Engineers at Caltech discovered that for sand dunes to produce sound they need a dry layer on top that amplifies internal frequencies during sand movement. Christopher Intagliata reports
<<Dune sounds>> The sound of supernatural spirits talking? Well, no. But that’s what it sounded like to Marco Polo as he travelled through China's Lop Desert in the 13th century. He also described the sound as “a variety of musical instruments.” And in reality, the mysterious noises are the product of what could be considered to be an unusual sort of instrument—the desert’s sand dunes. <<dune sound>>
"And it's a sound that's very similar to kind of the tones you'd get on a cello." Melany Hunt, a mechanical engineer at Caltech. The dunes sing when sand avalanches down the side, "which would then be somewhat equivalent to using the bow on the strings of the cello."
Hunt and her colleagues recorded the sounds produced closer to home, by California's Kelso and Eureka Dunes. They used four dozen geophones—microphones you stick in sand. And they mapped the dunes' structure with ground penetrating radar. Turns out, dunes that sing are built differently from silent dunes—they’re topped with an even layer of dry sand—about five feet thick—on top of all the damp sand below. And that layer of dry sand is like the body of an instrument—it traps and amplifies certain frequencies more than others. "And so really the size of the instrument is really going to set the range of frequencies that you get. And that's the same thing with this dune."
When sand avalanches, it shoots off sound waves of various frequencies. Those waves travel through the dry sand, hit the wet layer, bounce back, hit the air on top of the dune, bounce back, and so on. As that happens, a lot of the frequencies cancel out and dissipate. But certain waves have the goldilocks frequency, around 80 Hertz. As they travel back and forth, they amplify, creating that booming sound. The study is in the journal Physics of Fluids. [N. M. Vriend et al, Linear and nonlinear wave propagation in booming sand dunes]
The bad news? Since the 'instrument' requires a uniform, thick layer of dry sand, Hunt says the dunes tend to fall quiet in the cooler, wetter months. Meaning you'll have to brave summer heat if you want to hear the dunes when they're really in tune.
[The above text is a transcript of this podcast.]