Amazing Places

Listen to the Sound of "Singing Sand Dunes"

Picture this. You're on a strenuous hike with friends up and down some giant sand dunes when you look up and suddenly realize that you've wandered away from your group. But even though there's no one in sight, you can hear ... something. A wailing, humming moan — and it seems to be coming from every direction. No, you're not under attack by desert ghouls. It's only singing sand.

The Dunes Are Alive with the Sands of Music

Listen to this.

Singing Sand Dunes in Oman and Morocco

That monotonous hum can be heard from up to 6 miles (15 kilometers) away, and it sounds pretty ominous if you ask us. No wonder Marco Polo thought it was the sound of wailing, evil spirits. But there are actually a lot of different places you can hear the sound of singing sand. It's often heard in Morocco, Wales, Death Valley, and Copiapo, Chile. But for many years, scientists couldn't quite explain what caused the strange and deafening sound.

They could, however, explain exactly what conditions have to be met for the noises to start up. Large dunes only sing when it's windy, as sand accumulates at the top until it eventually spills over. The resulting sand-valanches are known to be linked with the booming sound, but it's also known that they aren't the only explanation. After all, every sand dune will have a slip now and then. But most dunes aren't so dramatic about it.

Besides the wind, it has to be a dry, hot day in the desert for the sands to start singing. Still, even as the picture of what makes sand sing comes into focus, a lot of questions remain. Why do only some dunes sing in dry, hot, windy conditions, which aren't exactly uncommon in desert climates? And as for the ones that do, why do they make so many different kinds of sounds at so many different frequencies?

Sand dunes in the Sahara Desert

Feel the Music Inside

The secret, according to a report from a Parisian team of biophysicists, lies in the sand itself. They traveled to two known singing dunes, one in Morocco and one in Oman. First, they had to measure the sounds these dunes made, and they did it with the very serious method of scooting on their butts to shift the top layers of sand. The Moroccan dune let out a low G-sharp at 105 Hz, while the Omani dune blasted nine different tones between 90 and 150 Hz. Then, they gathered about 330 pounds (150 kilograms) of the sand and took it back to the lab.

The Moroccan sands, as it turned out, were pretty much uniform in size and shape, measuring an average of 160 microns across — roughly three times the width of a human hair. But the Omani sands were a much more diverse bunch, ranging from 150 microns to 300 microns. If that's not proof enough that the size of the sand is what determines its sound, they settled it for sure by sieving out just the 200- to 250-micron sand particles and used them to create a clear, single tone. They still can't say for sure what causes the sound to happen in the first place, but the fact that it is so closely tied to the size and shape of the sand particles offers some tantalizing clues.

First, the bad news (if you think a theory being disproven is bad news). Because the scientists were able to recreate the sound in a lab with a relatively small amount of sand, they showed that the singing isn't dependent on the sheer size of the dunes. It was previously thought that the shifting sands caused reverberations that echoed through something called a waveguide, which conveys wave patterns while preserving their energy. In this case, the hypothetical waveguide would have been formed in the giant dunes, but no such structure turned out to be necessary. Still, the team thinks the answer lies basically in the billions upon billions of tiny collisions of sand particles. Speaking with National Geographic, lead author Stéphane Douady explained the team's suspicions that the sound really is just the grains of sand bouncing around. That song? It's "the sound of millions of little shocks."

Written by Reuben Westmaas April 25, 2018

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