Sound energy is transmitted through air as a disturbance of air molecules. When there are no air molecules ,as in a vacuum , there can be no sound.The disturbance of air molecules that makes up a sound wave consists of regions of compression ,in which the air molecules are close together and the pressure is high , alternating with areas of rarefaction , where the molecules are farther apart and the pressure is lower.Anything capable of creating such disturbances can serve as a sound source. A tuning fork at rest emits no sound , but if it is struck sharply , it give rise to a pure tone. As the arms of the tuning fork move, they push air molecules ahead of them, creating a zone of compression , and pull apart the molecules behind them, leaving a zone of rarefaction.AS they move in the opposite direction, they again create pressure waves of compression and rarefaction.The molecules in an area of compression, pushed together by the vibrating prong of the tuning fork, bump into the molecules ahead of them,push them together , and create a new region of compression. Individual molecules traval only short distances, but the disturbance passed from one molecules to another can traval many miles; it is in these disturbance (sound waves) that sound energy is transmitted. The sound dies out only when so much of the original sound energy has been dissipated that one sound wave can no longer disturb the air molecules around it. The tone emitted by the tuning fork is said to be pure because the waves of rarefaction and compression are regularly spaced.The waves of speech and many other common sounds are not regularly spaced but are complex waves made up of many frequencies of vibration.