  Diffusing-wave spectroscopy is a powerful technique which consists in
measuring the temporal correlation function of the intensity of light multiply
scattered by a medium. In this paper, we apply this technique to cold atoms
under purely ballistic motion and we investigate the transition between the
single and the multiple scattering regime. The intensity correlation function
changes from a simple Gaussian function, whose width reveals the sample
temperature, to a more complex decay shortened by the frequency redistribution
in the thick cloud. These features are quantitatively compared to simulations
with a phase-coherent and an incoherent model. Both show a very good agreement
with the experiments.
