Sonic sands

Many desert sand dunes emit a loud sound with a characteristic tremolo around a well-defined frequency whenever sand is avalanching on their slip face. This phenomenon, called the 'song of dunes', has been successfully reproduced in the lab, on a smaller scale. In all cases, the spontaneous acoustic emission in air is due to a vibration of the sand, itself excited by a granular shear flow. This review presents a complete characterization of the phenomenon—frequency, amplitude, source shape, vibration modes, instability threshold—based on recent studies. The most prominent characteristics of acoustic propagation in weakly compressed granular media are then presented. Finally, this review describes the different mechanisms proposed to explain booming avalanches. Measurements performed to test these theories against data allow one to contrast explanations that must be rejected—sound resonating in a surface layer of the dune, for instance—with those that still need to be confirmed to reach a scientific consensus—amplification of guided elastic waves by friction, in particular.