Sound waves are created by some type of mechanical vibration or oscillation that forces the surrounding medium to vibrate. A tuning fork is an excellent example of a vibrating sound source. When struck by a rubber mallet, the tines of the tuning fork vibrate, causing the air molecules around them to move back and forth at the same frequency, creating areas of compressions (where the molecules are close together and air pressure is slightly increased) and rarefactions (where the molecules are spread apart and thus the air pressure is reduced). These pressure changes then move away from the tines creating the longitudinal sound wave.