When a vibrating object moves forward, it pushes the molecules of the air in front of it and create region of high pressure and high density called Compression. As the compression produced in the air travels forward, the Vibrating body moves backward. They create a region of low pressure and low density in the Air commonly called rarefaction. The compression and rarefaction causes the particles in air to vibrate about their mean position. The Figure below illustrates the formation of compression and rarefaction by Tuning fork.