Vocal folds vibrate approximately between 80 Hz (most males) to 300 Hz (most females) during modal phonation. It can reach as high as 1000 Hz when the voice is produced in extreme high pitch or falsetto voice. According to the law of Talbot, human eyes can only sense five image frames (video) per second, and each image is retained on the retina for 0.2 s. Hence, vocal fold vibration is too fast for the eyes to follow. Therefore, conventional laryngoscopy cannot observe the vibration of vocal folds under continuous light.
Strobolaryngoscopic video of the normal larynx (MP4 2429 kb)