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.
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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.
