Liu and Nagel have reported recently [Phys. Rev. Lett. 68 (1992) 2301
] a set of measurements involving sound propagation in an unconsolidat
ed granular glass beads system (an artificial ''sand''). In order to r
ationalize qualitatively and quantitatively the reported results, we p
ropose the physical picture that sound waves are multiply scattered at
grain-grain contacts and are thus in the diffusive regime, close to A
nderson localization. This suggests a deep link between the quest for
experimental observation of strong localization for classical waves an
d the physics of unconsolidated granular media.