Since the main focus in the current study is on the inhomogeneity
of material damping properties, a description of wave propagating
through a heterogeneous medium have to be provided. In order
to assume heterogeneous damping distribution, the term l0 in
Eq. (3) should be written explicitly l0ðxÞ. The wavenumber k and
the spatial attenuation a in Eqs. (9) and (10) respectively have to
have following forms