Propagation of sound waves in air from a tuning fork
A wave motion, in which the particles of the medium oscillate about their mean positions in the direction of propagation of the wave, is called longitudinal wave.
Sound waves are classified as longitudinal waves. Let us now see how sound waves propagate. Take a tuning fork, vibrate it and concentrate on the motion of one of its prongs, say prong A. The normal position of the tuning fork and the initial condition of air particles is shown in the fig (a). As the prong A moves towards right, it compresses air particles near it, forming a compression as shown in fig (b). Due to vibrating air layers, this compression moves forward as a disturbance. As the prong A moves back to its original position, the pressure on its right decreases, thereby forming a rarefaction. This rarefaction moves forward like compression as a disturbance. As the tuning fork goes on vibrating, waves consisting of alternated compressions and rarefactions spread in air as shown in fig (d). The direction of motion of the sound waves is same as that of air particles, hence they are classified as longitudinal waves. The longitudinal waves travel in the form of compressions and rarefactions.