The standard approach to the wave propagation in an inhomogeneous elastic layer leads to the displacement in a form of a
product of a function of space and a harmonic function of time. This product represents the standing, and not the running
wave. The part
depending on the space variable is governed by the linear ordinary second order differential
equation. In order to calculate the propagation
speed in the present paper the inhomogeneous material is separated by a plane into two
parts. Between the two inhomogeneous parts the
virtual homogeneous elastic extra layer is added. The elasticity modulus and the mass density of the extra layer have the same values as
the inhomogeneous material on the separation plane. In further calculations the extra layer is assumed to be infinitesimally
thin. The virtual
layer allows to decompose the motion into two waves: a wave running to the right and a wave running to the
left. Energy conservation
equation is derived.
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