I rarely spend time near sand dunes so I was amazed to learn that sand can sing. In fact there are 35 places around the world where the dunes sing a low frequency hum in the bottom half of a cello’s range.
The droning happens naturally when the wind causes a sand avalanche. People can force the song by pushing sand downhill. The songs are well known but people have always wondered how and why they happen.
Singing dunes are crescent-shaped barchans with their backs to the wind and their horns pointing downwind. The slipface is inside the crescent (downwind) with its surface at the angle of repose and a stationary layer beneath.
Experiments have shown the importance of the grains themselves. If they’re spherical, 0.1 to 0.5 mm in diameter, and contain silica, they will sing in the lab when they slide down an incline.
This year physicists from Paris Diderot University discovered that grain size determines the tune. They studied two dunes: one in Morocco, one in Oman. The Moroccan dune has grains 150-170 microns and emits a 105 hz sound (for musicians that’s near G-sharp two octaves below middle C). The Omani dune has a variable grain size from 150 to 310 microns and its sound varies, too — from 90-150 hz (F-sharp to D).
Researchers took the Omani sand back to the lab and sifted it down to a nearly uniform size — 200 to 250 microns — and sent it down an incline. Voilà. The sand made a sound of 90 hz, close to the song of the Moroccan dune. (Click here for more information about the study.)
What are the songs like? In this video, filmed in Morocco, a man shows how he learned to make the sand sing. Turn up your speakers and you’ll be able to hear a variety of sounds as he puts the sand through its paces. The video is in French with subtitles, some of which are surprisingly translated as in the first sentence that says “Beware” when it means the less dangerous-sounding “Be aware.”
Thanks to science we’ve learned how the sand sings, but we still don’t know why.
(video from YouTube)