When Sand Dunes Go Boom

Sept. 19, 2007 — The age-old mystery of why the dunes of 30 or so sand fields worldwide make eerie booming and singing sounds may be solved.

Researchers at the California Institute of Technology have recorded the audio signatures of booming dunes, dissected the dunes with ground-penetrating radar and studied the mechanics of sand avalanches. What they discovered is that the deep tones made by the 30 or so singing dunes worldwide aren't very different from those made by a stringed instrument.

"I compare the tone of a dune to the tone of a cello," said Caltech's Melany Hunt, who has been studying the problem for several years with help from graduate and undergraduate engineering students. Among the students' tasks is to line up along the ridges of a dune near Death Valley in California and slide down on their bottoms to trigger a sand avalanche and the dune song.

"We've been sliding down dunes and the whole thing is vibrating," she said. Audio analyses of the dune noises show a surprisingly pure tone. "It looks right on — you even get the harmonics just like you do with a [cello] string."

It's as if there is some sort of "waveguide," like a giant string, in the dune that favors only certain frequencies of sound. That waveguide, it turns out, really exists. It's on the slip-face of the dunes — that's the steep side facing away from the prevailing wind.

Instead of a string, however, the waveguide is a layer of very dry, loose, avalanching sand sandwiched between the air above and the denser sand below. Inside that loose layer, sound moves much more slowly than in the air above or the packed sand below. And so it's inside of that bounded layer that a specific frequency is favored and the avalanching sand gets its pure tone.

"The sand moving down the face is the moving string, but it's really playing the whole body," Hunt told Discovery News.

Armed with their new model, Hunt's team has come up with a recipe for singing dunes: they must be big, dry and dense.

"Moisture is a big thing — they have to be really dry," said Hunt. And they have to be very big with the right internal structure, she said. The singing dunes her team studied with ground-penetrating radar had a very clear density difference a few feet under the surface.

The full report on the singing dunes appears in the latest issue of Geophysical Research Letters. The lead author is Nathalie Vriend, a Ph.D. candidate in mechanical engineering.

It remains to be seen how many other sand researchers accept the new explanation, however. Despite the lack of any critical application of the science, there has been quite a lot of disagreement about the mechanism behind the singing.

Over the last several years, for example, the prevailing theories were largely based on the idea that sand grain sizes set the tone — an approach Hunt believes is incorrect. But even among the advocates of the grain size approach, who are mostly French researchers working on Moroccan dune fields, there have recently been a lot of arguments and hard feelings recently over the specifics.

"This episode has destroyed several years of my life," French dune researcher Stephane Douady said in a recent Institute of Physics interview. Maybe it's time to just go out and slide down a dune for fun again.

Listen to the singing sand.

Wikipedia entry on singing sand dunes.

The Mystery of Desert Music

The Secret of the Booming Dunes