Paul Gollnick and Dr. Scott Thomson, Mechanical Engineering
Importance of Project
Sounds produced in human speech originate from the vocal folds (also known as vocal chords). In order to speak, the vocal folds must close together. Air is then forced between them causing the folds to vibrate and produce sound.
Unilateral Vocal Fold Paralysis (UVFP) is a condition in which one of the vocal folds is paralyzed. This condition causes a hoarse, breathy, and soft voice because the vocal folds are unable to close all of the way. A surgical procedure called vocal fold medialization can be performed to restore speech capability. In this type of procedure the doctor implants material into or behind the vocal fold to bring it closer to the other functional vocal fold. The addition of new materials into or just behind the vocal fold has an effect on the stiffness of the vocal fold. The purpose of this study is to investigate how changing the stiffness of one vocal fold affects the sound produced by the vocal fold pair, such as in the case of a UVFP victim.
Approach
To study how stiffness variation of one vocal fold affects speech, synthetic vocal folds were made out of a silicone rubber. Each synthetic vocal fold consists of two layers: a base layer and a cover layer. These layers simulate the anatomy and stiffness of human vocal fold tissue. A three-part silicone rubber mixture is poured into molds and cures to form the vocal folds. Stiffness is changed by varying the mixing ratio of the silicone rubber mixture. These vocal fold models were used to study different characteristics of sound generation, such as: onset pressure (the minimum pressure required to begin vibration as pressure is increased), offset pressure (the pressure at which the model stops vibrating as the pressure is decreased), glottal width (the spacing between the vocal folds during vibration), frequency of vibration, jet stream direction and modulus of elasticity (a material property that varies with stiffness).
Testing
To study the characteristics of sound production (see list above), the vocal folds were mounted in rectangular plastic blocks. This allowed the two vocal folds to be brought together such that pressurized air could be forced between them, causing them to vibrate and produce sound. The pressure of the air was controlled by a valve and monitored by a pressure gauge, which allowed for the observation of the onset and offset pressures. A high-speed camera was used to take pictures of the vocal folds as they vibrated, which allowed for the observation of the spacing between the vocal folds during vibration. A data acquisition system measured the frequency of vibration. A particle imaging velocimetry system was used to produce a vector field which visually shows the jet stream of the airflow coming out of the vocal folds. A tensile testing machine was used to measure the modulus of elasticity.
Procedure
The above testing was performed on 3 pairs of vocal folds. In each pair the first vocal fold had a stiffness similar to that of a normal human vocal fold. The outer layer of the second vocal fold had a different stiffness for each pair. The benchmark pair was a symmetric model; the stiffness of both vocal folds was the same (similar to that of a normal human vocal fold. The second pair had one vocal fold with an outer layer being slightly stiffer than that of a normal human vocal fold. The third pair had one vocal fold with an even stiffer outer layer. The second and third pairs of vocal folds are representative of the vocal folds of UVFP victims.
Results
It was observed that changing the stiffness of one vocal fold does have an effect on speech. Increasing the stiffness of one vocal fold had the following effects: it increased both the onset and offset pressures, it decreased the glottal width, it increased the frequency of vibration and it changed the jet stream direction (the jet stream flowed more at an angle toward the stiffer vocal fold).
Contributions by Others
I participated in this project from October 2007 to April 2008. I was involved from the early planning of the project through the initial testing. The results above were observed in the initial testing that I performed. Since leaving this project, further tests have been performed by another research assistant, Brian Pickup. Brian has built on the initial observations and is compiling the results of his findings into a paper. It is anticipated that the results from this project will be published in the Journal of Biomechanics next year (2009).
Recommendations
This work focused primarily on the effects of changing the stiffness of the cover layer of the vocal fold. I would recommend that further investigation be done on the effect of changing the stiffness of the base layer of the vocal fold. I would also recommend that a study be done on the effect of changing the stiffness of both the base and cover layers of the vocal fold.
Thanks
I just wanted to give a special thanks to ORCA for the funding to do this project. I feel like I have learned a lot and am grateful for the opportunity I have had to do research as an undergraduate. Thanks to Dr. Thomson for his mentorship throughout this project. He has been an excellent leader and teacher. I certainly could not have accomplished this research without Dr. Thomson’s expertise. Thank you again for this great learning opportunity.