You may imagine the desert as quiet rows of drifting dunes. But under the right conditions, some dunes can emit a thunderous boom, and smaller volumes of sand from these noisy dunes can squeak. Now scientists have found an artificial version of sand that reproduces the sound. The finding, reported in today's issue of Nature, may help explain how this odd phenomenon works.
Sand sings at fewer than 100 dunes worldwide. Individual grains from these dunes are uniform in size, polished smooth by the wind, and dried at a certain humidity. Intrigued by these properties, chemist Douglas Goldsback and physicist Marcel Leach of Laurentian University in Sudbury, Ontario, used an infrared spectroscope to probe the surface of sand grains taken from singing dunes in California, Hawaii, and Nevada. The duo noticed that each grain was coated with what appeared to be a thin layer of silica, a mixture of silicon and water. They hypothesized that the silica was somehow responsible for the noise.
Goldsback and Leach bought off-the-shelf silica gel spheres, a kind of artificial sand, with each sphere about 200 to 500 micrometers in diameter. They put the sand in a jar and shook. The sand indeed briefly rumbled and vibrated. "It's as though the jar were farting in your hand," says Leach, who for his real job studies ultrasonic fragmentation of materials. An infrared spectroscopy reading later showed that the surface layer looked just like a grain of real singing sand.
The sand may sing because the silica layer makes grains sticky. Leach says the silica probably works like the resin applied to a violin bow. "When you put resin on the bow, it increases the friction between the bow and the strings and makes a nice sound," he says.
Other experts in the field--a relatively small one, mind you--are anxious to get their hands on the artificial singing sand. "It's no trivial task to obtain samples of booming sand," says University of Michigan physicist Franco Nori. Leach has grand ideas for the artificial sand: He says that someday scientists might be able to use it to make an acoustic laser that would simultaneously vibrate many small particles at the same frequency, creating powerful bursts of sound.