surface shape was larynx specific, and varied significantly depending on the vocal fold cross-sectional geometry and its variation along the anterior-posterior direction. The results suggest that, in addition to vocal fold approximation, surgical interventions of voice disorders should also aim at restoring a rectangular and sufficiently thick medial surface.

Estimation of vocal fold geometry and stiffness from voice acoustics

Many speech applications require estimating vocal fold properties from the produced acoustics. While there have been many previous research on solving the inverse problem, they are often based on lumped-element models of phonation, whose model parameters are difficult to relate to realistic vocal fold properties. This study explores the feasibility of inferring physiologically realistic vocal fold properties, including vocal fold length, thickness, depth, anisotropic stiffness moduli, and the subglottal pressure, from the produced acoustics, using a three-dimensional phonation model and a Bayesian inference approach. To reduce the computational cost associated with the use of a three-dimensional model and the large number of inputs to be estimated, we explore the possibility of improving computational efficiency and estimation accuracy using the large amount of three-dimensional simulation data available from our previous research. Preliminary results show that joint likelihood probabilities can be reasonably estimated based on the available simulation data, significantly reducing computational costs of Bayesian inference. The approach is able to estimate the control parameters of voice production from the produced acoustics with reasonable accuracy. It is observed that certain control parameters, particularly vocal fold stiffness, consistently have large estimation errors than others. [Work supported by NIH.]

Evaluation of excised porcine larynx frequency changes due to changing vocal fold elastic properties

The work presented in this study investigates the impact of changing vocal fold elastic properties on the frequencies of vibration during phonation. Excised porcine larynx tests were conducted to examine the relationship between porcine vocal fold anterior-posterior strain and the resulting changes in the phonation frequency. During each test, two high speed cameras (4000 FPS) and a microphone (44100 Hz) were used to record changes in vocal fold dynamics while a servo motor stretched the tissue in the anterior-posterior direction. A digital image correlation tracking method was then used to measure anterior-posterior strain on the exposed porcine vocal fold superior surface. Strain measurements were related to changes in vocal fold elasticity based on previously reported mean elastic parameters which describe porcine vocal fold elasticity from 0%–40%. Additionally, an aeroelastic model of phonation which incorporates the measured vocal fold geometry, elastic properties, and air flow characteristics was implemented to further study the impact of changing porcine vocal fold elasticity on the vibration frequency. Phonation was first observed at a strain of 0.18 mm/mm with a fundamental frequency of 246 Hz and increased to 404 Hz at a strain of 0.35 mm/mm. Airflow conditions were kept constant during all experimental tests.The work presented in this study investigates the impact of changing vocal fold elastic properties on the frequencies of vibration during phonation. Excised porcine larynx tests were conducted to examine the relationship between porcine vocal fold anterior-posterior strain and the resulting changes in the phonation frequency. During each test, two high speed cameras (4000 FPS) and a microphone (44100 Hz) were used to record changes in vocal fold dynamics while a servo motor stretched the tissue in the anterior-posterior direction. A digital image correlation tracking method was then used to measure anterior-posterior strain on the exposed porcine vocal fold superior surface. Strain measurements were related to changes in vocal fold elasticity based on previously reported mean elastic parameters which describe porcine vocal fold elasticity from 0%–40%. Additionally, an aeroelastic model of phonation which incorporates the measured vocal fold geometry, elastic properties, and air flow characteristics was im...

Effect of medial surface shape on voice production in a MRI-based three-dimensional phonation model

Our previous studies using simplified vocal fold geometry have shown that medial surface shape, particularly the vertical thickness, has an important effect on the closure pattern of vocal fold vibration. The goal of this study is to investigate if similar effect can be observed for medial surface shape that occurs in realistic human phonation, using a parametric MRI-based three-dimensional vocal fold model. Manipulation of medial surface shape is achieved through a two-level control of the superior and inferior portion of the medial surface, based on experimental observations. Simulations show that, in general, both superior-medial bulging and inferior-medial bulging of the medial surface, which leads to an increased vertical thickness and a more rectangular glottal configuration, increase the closed quotient of the vocal fold vibration. Changes in medial surface shape also have significant effect on the phonation threshold pressure. This effect of the medial surface shape varies significantly across larynges, indicating the important effect of subject-specific laryngeal geometry. The results point to the importance of taking into consideration of the medial surface shape in clinical management of voice disorders. [Work supported by NIH.]

Vocal fold contact pressure in a three-dimensional body-cover phonation model

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.

Kinematic model for simulating mucosal wave phenomena on vocal folds

Mucosal waves have been found to be important for evaluating vocal fold vibrations in laryngological practice. While they are routinely evaluated visually, the knowledge on the physical phenomena related to mucosal wave propagation is limited. Kymographic imaging, in particular, reveals various mucosal wave features that deserve more understanding in order to advance functional diagnostics of voice disorders. Here, a kinematic model is presented which simulates mucosal waves on human vocal folds. The vocal fold geometry is based on a parametrically adjustable M5 model. A kinematic rule is used for simulating the propagation of the mucosal wave from the bottom of the vocal folds upwards and laterally over the upper vocal fold surface. The model maps the changes of the coronal shape of the vocal folds through vibration cycles. The vibration characteristics including the mucosal wave movements are then visualized using a synthetic kymogram graphically obtained through a local illumination method. The model can serve as an educational and research tool for studying the mucosal wave features and their appearance in laryngeal kymographic images.

3D multiscale imaging of human vocal folds using synchrotron X-ray microtomography in phase retrieval mode

Human vocal folds possess outstanding abilities to endure large, reversible deformations and to vibrate up to more than thousand cycles per second. This unique performance mainly results from their complex specific 3D and multiscale structure, which is very difficult to investigate experimentally and still presents challenges using either confocal microscopy, MRI or X-ray microtomography in absorption mode. To circumvent these difficulties, we used high-resolution synchrotron X-ray microtomography with phase retrieval and report the first ex vivo 3D images of human vocal-fold tissues at multiple scales. Various relevant descriptors of structure were extracted from the images: geometry of vocal folds at rest or in a stretched phonatory-like position, shape and size of their layered fibrous architectures, orientation, shape and size of the muscle fibres as well as the set of collagen and elastin fibre bundles constituting these layers. The developed methodology opens a promising insight into voice biomechanics, which will allow further assessment of the micromechanics of the vocal folds and their vibratory properties. This will then provide valuable guidelines for the design of new mimetic biomaterials for the next generation of artificial larynges.

Vocal instabilities in a three-dimensional body-cover phonation model.

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.

Voice instabilities in a three-dimensional continuum model of phonation

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 are classified into three types corresponding to regular, quasi-steady or subharmomic, and chaotic phonation. The results show that the presence of a vocal tract significantly increases the occurrence of irregular vocal fold vibration, which naturally occurs even under symmetric vocal fold conditions, in particular for vocal folds with very low transverse stiffness in the coronal plane or a soft vocal fold body layer. The occurrence of voice instability is suppressed by increasing the transverse stiffness, increasing the body-cover ratio in the longitudinal stiffness, or decreasing the subglottal pressure. The implications of these observations on the production of certain voice qualities are discussed. [Work supported by NIH.]

An empirical model linking intrinsic laryngeal muscle activation to vocal fold geometry and stiffness

Although vocal fold vibration and the resulting voice production depend on the geometry and mechanical properties of the vocal folds, control of voice is achieved through laryngeal muscle activation, which postures the vocal folds into desired geometry and mechanical properties. Such posturing process is known to cause simultaneous changes in vocal fold position, geometry, stiffness, and tension, which has been hypothesized to be a possible origin of vocal registers. Understanding such co-variations in vocal fold geometry and mechanical properties is thus critical to our understanding of human voice production and control, and the development of muscularly controlled, computationally efficient models of phonation. In this study, an empirical model is developed to link the glottal gap, vocal fold length, vocal fold vertical thickness, and the mechanical properties of the vocal fold to the activation levels of the cricothyroid, thyroarytenoid, lateral cricoarytenoid, interarytenoid, and posterior cricoarytenoid muscles, based on data from in vivo larynx experiments. Combined with a continuum model of phonation, this model is used to investigate muscular control of pitch and vocal registers and the preliminary results are discussed.

Effect of level difference between left and right vocal folds on phonation: Physical experiment and theoretical study

The vocal folds, which are constituted by muscles covered with a mucous membrane, generate a primary sound called the voice source, as airflow passes them. In some voice disorders, asymmetry between left and right vocal folds was observed. We focus on level difference, which is defined as the distance between the upper surfaces of the bilateral vocal folds in the inferior-superior direction and is caused by symptom of such disorders. Physical models of the vocal folds were utilized to study the effect of the level difference on the phonation threshold pressure. For three types of different self-oscillating synthetic models, our experiments reveal that the phonation threshold pressure increases significantly as the level difference is extended. Furthermore, based upon a small amplitude approximation of the vocal fold oscillations, a theoretical formula was derived for the phonation threshold pressure. Our theory was in good agreement with the experiments, especially when the phase difference between the left and right vocal folds is not too large. From these results, we conclude that the level difference affects voice production, therefor the effect of the vocal fold geometry needs to be taken into account for the observation of voice disorders.