Many sand dunes – at least seven in the United States – make loud booming noises when they avalanche. Records of the sound are centuries old, but the cause remains a mystery. This study examines properties of both the sand and the sound.
Properties of the sand reveal clues about the source of the booming. Sand must be extremely dry to boom, but low moisture content alone is not sufficient to facilitate booming. Although the mean grain diameters of both booming and silent dune sands range from 0.20 – 0.40mm, the booming samples have smaller standard deviations. However, synthetic sands with similar size distributions do not boom, so a narrow size distribution cannot be solely responsible for the booming. Studies of the roundness and sphericity of the grains are currently underway.
Air microphone and geophone recordings of the booming indicate that the fundamental frequency varies between 80–105 Hz depending on the dunes. This is consistent with previous measurements. Laboratory recordings of the “burping” sound that booming sand makes when shaken in a jar reveal a broad peak between 150–300 Hz.