Explain propagation of sound waves in air.

Explanation

Because sound waves are longitudinal waves which move ahead through condensations and rarefactions. Where condensation is formed, there particles of air come very closer. So number of particles per unit volume increases there, le.. the density (number density) increases. Pressure also depends on number of particles per unit volume. Hence, pressure also increases there.
Where rarefaction is formed, situation reverse to that discribed above is formed.

Thus, change in density and pressure in medium (in air) is seen during propagation of sound in medium in air).