Sound is a mechanical wave that propagates through a medium such as air, water or other solid or liquid matter. Sound waves are longitudinal waves where the vibration of particles is parallel to the direction of wave propagation. When atoms in the medium are set in vibration through an oscillation, it creates alternating high and low pressure regions called compressions and rarefactions that propagate as a sound wave. The characteristics of a sound wave include wavelength, amplitude, velocity, frequency, timbre and time period. Sound waves travel fastest in solids, slower in liquids, and slowest in gases.