These are the waves in which the individual particles of the medium oscillate along the direction of wave propagation.
We consider a long hollow cylinder AB closed at one end and having a movable piston at the other end. If we suddenly move the piston rapidly towards right, a small layer of air just near the piston-head is compressed and after being compressed, this layer moves toward right and compress the next layer and soon the compression reach is the other end. Now, if the piston is suddenly moves toward left, the layer adjacent to it is rarefied resulting in the fall of pressure.
The air from the next layer moves in to restore pressure. Consequently the next layer is rarefied. In this way a pulse of rarefaction moves toward right.
If we continuously push and pull the piston in a simple harmonic minor, a sinusoidal sound wave travels along the cylinder in the form of alternate compressions and rarefactions, marked C, R, C, R,………etc. As the oscillations of the element of air are parallel to direction of wave propagation, the wave is a longitudinal wave. Hence sound wave produced in air are longitudinal waves.
A Compression is a region of the medium in which the distance between any two consecutive particles of the medium is less than the normal distance. Hence temporarily there will be decrease in volume, increase in density.
A Rarefaction is a region of the medium in which the distance between any two consecutive particle of the medium is more than the normal distance. Consequently, there will be increase in volume and decrease intensity of the medium temporarily.