Sound is produced by pressure variations about a mean value within a medium. In air, sound propagates via very small rapid changes in the air pressure above and below atmospheric pressure. These pressure variations travel in the form of a pressure wave with an inherent amount of energy. When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e., at a frequency between 20 hertz and 20 kilohertz), sound is potentially audible by humans even though the pressure variation can be, and typically is, very small with respect to the mean pressure (e.g., atmospheric pressure). Louder sounds are caused by greater variation in pressure – 1 Pascal, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1 to 4 kilohertz) where the ear is most sensitive.

These pressure variations in air are typically induced by a vibrating object. A speaker cone serves as a good illustration. As the cone moves forward, the air immediately in front is compressed causing a slight increase in air pressure. As the cone moves back past its rest position, it causes a reduction in the air pressure known as rarefaction. The process continues so that a wave of alternating high and low pressure is radiated away from the speaker cone at the speed of sound.