Compressions and rarefactions define sound as a longitudinal wave through a medium.

Evidence

The region where the golf balls are pushed together is known as a “compression,” whereas the region where they are pulled apart is known as a “rarefaction,” and the golf balls themselves are the propagating medium. In a real propagating medium, such as air, a disturbance would naturally consist of either a compression followed by a rarefaction or a rarefaction followed by a compression in order to allow the medium to return to its normal state. A picture of what happens is shown in Figure 1.2. Because of the way the disturbance moves—the golf balls are pushed and pulled in the direction of the disturbance’s travel—this type of propagation is known as a “longitudinal wave.” Sound waves are therefore longitudinal waves that propagate via a series of compressions and rarefactions in a medium, usually air. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 52

Categorization

• DEFINE: Defines compression, rarefaction, and longitudinal wave as the core vocabulary of sound propagation.
• XREF: The compression-rarefaction oscillation maps onto the figure-ground dynamic from The Soundscape; every sonic event is a paired push and pull that returns the medium to rest.