Explain propagation of sound waves in air.

Explanation

As the sound wave passes through air, compression and expansion of small regions of air medium take place periodically turn by turn, along the direction of propagation of wave. A small region of air medium in which air particles tend to move towards each other, causing increase in density `(DeltaP)` and hence increase in pressure `(DeltaP),` because according to ideal gas equation we have `PV = nRT implies P ((M )/(rho))= nRT (because rho = (M)/(V)) implies P = ((n RT)/(M)) rho implies`
At constant temperature `Delta P prop Delta P.`

In such a high prcssure region, where density is increased temporarily, "Condensation" or "Compression" is said to be formed. It is shown by symbol C in above figure.
Now, since pressure is force exerted normally (perpendicularly) on unit area, restoring force is developed in above region, which is directly proportional to displacement of a particle from its mean position. Consequently, air particles in above region tend to move outward (parallel to direction of propagation of wave) towards adjoining region on left & right, causing compressions in them. When this happens, density of air in the middle region (from where air particles have moved out) decreases temporarily.
In such a low pressure region, "rarefaction" or "expansion" is said to have be formed. It is shown by symbol R in above figure.
As explained above, each small region of air medium coming successively in a direction away from origin of sound, undergoes compression and rarefaction periodically. This is how disturbance moves away from source of sound, which indicates propagation of sound waves which are mechanical and longitudinal.