Sound is a mechanical wave resulting from the back and forth vibration of the particles of the medium through which the wave is moving. The compressions and rarefactions of sound waves move parallel to the direction of wave propagation, making sound a longitudinal wave.
Sound can also be defined subjectively as the human interpretation of these mechanical waves by the auditory sense. This definition is somewhat restrictive, as it excludes subsonic and supersonic frequencies that the human ear cannot perceive. Unlike electromagnetic waves, which can travel in a vacuum completely devoid of material, a compressible medium must be present for sound to propagate through it. Sound waves can propagate through practically all forms of matter, provided that this matter is packed densely enough.
The particles of the medium do not travel with the sound wave. Instead, they vibrate around their fixed positions, transferring energy to their neighboring particles. The behavior of a sound wave is affected by the density of the medium, the temperature of the medium and the pressure of the medium if it is a gas. Motion of the medium independent of the sound wave may also affect the way sound propagates. For example, wind flowing in the same direction as a sound wave enables the wave to carry farther.