Wave propagation is actually the transport of energy in a medium that also acts as a sink for the energy. In order to describe this process more accurately than is done in the classical approximation, it is necessary to modify the lossless equations of motion to account for losses in an inhomogeneous, arbitrary medium. This can be accomplished by the introduction of a generalized loss term (loss function) in the wave equation of motion. The inclusion of this term introduces into the force balance equation an effective dissipative force field that is the equivalent of the losses. The development of a general series solution in powers of the parameter (kx) for the complex wave vectorkand the displacementxyields a description of the wave field that incorporates the nonlinear effects of losses in its form. This approach gives insight into the higher order processes occurring in an arbitrary, inhomogeneous, lossy medium and shows how a frequency shift occurs in the wave as a result of travel through the medium.