Reflected sound may encounter the original sound wave and, depending on the relative timing of the two, they may either reinforce or cancel one another. Sound waves may also be diffracted, which means that, depending on the frequency of the sound, they are able to wrap around small or medium-size objects. Any motion that is repeated is called periodic motion. Examples of periodic motion are the moon's orbit of the earth, a beating heart, the whirring tail of a frightened rattlesnake, the wings of a hummingbird, and the movement of the valve on a revolving bicycle tire. A vibrating body in contact with the atmosphere will produce sound waves.
Most sound generators produce recurrent waves which are generally similar to each other.
These waves are propagated at a definite velocity. This velocity depends on the medium of propagation. One cycle of a sound wave in air consists of one compression of the air together with the subsequent rarefaction that occurs. The equilibrium state of the air molecules is the state in which they were before the compression under observation occurred. Sound originates when a vibrating source disturbs the air in a quasi-periodic fashion and sends a sound wave traveling through it.