Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. The sound may be produced in a variety of ways, normally as a result of some mechanical disturbance on an object, causing it to vibrate - the propagation of sound. Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). During their propagation, waves can be reflected, refracted, or attenuated by the medium. The travelling of sound is called propagation of sound. Sound is propagated by the to and fro motion of particles of the medium. When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its equilibrium position.