Excessive variation of air pressure can be associated with poor motor control of the vocal folds or of the respiratory system. A high frequency, low current signal is passed between the vocal folds via electrodes located on the external neck over the thyroid lamina. When the vocal folds touch, greater current flows than when they are open. Electroglottographic recordings can be used to determine when the vocal folds are closed and how fast they are closing. The laryngography is a device to record the electroglottographic signal Photoglottography-is designed to obtain estimates of variations of glottal area during phonation.
Light is directed from above, usually from a fiberoptic light sources passed through the nose.
The light passes through the glottis and is detected by a light-sensitive device usually positioned over the skin of the trachea immediately beneath the vocal folds. As the vocal folds vibrate, their area of opening will vary and so will the amount of light passing through the glottis. It yields a good approximation of glottal area. Measurements obtained Speed quotient - the speed of the opening phase of the vocal folds divided by the speed of their closing phase Open quotient. These reflect efficient vocalization.