A booming sand dune manifests itself by initiating an avalanche from the leeward face of a large dune. The resulting low-frequency booming noise or music is loud and resembles a low-flying propeller airplane. The sound is surprisingly monotone with one dominant audible frequency. The sound is sustained and may continue for up to a minute after initiation, even after all visible motion has ceased. Moving a hand through the dry sand of a booming dune shears the upper layer and generates another acoustic phenomenon, the burping emission - pulse-like, short bursts of sound.
Booming dunes are silent in the wintertime when moisture from precipitation is retained in the dune. The burping property depends on sand grain characteristics and can be generated all year around. In the summer time when the larger dunes produce their music, the smaller dunes in the dune field remain silent. This indicates that structural properties of the dune are critical for the generation of the singing sand. Also, booming can only be generated at slopes at the angle of repose (30 degrees) on the leeward face of dune, the same sand on the shallower windward side cannot generate the music.
Many qualitative accounts of this booming phenomenon have been made over the years. Marquess the Curzon of Kedleston (1923) listed in his Tales of Travels 33 observations of singing dunes from desert explorers including Marco Polo in China, the Emporer Baber in Afghanistan and Charles Darwin in Chile. These travelers all pondered about the question why the dunes they visited boomed while others with similar characteristics remain silent. Bagnold (1966) related the booming frequency to the sand grain diameter based on shearing and dilatation in a slip plane. Haff (1979) performed extensive experiments on the burping phenomenon in the laboratory. Patitsas (2003) proposed the sand avalanche as a fluidized layer and explained the sound emission due to slip channels. Recently, Andreotti (2004) and Bonneau et al. (2007) stated that the collision of grains excite waves outside the shearing layer. These surface waves synchronize the collisions between grains to produce a wave-particle mode locking phenomenon. Douady et al. (2006) presented a related analysis with a resonance within the shearing layer only synchronizing the grains during a sand avalanche.
These theories are difficult to validate as the documented quantitative data on booming dunes compromises only a few booming locations. Slight seasonal variations in frequency observed for the same location indicate that seasonal dependence plays a role and that solely grain characteristics are not to explain these variations. None of the existing explanations are able to explain why certain dunes boom while many others in the world remain silent.
Recent field research (Vriend et al., 2007) done at Caltech indicates that the dominant booming frequency ranges from 70 - 105 Hz, depending on the desert location and the time of year. Quantitative field research at four additional locations in different seasons invalidate the dependance of the booming frequency on grain diameter. An internal natural waveguide within the subsurface structure of the dune explains the geographical and seasonal variations in booming frequency. The source of the acoustic emission is the shearing of burping sand, but the waveguide in the subsurface structure sets the booming frequency and amplifies the sound. Field surveys with ground penetrating radar and seismic refraction confirm the existence of the waveguide for booming dunes. Subsurface soil sampling shows that a firm layer exists at a depth of approximately 2 meters with a higher water strata and chemically altered sand. The large dunes in the wintertime are saturated with moisture, loose the velocity contrast across the interface and hence eliminate their natural waveguide. Smaller dune lack the correct subsurface structure even in the summer and are not able to amplify the burping sound. Higher harmonics in the frequency signal are explained by higher mode excitation of resonance in the waveguide. The acoustic characteristics of the waveguide model are consistent with all field measurements.