As singers and speakers, we care about efficient ways to use our voices. Questions regarding the effective use of air pressure and airflow arise. For example: To get louder, is it just about pushing harder with more lung pressure? Or, are there other ways to tune up the system to get more bang for the buck? Let’s get some insights into various pressures in our airways.

In vocalization, we talk about lung pressure, subglottal pressure, intraglottal pressure, transglottal pressure, phonation threshold pressure, and oral pressure. These pressures determine the movement of air or tissue along the vocal tract. The pressures are positive in the sense that they produce forces against airway walls and the air itself. Because they all fluctuate around the atmospheric pressure, they can be expressed as positive or negative relative to atmospheric pressure. During inspiration, relative pressures are mostly negative, while during expiration, they are mostly positive.

In phonation, all airway pressures have two components, a somewhat steady component due to gradual air transport from the lungs to the lips and an oscillating (acoustic) component due to forward and backward wave propagation. The airway pressures are regulated with lung pressure and with impedances along the airway, like semi-occlusions or narrow portions that restrict airflow and thereby create a pressure difference across the constriction. 

Consider the glottis as an airway impedance (Fig. 1). Two pressures are critical, the transglottal pressure and the mean intraglottal pressure. Transglottal pressure (PSG-PIN), is short for transglottal pressure difference. This difference drives the airflow through the glottis (vertically in the figure). Mean intraglottal pressure P drives the vocal fold tissue in a perpendicular direction (see net velocity arrow). It is roughly calculated as P = (PSG+PIN)/2.

Transglottal and mean intraglottal pressures can be very different. Any upper airway constriction can lower the transglottal pressure, while at the same time raising the mean intraglottal pressure. The key is the supraglottal pressure PIN, which can be thought of as a back pressure on the vocal folds. PIN is highly dependent on vocal tract shape and varies with fundamental frequency. It can be positive or negative relative to lung pressure in the vibration cycle.

The influence of PIN on the transglottal and mean intraglottal pressures is critical. For example, if PIN is negative at some moment in time due to an acoustic wave rarefaction above the glottis), the transglottal pressure (PSG-PIN) is larger than the subglottal pressure PSG. However, the mean intraglottal pressure (PSG+PIN)/2 is reduced because the two pressures tend to cancel each other. This condition will drive a lot of glottal airflow, but little pressure will be applied to the vocal fold surfaces to drive the tissue.