The minimum level of lung pressure needed to sustain vocal fold oscillation at a specific pitch is referred to as the phonation threshold pressure. If the lung pressure supplied to the folds is less than this level, air will flow through the glottis (if it is open) but the airflow is steady and does not cause the vocal folds to begin self-sustaining oscillation. At the key pressure, a bifurcation, or sudden qualitative change in the behavior of the system, occurs. In the case of the vocal system, the change is from a condition of no oscillation - or oscillation which quickly damps itself out - to a new condition in which oscillation is occurring and is self-sustaining. This key pressure is called phonation threshold pressure or PTP.

Conditions of PTP Variability
This key threshold pressure is not static, but can vary widely from person to person, and can also vary within the same person based on several factors:

• Hydration. Proper hydration of the vocal fold mucosa (the outer layers of the folds) is vital for proper functioning of the voice in all areas, but is especially relevant to the phonation threshold pressure. According to research by Dr. Katherine Verdolini and others, fully hydrated mucosa are both more mobile and more easily deformed. In other words, they will move more easily with less force being applied, and they can change shape and then spring back to their original shape much better than dried-out mucosa. These properties mean that the well-hydrated mucosa will begin to vibrate with less air pressure than would be required for drier tissue.

• Vocal skill. As explained in other tutorials, proper oscillation often depends on the shaping of the vocal tract as well as the larynx and other factors, especially in extreme pitch ranges. The vocalist's skill in manipulating the shape of the vocal tract to adapt to such situations will affect the amount of breath pressure needed to start oscillation; a less skilled vocalist may require more pressure than one who is highly trained.

• Pitch. In general, the phonation threshold pressure required will be increased for higher pitches than for the middle of a vocalist's pitch range. There can be an exception for extremely low pitches as well; these can require a slightly higher pressure than the middle of the range, but still not nearly as much as the highest pitches.

• Fatigue. It has been hypothesized that a voice that is tired is associated with a higher phonation threshold pressure. Current research seeks to quantify relationships between PTP, vocal fatigue and perceived phonatory effort. Early results indicate that, indeed, PTP will be increased when a speaker's voice has become fatigued as compared to the less fatigued condition.