This chapter delves a little more deeply into a particular experimental investigation from the seventeenth century. Robert Boyle’s air-pump allowed him to evacuate (nearly) all of the air from an enclosed chamber. He sought to investigate various phenomena, including the recent discovery that, in a tube filled with mercury and open at one end and then inverted into an open dish filled with mercury, an apparently empty space will appear at the top (closed) end of the tube. Boyle’s experiments are credited with having led to the modern conception of air pressure, but his conclusions were met with controversy.

In the production of speech, airflow or heightened air pressure is required in order to produce the individual sounds of speech. Most usually this is achieved through the use of outgoing (egressive) air flowing from the lungs. Air pressures used in speech are generally not great, but involve a pressure differential from ambient air pressure. For the production of a small number of speech sounds, a negative air pressure is required, and for some speech sounds airflow is ingressive. Normal ambient air pressure at sea level is 101.325 kilopascals (kPa) or 14.7 pounds per square inch (psi). When air is permitted to flow unimpeded, air will flow from an area of higher pressure to an area of lesser pressure in order to equalize pressures. In speech, air may flow, causing the vocal folds to vibrate. Air may be forced to move through a constricted space such that it generates noise. Airflow may be blocked and a heightened pressure built up behind the blockage, which is then released with an explosive effect.