Understanding the Aeroacoustics of the Human Voice
By Emil Venere
Engineers are trying to better understand and duplicate the aerodynamics and acoustics of the human voice, in part to help prevent profound changes to the voice after throat surgery.
"Our main interest is to try to predict the consequences of surgery and plan the operation to minimize the effects on voice," said Luc Mongeau, an associate professor of mechanical engineering at Purdue University, in West Lafayette, IN.
With funding from the National Institutes of Health, he and associate professor Steven Frankel have created plastic and mathematical models to better analyze and recreate the voice-production process, which depends on turbulent air- flow through the glottis. The researchers are trying to predict the aeroacoustics, or the aerodynamic sound, produced by air flow. In addition to preserving a person's voice, the findings may help engineers figure out how to better synthesize and characterize the voice for robotics and voice-recognition purposes.
The Purdue engineers have detailed some recent findings of their work in a paper that focuses on the aerodynamics behind "impulsively started" jets of air that are central to human speech (Physics of Fluids, March 2000).
"Impulsive is when you build up pressure and then release it all of a sudden," Mongeau explained.
The voice process begins when the lungs exert air pressure and the vocal cords open, releasing successive, pulsing jets of air. Each jet of air is attached to a leading vortex that eventually detaches from the jet. The time it takes for the ring to detach from the jet--about one-thousandth of a second--is critical to the formation of speech.
"We looked at that process with a magnifying glass in a big computer simulation to try to understand that type of flow better," Mongeau reported. "We want to know how much jet development you have during that period of time. Is that sufficient for a single vortex to form and detach or would it stay attached until the formation of another one?"
He used the example of smoking a cigarette and making smoke rings.
"If you make them very slowly, the rings have time to go away; and you can watch them dissipate," he said. "But you could also puff them in close succession, and that's when you get what I call a 'vortex train,' one vortex following another, that looks like a caterpillar."
The computational results discussed in the paper support previous work by other researchers. They also reveal something new: Each individual jet becomes unstable and forms tiny eddies that influence the detachment of the ring. The numerous eddies affect the eventual qualities of the human voice.
In addition to their work involving computational simulations, the Purdue researchers have designed an artificial larynx. As air flows through the model, its rubbery walls rapidly are adjusted by small rods to simulate how the tissue responds during speech.
"We are extending this work one step further and taking into account the motion of the walls so that the vocal cords are now moving," said Frankel. "The air pushes on the walls; and the walls spring back, pushing on the air. There is an interaction between the two."
Eventually, the modeling research will have to extend beyond the larynx if engineers are to fully understand the physiology of speech. The strategy is different than a more conventional approach to speech synthesis, which ignores human physiology. The physiological approach strives to model speech production by taking into account the positions of the vocal cords, tongue, lips and other articulators involved in speech, Mongeau said.
Such a technique enables scientists to "develop a sort of code- book so that for a given sentence you track the motion of all the different articulatory parameters," he stated. "You generate the speech output from the articulatory parameters."
For More Information
Steven Frankel, on-line: http://widget.ecn.purdue.edu/~frankel/
Luc Mongeau, on-line: http://www.ecn.purdue.edu/Herrick/Faculty/mongeau
Emil Venere is a staff writer with the Purdue University News Service.