Sound Wave

Because good sound quality is the goal of any sound system, it is helpful to be familiar with some general aspects of sound: how it is produced, transmitted, and received. In addition, it is also useful to describe or classify sound according to its acoustic behavior. Finally, the characteristics of “good” sound should be understood.
Sound is produced by vibrating objects. These include musical instruments, loudspeakers, and, of course, human vocal cords. The mechanical vibrations of these objects move the air which is immediately adjacent to them, alternately “pushing” and “pulling” the air from its resting state. Each back-and-forth vibration produces a corresponding pressure increase (compression) and pressure decrease (rarefaction) in the air. A complete pressure change, or cycle, occurs when the air pressure goes from rest, to maximum, to minimum, and back to rest again. These cyclic pressure changes travel outward from the vibrating object, forming a pattern called a sound wave. A sound wave is a series of pressure changes (cycles) moving through the air.

(from Audio System Guide for House of Worship, Shure Educational Publication)