Sonic sands: uncovering the secret of the booming dunes.

Marco Polo, Charles Darwin, and other adventurers marveled at the loud, thrumming sounds that emanate from sand dunes in certain desert locales around the world. Now, researchers say that they've solved the mystery of how the dunes produce their mysterious tones.

So-called booming dunes, which generate droning sounds that can last a minute or longer, have been found at around 30 places worldwide. Their often deafening tones are typically loudest at a single frequency between 70 and 105 cycles per second, says Melany L. Hunt, a mechanical engineer at the California Institute of Technology in Pasadena. (Listen to a booming dune at www.sciencenews.org/ articles/20070908/boom.wav.)

The sound originates when sand avalanches down the lee face of a dune, explains Hunt. Past studies suggested that the sound's frequency is related to the average size of the sliding sand grains, but new data gathered at several booming dunes in California and Nevada don't support that theory, say Hunt and her colleagues in the Aug. 28 Geophysical Research Letters.

Instead, the team's field studies suggest that a heretofore unrecognized property of such dunes--the thickness of a surface layer of dry sand--dictates the frequency of their tones.

In 2006, Hunt and her colleagues laid a string of sensitive microphones along the face of a 45-meter-tall dune at Dumont Dunes, just south of California's Death Valley. The researchers mapped the dune's internal structure by analyzing vibrations produced when they placed a metal plate on the dune and struck it with a hammer. The scientists also triggered avalanches.

The dune repeatedly boomed during tests in September 2006, when a 1.5-m-thick layer of dry sand covered the dune's face. Data indicated that sound traveled through this surface layer at about 200 meters per second (m/s). At depths exceeding 1.5 m, where the sand was moist, vibrations traveled at speeds of 300 to 350 m/s, Hunt notes.

This abrupt difference causes the boundary between moist and dry sand to reflect sound waves, the team explains. The sound of avalanching sand bounces back and forth within the dry layer, creating a resonance that boosts the sound's intensity. Hunt says that the frequency of the sound that eventually escapes the dunes should be related to the layer's thickness.

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In December 2006, when the dune had no dry layer, the sands remained silent, says Hunt.

Michael Bretz, a physicist at the University of Michigan in Ann Arbor, says that Hunt and her colleagues "did a pretty nice job of collecting data." The booming dunes act somewhat like a stringed musical instrument, he notes. An avalanche, like a string of a violin, generates only a small sound. The dry layer of sand on the surface of the dune, like the body of the violin, acts as a chamber that magnifies the sound.