In the last lecture we concluded our discussion of the heat equation. We move on to the wave equation We will use sound propagation in a gas such as air as an example.
Sound waves are elastic waves that propagate in a fluid or solid.
In fluids long wavelength sound waves consist of an alternating pattern of rarefaction and compression. In a solid transverse waves can also propagate. In ordinary sound the changes in pressure tend to be very small. The mechanism for a sound wave is that gas motion generates a change in the density, which causes a change in pressure. There will then be an unbalanced force which accelerates the gas and causes the cycle to repeat.