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Biomechanics of Voice Production

H.E. Gunter, R. Howe *, R. Hillman**, K. Stevens***
*Biorobotics Laboratory, Harvard University
**Voice & Speech Laboratory, Massachusetts Eye and Ear Infirmary
***Speech Group, Massachusetts Institute of Technology

Support provided by the Whitaker Foundation

The human vocal apparatus and its control capabilities are unique in the animal kingdom and enable our exceptional abilities to speak and sing. In addition to the importance of daily communication in our personal and working lives, an estimated 23% of all people in the workforce have occupations in which voice use is essential (Titze et al.,1997). Pathologies of the speech system may increase the effort needed to speak, decrease the quality of sound produced, cause pain, or eliminate the ability to speak. The etiology, and pathophysiology of many of these pathologies is poorly understood and treatment is empirically based.


The larynx is a specialization of the superior trachea that contains the vocal folds. During normal breathing the folds are separated to facilitate airflow into and out of the lungs. During voice production the folds are adducted (brought together) to the midline. A sufficient, but not excessive, pressure buildup in the lungs, below the adducted folds, will initiate coupled vibration of the airflow between the folds and the vocal fold tissue as shown in the figure below. A motorboat imitation made by forcing air between closed lips to make them vibrate is a similar phenomenon. The air flow forms a sound wave that is filtered by the pharynx, nasal and oral cavities to produce the speech sound heard by a listener.
[pic] Structure dictates function. Alterations to the vocal folds due to changes in laryngeal musculature activation or a change in vocal fold structure alter the vocal fold vibration waveform and the air flow waveform and ultimately change the sound heard by an listener.

Practices that are clinically associated with the formation of lesions such as nodules and polyps, including loud voicing and voice misuse are thought to increase mechanical stress levels in vocal fold tissue. The lesions are thought to be a reaction to these high stress levels. Therefore, better understanding of mechanical stress determinants has the potential to improve prevention and treatment of benign vocal fold lesions.

Background Links

"How Humans Speak, Sing, Squeak and Squeal" is an outstanding tutorial on the principles of voice production developed by the National Center for Voice and Speech.

The University of Iowa has an interesting array of voice case studies that illustrate the connection between structure and function.

Theoretical Models
The goal of this project is to address questions regarding the etiology and treatment of common voice pathologies. To this end a mathematical model that permits systematic manipulation and control of physiologically relevant variables is being constructed using finite element methods. The model contains representations of the soft tissue vocal folds and of the airflow between them.

The model parameters are based on laryngeal geometric and material measurements contained in the literature. Validation will be based on comparison with original and published experimental measurements of geometry, aerodynamic flow rate and contact pressure. This work was presented at the 142nd meeting of the Acoustical Society of America and has been submitted for publication in the Journal of the Acoustical Society of America

. The validated model is being used to investigate the role of mechanical stress in vocal fold pathology development. This work will be presented at the International Conference on Voice Physiology and Biomechanics in Septemeber, 2002. Future applications will include prediction of the functional outcomes of proposed surgical treatments.


[pic] Experimental Measurements
Another part of the project involves the development of techniques with which to measure vocal fold impact force during voice production in vivo. These measurements will directly contribute data towards the question of the role of mechanical stress in pathology development and will provide important values for use in validation of the theoretical models.

A force sensor that can be placed between the vocal folds during voice production has been designed and constructed. Early in vivo results are encouraging. Future efforts will focus on measureing collision force while subjects generate voiced sounds at varying intensities and pitches.

Titze, I. R., Lemke, J., and Montequin, D. (1997). "Populations in the U.S. workforce who rely on voice as a primary tool of trade: a preliminary report." Journal of Voice, 11(3), 254-9.

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