Sound is a sequence of oscillating waves of pressure. They can propagate through compressible media such as water or air. Sound also has the ability to propagate through solids. During the propagation process, waves can be reflected, refracted or attenuated by the medium. Sound is perceptible by the human ear at frequencies about 20 Hz to 20, 000 Hz.
Sound waves can travel through all forms of matter, solids, liquids, gases or plasmas. The matter which supports sound wave travel and propagation is called a medium. Sound cannot travel through a vacuum due to the non presents of matter. In air at standard temperature and pressure the wavelengths of sound can range from 17m to 17mm.
The behavior of sound propagation is affected by three things. The first is the relationship between density and pressure. This relationship affects the speed of sound within a medium. The second is the motion of the medium itself. If the medium is moving, the sound is transported further. Lastly, the viscosity of the medium affects the motion of the sound waves. The viscosity affects the rate at which sound is attenuated. If the sound waves are not moving through a medium with constant physical properties then the sound waves may be refracted.