Theoretical and Numerical Analyses of Laryngeal Biomechanics: Towards Understanding Vocal Hyperfunction
MetadataShow full item record
Speech is a cornerstone in human communication and an irreplaceable tool for several academic, legal, and artistic careers. Producing intelligible speech is an extremely complex process, involving coupling between the air flow driven by the pressure built up in the lungs, the vibrating viscoelastic tissues in the larynx, namely the vocal folds, and the subglottal and supraglottal (vocal) tracts, including the nasal and oral passages. Therefore, the occurrence of a pathology in one of the organs responsible for voice production may result in deteriorated speech production and, consequently, a negative impact on the daily life and/or professional career of the individual. A specific class of voice disorders that is common among adults is vocal hyperfunction, which is associated with the misuse of vocal organs, resulting in inefficient voice production and, in some cases, vocal trauma. Researchers over the years have conducted clinical and numerical analyses of vocal hyperfunction and have developed assessment tools and therapeutic procedures for vocal hyperfunction; however, a comprehensive understanding of the underlying mechanisms of vocal hyperfunction remains unachieved. Fortunately, easily collected clinical measurements shed some light on potential mechanisms underlying vocal hyperfunction, and numerical and theoretical modelling campaigns of laryngeal biomechanics have shown some success in partially elucidating the biomechanics of voice production. Therefore, a potential route to pursue for a better understanding of the mechanics of vocal hyperfunction lays behind numerical and theoretical analyses guided by available clinical and experimental data. The aim of this thesis is to explore and elucidate, through four research projects, some of the underlying mechanisms associated with voice production in general and vocal hyperfunction in particular, where we resort to 1) data collected using some promising assessment tools and standard clinical measurements, and 2) models of voice production and larynx biomechanics, where theoretical and numerical analyses are conducted guided by the aforementioned clinical measurements. The first project analyses theoretically and numerically the underlying laryngeal factors altering fundamental frequency, for both healthy speakers and speakers with phonotraumatic vocal hyperfunction, during phonation offset, where clinical data of relative fundamental frequency are resorted to in modeling and analysis. We show that the clinically observed drop in fundamental frequency during phonation offset is potentially due to the decline in vocal fold collision forces, which is induced by increasing the glottal gap. We also show how the fundamental frequency drop rate can be modulated by the activation of certain laryngeal muscles, which we speculate to underlie the differences between healthy and hyperfunctional speakers. Besides, we illustrate how certain manifestations of vocal hyperfunction can also affect the drop rate during phonation offset. The second project extends the first one, where phonation onset is explored with similar numerical and theoretical approaches. We illustrate that, when all laryngeal and aerodynamic parameters are fixed in time, fundamental frequency tends to rise due to the increased vocal fold collision levels, and that matches with the clinical observations of the onset of initial or isolated vowels and, in some cases, vowels preceded by voiced consonants. On the other hand, we show through numerical simulations that the decline in fundamental frequency in the case of onset of vowels preceded by voiceless consonants requires involvement of laryngeal muscles, which we speculate to manifest the differences between healthy speakers and patients with vocal hyperfunction. In the third project, we attempt to elucidate the influence of extrinsic laryngeal muscles on posturing mechanics and phonation, and link findings with clinical observations collected from patients with vocal hyperfunction. We show how the vocal fold tension and phonation fundamental frequency vary with varying the magnitude, direction, and location of the net pulling force exerted by the extrinsic laryngeal muscles. Using the previous analysis in combination with clinical data, we pinpoint potential roles of specific extrinsic muscles in modulating fundamental frequency and we suggest some potential roles for extrinsic laryngeal muscles in hyperfunctional phonation. Finally, in the fourth project, we study the mechanics underlying curved and incomplete glottal closure configurations that are observed in some patients with vocal hyperfunction, where we develop and analyse a composite beam model for the vocal folds and we integrate it with a posturing model to enable exploring the effects of certain laryngeal maneuvers. The model predictive capability is adequate, matching clinical observations and simulations produced by high-fidelity models, yet providing useful insights into the underlying mechanism of curved glottal configurations due to its relative simplicity. Our analyses, based on the proposed model, show that the vocal fold layered structure and its interaction with the mechanical loading, resulting during laryngeal maneuvers, induce bending moments that result in different curved (convex and concave) vocal fold shapes that are associated with incomplete glottal closure patterns. We suggest, based on the conducted analyses, some potential laryngeal mechanisms that may be at play in patients with vocal hyperfunction.
Cite this version of the work
Mohamed Serry (2023). Theoretical and Numerical Analyses of Laryngeal Biomechanics: Towards Understanding Vocal Hyperfunction. UWSpace. http://hdl.handle.net/10012/19526