Sound Waves   

Sound waves are succesive or fluctuating levels of pressure, propagated in some elastic medium, usually air. These fluctuations consist of a series of compressions and rarefactions of the molecules of the medium or in other terms a rapid succession of raisings and lowerings of air pressure from its static norm. Sound waves which are the result of greater swings of pressure or compressions/rarefactions are said to exhibit greater amplitude than those with smaller changes in pressure. There is a corresponding subjective perception of loudness with amplitude, where sounds of larger amplitude are generally perceived as being louder than those softer sounds of smaller amplitude. Sound waves fluctuate at some rate of frequency between 20 and 20,000 times each second. This is the approximate range of human hearing with the lower frequencies corresponding roughly to lower perceived pitch and the higher frequencies to higher perceived pitch.
The amount of time it takes for a sound of some frequency to complete one cycle is called the period of the wave and the amount of space one cycle takes up in the medium (usually air) is called the wavelength.