Sounds in air or gases are due to small changes in pressure in terms of time and space (pressure waves) caused by disruptions in pressure and are superimposed on the static pressure.
They are propagated at the speed of sound. If the source of the disruption is moving, the relationship between the velocity v of the source of the disruption and the speed of sound c is referred to as the Mach number Ma.
Important sources of sound in air are as follows:
- oscillating surfaces of (solid) bodies (e.g. mounting wall, pump housing),
- vortex formation when gases flow out of openings (e.g. discharge opening on pneumatic valves),
- vortex formation when things flow against solid bodies (e.g. fan blades, aircraft).