Sound is a form of mechanical energy which produces sensation of hearing. a turning fork is an acoustic resonator at constant pitch when set in to vibration. if the particles of the medium move to and fro along the direction of propagation of the wave, the waves are called longitudinal waves. sound waves are longitudinal. the region of high density of particles in the medium during propagation of sound is called compression and low density region is called rarefaction. the distance between two consecutive compressions or rarefactions is called wave lengths. loudness of sound is defined a the degree of sensation produced in the ear. sound of frequency between 20HZ - 20KHZ  is called as sonic or audible limit.