  In this paper the validity of the diffusion approximation for multiple
scattering of classical waves in random medium in different regimes is
investigated, with emphasize to weak localization effects. Many principle
topics are discussed once again. We obtain new expressions for the incoherent
and coherent multiple scattering intensities for the more general case of large
scattering angles and various observation points inside the medium with moving
scatterers and absorption. (The results, we reached, assuming white-noise
disorder for the impurities correlator, contain one-dimensional integration in
its final analytical form). Also new self-consistent formulas for incoherent
and coherent intensities for fixed multiplicity orders of scattering are given
and their lineshapes are graphically represented. Through these formulas we
clearly demonstrate that the impurities motion reduces the contribution to the
intensity of the scatterings with large multiplicity orders. The total
scattering intensities in $4\pi$ solid angle are calculated for the general
case and compared with their diffusion approximation. For coherent intensities
it is shown that a drastic deviation from the exact results occurs. It turns
out that the region of angles, for which the observed lineshape of the weak
localization effect tends to the incoherent background, give the main
contribution to the total coherent scattered intensity. The last quantity
happens to depend linearly instead of predicted quadratic dependence on the
perturbation parameter, 1/kl.
