  A unified treatment for the propagation of classical waves in inhhomogeneous
media is proposed. We deal with four kinds of waves, they are the acoustic wave
in fluid, the elastic shear wave in two diemnsional solid, and the E- and
H-polarized electromagnetic waves in two diemnsional lossless medium. We first
show that a universal wave equation governing the wave motion of all these four
kinds of waves can be derived. We then introduce an auxiliary field, and give
the universal expressions for the energy densities and energy flows of these
waves. This unified treatment provides intuitive insights, which may be helpful
in understanding the essential physics of various wave phenomena, or useful for
designing new photonic and sonic devices.
