Using the /i/ sound, I ask the patient to see how long she can say /i/ on one breath at the above determined comfortable speaking pitch and comfortable volume. I am not controlling pitch and volume as precisely as a researcher might with computerized testing equipment, but as with many things in life, we do not need to measure a 2x4 piece of wood to the nearest 1/64th of an inch in order to build a wall in a house that will function quite well for many years.
This test, the maximum phonation time (MPT) is a rough measure of how completely closed the vocal cords are. The more closed they are, the less air wasted and the longer the sound can be maintained. As a rough guide, with an MPT of less than 10 seconds duration most people will complain of being out of breath with talking. Vocally healthy people can typically go beyond 20 to 30 seconds. There are many variables which impact this test including lung capacity and vocal strategies used to produce the sound, but the more that the pitch and volume are kept constant, the more the test measures vocal cord approximation. This is especially true and helpful for one individual over time, since after some treatment for the voice, the change in MPT after the intervention is likely from the intervention.
Additional qualitative findings are typically most audible near the end of the maximum phonation time, when there is reduced breath support. Vocal vibratory impairments become more noticeable with less air flow or less air pressure to drive the vocal cord vibrations. Essentially, the compensation provided by high subglottic pressure diminishes and impairments such as stiffness and glottic gaps become more audible.