Propagation Of Sound Wave Through Air
A small region of air can be considered as a spring. It is connected to the neighbouring regions or springs. As sound wave travels through air, it compresses or expands a small region of air. This changes the density and pressure of the region.
According to Boyle’s law
Change in Pressure (∆P) ∝ Change in Density (∆ρ)
As the pressure is force per unit area. So a restoring force proposal to the change in density is developed, just like in an extended or compressed spring. If a region is compressed, its molecules tend to move out to the adjacent region, thereby increasing the density or creating compression in that region.
The density of air in the first region decreases and is called rarefaction. But when a region is rare field, the air of the surroundings air rushes in. This shifts the rarefaction to the adjacent region. Thus compressions and rarefactions move from one region to another. This makes possible the propagation of disturbance in air.
Propagation Of Sound Wave In A Solid
In a crystalline solid, various atoms can be considered as end points, with springs connected between pairs of them. In this, each atom or group of atoms is in equilibrium due to forces from the surroundings atoms because the forces exerted by the other atoms are cancel out.
When the sound wave propagates, the atom is displaced from its equilibrium position and a restoring force is developed. This disturbance produced by the force travels to the next atom and so on. Thus the wave propagates through the solid.