where q is the mass density in kg
m3 ; u is the displacement in m, t is
time in sec, l is the component of the second complex LAME´
parameter representing stiffness, and r is LAPLACE operator.
For the sake of clarity, only a plane distortional wave propagating
in a viscoelastic medium in the x-direction with its particle
motion in the z-direction will be considered [28]: