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Due to basic stroboscopic principles and the nature and behavior of human vocal folds, the technology has its limitations, Deliyski says, especially for the visual evaluation of human vocal folds. "Stroboscopy simply cannot capture the true cycle-to-cycle vibratory behavior of the vocal folds, and as a result, the intra-cycle vibration seen in stroboscopy displays an illusory ‘slow motion.' Stroboscopy has no benefit to people whose voice disorder causes irregular vocal fold vibration, as the stroboscopic images produced cannot be used to accurately diagnose disorders. This limitation would affect approximately half of the patients with voice disorders."
Researchers actually tried high-speed cameras in the 1930s when Bell Labs and then Kodak produced them. It was highly impractical. For example, each picture had to be developed. As it has with most things, technology is making a difference today.
How fast do the folds vibrate? Very fast. With men, it's about 120-150 cycles per second; for women, it's 250-400. And if the pitch is raised, say to a woman's falsetto, it becomes almost 1,000 cycles per second.
"To understand most aspects of the vibration you need to capture at least 15 images per each vocal fold cycle," Deliyski, explains, "so you need 10,000-15,000 images per second. This is the challenge: You also need good resolution to see the anatomic structures. Brightness of the tissue is important. You need a camera recording at a speed of 10,000-15,000 pictures per second. As you increase frame rate, you sacrifice resolution and sensitivity.
"What's new in my research goes beyond the vibrations, linking them to voice disorders so we can diagnose clinically and better understand vocal-fold tissue motion. The idea of studying the vibrations of the vocal folds is an old idea. People have been trying to understand it for over 100 years. Now, several stars have aligned. We have the availability of a very fast camera with sufficient sensitivity to record the vocal folds. It has to be very fast and have very good sensitivity because excessive light may be dangerous to the vocal folds tissue.