Secrets of the Singing Sand Dunes

July 25, 2006
Findings
Secrets of the Singing Sand Dunes
By KENNETH CHANG

The dunes at Sand Mountain in Nevada sing a note of low C, two octaves below middle C. In the desert of Mar de Dunas in Chile, the dunes sing slightly higher, an F, while the sands of Ghord Lahmar in Morocco are higher yet, a G sharp.

Since at least the time of Marco Polo, desert travelers have heard the songs of the dunes, a loud — up to 115 decibels — deep hum that can last several minutes. (You can listen to them here. **check out the video at the NYT page--good example) While the songs are steady in frequency, the dunes do not have perfect pitch. At Sand Mountain, for example, dunes can sing slightly different notes at different times, from B to C sharp.

Scientists already knew that the sounds were generated by avalanches, but were not sure how. One thought had been that the force of an avalanche could cause an entire dune to resonate like a flute or a violin. But if that were true, dunes of different sizes and shapes should produce a cacophony of notes instead of one characteristic tone.

Now, after five years of research, visiting sand dunes in Morocco, Chile, China and Oman, a team of scientists from the United States, France and Morocco say they have the answer.

In a paper that will appear in Physical Review Letters, the scientists say that collisions between sand grains cause the motions of the grains to become synchronized. The outer layer of the dune vibrates like the cone of a loudspeaker. The particular note depends primarily on the size of the grains.

Indeed, no dune was required at all. The scientists shipped sand from a Moroccan desert to a Paris laboratory and reproduced the singing by pushing the sand around with a metal blade.

“It’s not at all like any other instrument we know,” said one of the scientists, Stéphane Douady of the National Center for Scientific Research in Paris.

The most beautiful dune tune comes from the sands of Oman. “Very pure sound,” Dr. Douady said. “This one is really singing.” The least musical bits of silicon were those from China, which hardly sang at all.