When a source of sound vibrates, it creates a periodic disturbance in the medium near it. The disturbance then travels in the medium in the form of waves. This can be understood by the following example —
Take a thin metal strip. Keeping it vertical, fix it’s lower end. Push it’s upper end to one side and then release it. As it vibrates (i.e., moves alternatively to the right and left) sound is heard.
When the strip advances to the right from a to b, it pushes the particles of air in layers in front of it. So the particles of air in these layers gets closer to each other i.e., air of these layers gets compressed.
The particles of these layers while moving forward, push and compress the layers next to them, which then compress the next layers and so on. Thus, the disturbance moves forward in form of compression. The particles of the medium get displaced, but they do not move along with the compression.
As the metal strip starts returning from b to a as shown in figure, after pushing the particles in front, the particles of air near the strip starts returning back to their mean positions due to the elasticity of the medium.
When the strip moves to the left from a to c, it pushes back the layers of air near it towards it’s left and thus produces a space of very low pressure on it’s right side. The air layers on the right side of the strip expand in this region thus forming the rarefied air layers. This region of low pressure is called the rarefaction R.
When the strip returns from c to it’s normal position a, it pushes the rarefaction R forward and the air layers near the strip again pass through their mean positions due to the elasticity of the medium.
In this manner, as the strip moves to the right and left repeatedly, the compressions and rarefactions regions are produced one after the other which carry the disturbance with it with a definite speed depending on the nature of the medium. Gradually due to friction, the strip losses its energy to the medium and the disturbance dies out.
One complete to and fro motion of the strip forms one compression and one rarefaction which together constitute one wave. This is how sound waves propagate through a medium such as air.
