Singing Sands
A low hum echoes across giant sand dunes. It buzzes and moans. It booms and roars. There’s nothing but sand in sight. So what’s making this eerie noise? Once, this mysterious music sent shivers down the spines of ancient explorers. Many thought evil spirits made the music. Today, scientists have a different explanation. The sands, they say, are singing. The music starts with shifting sand. To prove it, one scientist had volunteers slide down a dune. The dune moaned. Animals and the wind can make sand sing, too. A mystery remains, though. How does the sand make its sound?
Searching for a Cause Millions of dunes exist in the world, but only about 40 make music. T ey don’t sing all the time, and they don’t all make the same sounds. To understand why, some scientists decided to do an experiment. T ey hauled some sand from a singing dune back to a lab. T e scientists made the sand move. Yet in
the lab, the sand was silent. T at observation led scientists to one conclusion. Making music isn’t just about moving grains of sand. So the scientists did other experiments.
T ey changed the temperature of the sand. T ey added and removed moisture. T ey learned sand must be hot and dry to sing.
More Questions Physicist Nathalie Vriend wondered what controlled the volume of the sand songs. Some dunes are really loud. T eir songs can be heard from 10 km away. She dug deep inside a singing dune. About
2 m down, she hit a layer of sand as hard as concrete. Sound gets louder when it bounces off a hard layer. So she realized that this layer may act like a loudspeaker. Other scientists wondered why diff erent
dunes sang diff erent notes. A team of French physicists decided to find out.
16 NATIONAL GEOGRAPHIC EXTREME EXPLORER ★ ★
Making Models T e French scientists traveled to a dune in Morocco that sings one note. T en they went to a second dune in Oman that sings nine notes. T ey dug up more than 150 kg of sand from the two dunes. T ey were careful not to mix the two batches of sand together. In a lab, the scientists made small models
of each dune. T en they ran experiments on these mini dunes. T ey found that the lab dunes played the same musical notes as the ones in nature. Next, the team examined the grains of sand
in each dune. T ey measured the grains. T ey found some patterns. All the grains from the dune in Morocco were one size. T e grains from the dune in Oman came in many sizes. T e scientists concluded that the size of the grains determines the notes in the song.
A Mystery Remains T e scientists have learned a lot. Yet the main mystery remains. T ey still don’t know exactly how the sound is made. T e noise may come from millions of
grains of sand banging into each other. Each bump makes a sound so small no one could possibly hear it. Put a zillion moving grains of sand together, though, and you get a chorus.
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