The science behind the lonely singing of the sand

Out in the desert, the sand sometimes hums or 'sings' as people walk along the top of dunes. Find out what causes this beautiful yet creepy sound, and listen to audio of the sand song.

Many people think that out in the desert, there's nothing to hear but the footsteps of camels, the panting of lost souls and the occasional 'caw' of a vulture. They'd be wrong. Sometimes, people walking along the top of a dune will hear a deep hum. As they walk further and further along it, the hum builds to a roar, with high-pitched ringing, like feedback on a microphone, building over it. It's not a distant highway. It's the singing sands.

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Unconnected sand grains have been known to make all kinds of noises, from cricket-like clicks the gunshot-loud bangs, but the 'singing' has to be the most eerie. The low-pitched hum with its attendant high ringing sound can be as loud as 105-115 decibels - about as loud as a motorcycle or a power saw. It was known for some time under what conditions this singing occured. Dunes sang on dry days, and the singing tended to emanate from the side of the dune that was sheltered from the wind. Smaller dunes only sing on windless days.

Initially it was thought that the entire dune vibrated, and that vibration was what caused the tone and loudness of the song. A little more study showed that that was not the case. Even as a dune changed size and shape, the tone it emitted was the same. In fact, the song was determined by the sand itself, not the dune it was one.

Sand dunes are built up by the wind. It carves out a shallow bowl on one side of the dunes, rolling sand slowly upwards. When the sand hits the top of the dune, the wind just keeps it going, hurling it out into space to land some distance away from the peak of the dune. Because of this, the sheltered side of sand dunes tends to be a steep drop, not a gentle slope. Sand isn't a very stable building material, so it can't support that steep a slope. Once it builds up to about 35 degrees, it's in the danger zone. A little too much force, and the top of the dune collapses down the sheltered side, causing an avalanche.

This avalanche can have a lot of force, but not nearly enough to vibrate the whole dune. Instead, the small grains of sand, all approximately the same size, bounce off of each other as they fall. As they continue to jostle they form a standing wave along the surface of the dune. The entire surface of the dune vibrates as if it were a stereo speaker, or those cups on the dashboard in Jurassic Park when the T-Rex was approaching. The wave travels across the dune at approximately 40 meters per second, and the dune gives off a long, low-pitched hum. The exact tone of this hum is determined by the size, and quality of the sand itself. Perfectly round glass beads won't make a sound, and rough, jagged sand is equally silent. The grains have to be the right size, round enough, and just smooth enough to bounce off each other in the right way to keep the collisions, the wave, and the sound coming.

To hear more examples of singing sand, head on over here.

Via NY Times, NY Moon, Live Science, and Physics World.