Articulatory Phonetics - Airflow

Airflow

For all practical purposes, temperature can be treated as constant in the articulatory system. Thus, Boyle's Law can usefully be written as the following two equations.

What the above equations express is that given an initial pressure and volume at time 1 the product of these two values will be equal to the product of the pressure and volume at a later time 2. This means that if there is an increase in the volume of cavity, there will be a corresponding decrease in pressure of that same cavity, and vice versa. In other words, volume and pressure are inversely proportional (or negatively correlated) to each other. As applied to a description of the subglottal cavity, when the lung pistons contract the lungs, the volume of the subglottal cavity decreases while the subglottal air pressure increases. Conversely, if the lungs are expanded, the pressure decreases.

A situation can be considered where (1) the vocal fold valve is closed separating the supraglottal cavity from the subglottal cavity, (2) the mouth is open and, therefore, supraglottal air pressure is equal to atmospheric pressure, and (3) the lungs are contracted resulting in a subglottal pressure that has increased to a pressure that is greater than atmospheric pressure. If the vocal fold valve is subsequently opened, the previously two separate cavities become one unified cavity although the cavities will still be aerodynamically isolated because the glottic valve between them is relatively small and constrictive. Pascal's Law states that the pressure within a system must be equal throughout the system. When the subglottal pressure is greater than supraglottal pressure, there is a pressure inequality in the unified cavity. Since pressure is a force applied to a surface area by definition and a force is the product of mass and acceleration according to Newton's Second Law of Motion, the pressure inequality will be resolved by having part of the mass in air molecules found in the subglottal cavity move to the supraglottal cavity. This movement of mass is airflow. The airflow will continue until a pressure equilibrium is reached. Similarly, in an ejective consonant with a glottalic airstream mechanism, the lips or the tongue (i.e., the buccal or lingual valve) are initially closed and the closed glottis (the laryngeal piston) is raised decreasing the oral cavity volume behind the valve closure and increasing the pressure compared to the volume and pressure at a resting state. When the closed valve is opened, airflow will result from the cavity behind the initial closure outward until intraoral pressure is equal to atmospheric pressure. That is, air will flow from a cavity of higher pressure to a cavity of lower pressure until the equilibrium point; the pressure as potential energy is, thus, converted into airflow as kinetic energy.