Videostroboscopy 

Why?

The vocal cords vibrate very quickly when they are activated during phonation to produce sounds.  This technique allows the rapid speed of movement of the vocal cords to be slowed down so that a detailed assessment of the rhythmic oscillations of the surface of the vocal folds can be made.

How?

The test involves placing a small external surface plate about the size of a stethoscope over one side of the voice box.  This enables the frequency of the oscillations of the vocal folds to be measured.  An exact adjustment of the wavelength of the special strobe xenon illuminating light can be made, which results in an image of the vocal cords vibrating at a very much slower rate.