Comparison of a fiber-gel finite element model of vocal fold vibration to a transversely isotropic stiffness model

Vocal Tradeoffs in Anterior Glottoplasty for Voice Feminization

OBJECTIVES/HYPOTHESIS

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 (f_o ) 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.

STUDY DESIGN

Computational simulation.

METHODS

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 f_o , SPL, and glottal flow waveforms.

RESULTS

f_o 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 f_o . Any fixation increased aperiodicity in the acoustic signal.

CONCLUSIONS

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.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:1081-1087, 2021.

Mapping Thyroarytenoid and Cricothyroid Activations to Postural and Acoustic Features in a Fiber-Gel Model of the Vocal Folds

Any specific vowel sound that humans produce can be represented in terms of four perceptual features in addition to the vowel category. They are pitch, loudness, brightness, and roughness. Corresponding acoustic features chosen here are fundamental frequency (fo ), sound pressure level (SPL), normalized spectral centroid (NSC), and approximate entropy (ApEn). In this study, thyroarytenoid (TA) and cricothyroid (CT) activations were varied computationally to study their relationship with these four specific acoustic features. Additionally, postural and material property variables such as vocal fold length (L) and fiber stress (σ) in the three vocal fold tissue layers were also calculated. A fiber-gel finite element model developed at National Center for Voice and Speech was used for this purpose. Muscle activation plots were generated to obtain the dependency of postural and acoustic features on TA and CT muscle activations. These relationships were compared against data obtained from previous in vivo human larynx studies and from canine laryngeal studies. General trends are that fo and SPL increase with CT activation, while NSC decreases when CT activation is raised above 20%. With TA activation, acoustic features have no uniform trends, except SPL increases uniformly with TA if there is a co-variation with CT activation. Trends for postural variables and material properties are also discussed in terms of activation levels.

Effect of Vocal Fold Implant Placement on Depth of Vibration and Vocal Output

OBJECTIVE

Most type 1 thyroplasty implants and some common injectable materials are mechanically stiff. Placing them close to the supple vocal fold mucosa can potentially dampen vibration and adversely impact phonation, yet this effect has not been systematically investigated. This study aims to examine the effect of implant depth on vocal fold vibration and vocal output.

STUDY DESIGN

Computational simulation.

METHODS

Voice production was simulated with a fiber-gel finite element computational model that incorporates a three-layer vocal fold composition (superficial lamina propria, vocal ligament, thyroarytenoid muscle). Implants of various depths were simulated, with a "deeper" or more medial implant positioned closer to the vocal fold mucosa and replacing more muscle elements. Trajectories of surface and within-tissue nodal points during vibration were produced. Outcome measures were the trajectory radii, fundamental frequency (F₀ ), sound pressure level (SPL), and smoothed cepstral peak prominence (CPPS) as a function of implant depth.

RESULTS

Amplitude of vibration at the vocal fold medial surface was reduced by an implant depth of as little as 14% of the total transverse vocal fold depth. Increase in F₀ and decrease in CPPS were noted beyond 30% to 40% implant depth, and SPL decreased beyond 40% to 60% implant depth.

CONCLUSIONS

Commonly used implants can dampen vibration "from a distance," ie, even without being immediately adjacent to vocal fold mucosa. Since implants are typically placed at depths examined in this study, stiff implants likely have a negative vocal impact in a subset of patients. Softer materials may be preferable, especially in bilateral medialization procedures.

LEVEL OF EVIDENCE

N/A Laryngoscope, 130:2192-2198, 2020.

A computational study of depth of vibration into vocal fold tissues

The effective depth of vocal fold vibration is self-regulated and generally not known a priori in vocalization. In this study, the effective depth was quantified systematically under various phonatory conditions using a fiber-gel finite element vocal fold model. The horizontal and vertical excursions of each finite element nodal point trajectory were recorded to compute trajectory areas. The extent of vibration was then studied based on the variation of trajectory radii as a function of depth in several coronal sections along the anterior-posterior direction. The results suggested that the vocal fold nodal trajectory excursions decrease systematically as a function of depth but are affected by the layered structure of the vocal folds. The effective depth of vibration was found to range between 15 and 55% of the total anatomical depth across all phonatory conditions. The nodal trajectories from the current study were compared qualitatively with the results from excised human hemi-larynx experiments published in Döllinger and Berry [(2006). J. Voice. 20(3), 401-413]. An estimate of the effective mass of a one-mass vocal fold model was also computed based on the effective depth of vibration observed in this study under various phonatory conditions.

Sensitivity analysis of muscle mechanics-based voice simulator to determine gender-specific speech characteristics

The purpose of this study was to investigate the gender differences in voice simulation using a sensitivity analysis approach. A global, Monte Carlo-based approach was employed, and the relationships between biomechanical inputs (lung pressure and muscle activation levels) and acoustic outputs (fundamental frequency, f₀, and sound pressure level, SPL) were investigated for male and female versions of a voice simulator model. The gender distinction in the model was based on an anatomical scaling of the laryngeal structures. Results showed strong relationships for f₀ and SPL as functions of lung pressure, as well as for f₀ as a function of cricothyroid and thyroarytenoid muscle activity, in agreement with previous literature. Also expected was a systematic shift in f₀ range between the genders. It was found that the female model exhibited greater pitch strength (saliency) than the male model, which might equate to a perceptually more periodic or higher-quality voice for females. In addition, the female model required slightly higher lung pressures than the male model to achieve the same SPL, suggesting a possibly greater phonatory effort and predisposition for fatigue in the female voice. The methods and results of this study lay the groundwork for a complete mapping of simulator sound signal characteristics as a function of simulator input parameters and a better understanding of gender-specific voice production and vocal health.

WHERE HAS ALL THE POWER GONE? ENERGY PRODUCTION AND LOSS IN VOCALIZATION

Human voice production for speech is an inefficient process in terms of energy expended to produce acoustic output. A traditional measure of vocal efficiency relates acoustic power radiated from the mouth to aerodynamic power produced in the trachea. This efficiency ranges between 0.001 % and 1.0 % in speech-like vocalization. Simplified Navier-Stokes equations for non-steady compressible airflow from trachea to lips were used to calculate steady aerodynamic power, acoustic power, and combined total power at seven strategic locations along the airway. A portion of the airway was allowed to collapse to produce self-sustained oscillation for sound production. A conversion efficiency, defined as acoustic power generated in the glottis to aerodynamic power dissipated, was found to be on the order of 10%, but wall vibration, air viscosity, and kinetic pressure losses consumed almost all of that power. This sound, reflected back and forth in the airway, was dissipated at a level on the order of 99.9 %.

Vocal fold contact patterns based on normal modes of vibration

The fluid-structure interaction and energy transfer from respiratory airflow to self-sustained vocal fold oscillation continues to be a topic of interest in vocal fold research. Vocal fold vibration is driven by pressures on the vocal fold surface, which are determined by the shape of the glottis and the contact between vocal folds. Characterization of three-dimensional glottal shapes and contact patterns can lead to increased understanding of normal and abnormal physiology of the voice, as well as to development of improved vocal fold models, but a large inventory of shapes has not been directly studied previously. This study aimed to take an initial step toward characterizing vocal fold contact patterns systematically. Vocal fold motion and contact was modeled based on normal mode vibration, as it has been shown that vocal fold vibration can be almost entirely described by only the few lowest order vibrational modes. Symmetric and asymmetric combinations of the four lowest normal modes of vibration were superimposed on left and right vocal fold medial surfaces, for each of three prephonatory glottal configurations, according to a surface wave approach. Contact patterns were generated from the interaction of modal shapes at 16 normalized phases during the vibratory cycle. Eight major contact patterns were identified and characterized by the shape of the flow channel, with the following descriptors assigned: convergent, divergent, convergent-divergent, uniform, split, merged, island, and multichannel. Each of the contact patterns and its variation are described, and future work and applications are discussed.