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Sound Clip: Booming Sands

Only around 30 “booming” dunes are known worldwide—in deserts and on beaches in Hawaii, North and South America, Africa, the Middle East, and Asia. The sounds of booming dunes have been compared to a variety of instruments, including violins, cellos, trumpets, bells, organs, and didgeridoos, which are Australian aboriginal wind instruments. Booming sand dunes sound like instruments because, as opposed to squeaking sands, the noise they produce lasts longer (up to 15 minutes in large dunes) and it emanates at a single, steady musical tone. How all the factors that produce booming sands mingle to affect the nature of their song is still a mystery, but here is what’s known:

Booming sand starts with a dune made of sand that has traveled long distances from its original source. The sand’s lengthy, windy journey means that grains deposited on the surface of the dune are extremely round, smooth, and uniform.

Next, a desert rainstorm must take place, washing dust and other foreign particles from among the surface grains. The topmost sand must then dry out over a period of weeks while the sand beneath remains relatively moist.

In squeaking sand, noise-producing displacement can occur merely by stepping on the sand with your foot. Booming sand requires the shearing action of a sand avalanche. Avalanches can begin only after a dry desert sand dune has built up to an angle of about 35 degrees and only when a sufficient amount of wind provides enough force to begin the avalanche.

During an avalanche, surface layers of dry sand slip over the lower layers of the dune. Individual grains of loose surface sand bounce up and down over the compacted lower layers of the dune, which have a higher moisture content. The interaction between the upper- and lower-layer sand grains produces vibrations that make sound, much as a violin string does as a bow passes over it.

Booming sand makes loud, low-frequency sounds of 50 to 300 hertz. During a large avalanche, the booming can be heard more than six miles away and standing near its locus can be deafening.—Lexi Krock

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