A few dozen sand dunes around the world can make a sound like a low organ note or a distant propeller plane. Push sand down the steep side of one on a dry day and the whole hill starts to hum, loud enough to feel in your chest, sometimes for minutes. Travelers wrote about it for centuries and blamed spirits. Scientists have only recently pinned down most of what is going on, and they still argue about the last piece. Here is what makes a dune sing, where you can hear one, and why most sand stays silent.
What it sounds like
People who hear it reach for big comparisons. In 1936, a researcher named Lewis studied roaring sands in the Kalahari Desert in southern Africa. He wrote that the steady hum was like “the noise made by an aeroplane at a distance in steady flight.” Other travelers compared it to a bass organ pipe, a kettle drum, or a didgeridoo.
Measurements back that up. When Caltech engineers recorded booming dunes in California and Nevada, the sound was close to a single musical note, somewhere between 70 and 105 vibrations per second, with fainter higher notes stacked on top. That is about the range of the lowest strings on a cello. One recording from Dumont Dunes in California lined up neatly with a cello playing a low F. The loudest booms have been put at around 105 decibels, louder than a busy city street.
The sound does not come from wind whistling over the crest. It comes from moving sand. It usually starts when a slab of sand slides down the steep downwind face of a dune, either on its own or because someone set it off. Once it gets going, it can keep humming even after the sliding slows, and people standing well away from the avalanche can feel the ground buzz.
Centuries of spooked travelers
Writers have described these sounds for well over a thousand years. Near Dunhuang in western China, a range of dunes on the old Silk Road is called Mingsha Shan, usually translated as Singing Sand Mountain. It was once known as the Gods’ Sand Dunes.
Marco Polo crossed the desert of Lop, which ends near Dunhuang, in the 1270s. In his account, desert spirits lured travelers away from their caravans. “Sometimes you shall hear the sound of a variety of musical instruments, and still more commonly the sound of drums,” he reported. Travelers kept bells on their animals and, at night, put up a sign pointing to the next day’s route, just in case.
Charles Darwin heard about one in 1835, in the Copiapó valley of northern Chile. Locals told him about a sandy hill they called El Bramador, “the roarer or bellower.” Darwin admitted he did not pay much attention at the time, and he never heard it himself. But he wrote down the key detail: the noise came only when people climbing the hill set the sand in motion.
In 1923, the British statesman Lord Curzon gathered dozens of these reports into a 78-page chapter called “The Singing Sands.” He noticed what they had in common. The sound came from sand sliding down a slope at about 31 degrees, the steepest angle loose dry sand can hold before it starts to slide. Scientists call that the angle of repose.
Not just any sand
Most dunes never make a sound. The ones that do share a short list of traits. The grains are mostly quartz, the main mineral in sand. They are rounded and polished by wind, and very close to the same size, usually somewhere around 0.1 to 0.5 millimeters across. There is very little dust or fine powder mixed in.
The sand also has to be dry. Even a little moisture makes grains stick together instead of sliding freely, and that can silence a dune completely. That is why booming is seasonal. Researchers in California did their booming tests in summer, and found the inside of the dunes behaved differently in winter, when they could feel dampness just under the surface.
This is also why only a few dozen booming sites are known, roughly 35 to 40 worldwide. In an 1890 letter to the journal Nature, an early investigator named Carus-Wilson put it this way: “only observers are rare — not the sands.” Since then, field teams have found booming dunes in the Sahara, Namibia, Chile, China, the Middle East, and the American West.
So how does a hill hum?
The first serious attempt to explain it came from Ralph Bagnold, a British army officer who spent the 1920s and 1930s exploring the Libyan Desert by car. In his 1941 book on how wind moves sand, he suggested that the note depends on how big the grains are and how fast they tumble over one another, the way running a fingernail along a comb makes a pitch that depends on the gaps between the teeth.
Two French teams picked up that idea in the 2000s. Bruno Andreotti, working on dunes near Tarfaya in Morocco, and a group led by Stéphane Douady, working there and near Copiapó in Chile, not far from Darwin’s roaring hill, argued that the grains in a sliding layer fall into step with each other. Millions of tiny collisions start happening in rhythm, like a crowd that begins clapping together. That shared rhythm sets the note. Douady’s team also reproduced the effect in a lab experiment.
A team at Caltech, led by engineer Melany Hunt and her graduate student Nathalie Vriend, came at it from a different angle. Over five summers they visited four booming dunes: Kelso Dunes and Dumont Dunes in California, Eureka Dunes in Death Valley, and Big Dune near Beatty, Nevada. They buried up to 96 small vibration sensors in a line down a dune’s face and tapped the sand to see how sound traveled through it.
What they found was a layer cake. On top sits a loose, dry layer, usually about 1 to 2 meters (3 to 6 feet) deep, where sound moves slowly, around 200 meters per second. Below it is firmer sand, packed and lightly cemented by minerals that rainwater carries down over the years. Sound moves much faster there, about as fast as it does in air.
That loose top layer acts like the body of a guitar. Vibrations from the sliding sand bounce back and forth between the air above and the hard sand below. Most of them cancel out, but one pitch fits the layer’s thickness just right and gets louder with each bounce. In their view, the avalanche is the bow and the dune itself is the instrument. A thinner or damper top layer changes the note or kills it, which fits the seasonal pattern.
The French researchers disagreed in print, and the debate has not fully settled. The most likely answer is that both sides have part of it. The grains moving together seem to start the sound, and the dune’s layers seem to shape and amplify it into one steady boom.
The squeaky beach is a different thing
You may have heard sand squeak on a beach, a sharp chirp when you scuff your feet. That is related but not the same. Beach squeaking is a short burst at a much higher pitch, around 1,000 vibrations per second, and it comes from a small patch of sand being squeezed underfoot. Well-known squeaky beaches include Porth Oer, nicknamed Whistling Sands, in Wales, and Singing Beach in Manchester-by-the-Sea, Massachusetts.
Booming needs a whole hillside on the move. A handful of sand squeaks. A sliding dune face sings.
How to hear one yourself
In the United States, Kelso Dunes in the Mojave National Preserve, near Baker, California, is the easiest to reach. The dunes rise up to about 650 feet above the desert floor, and the trailhead sits at the end of a 3-mile graded dirt road off Kelbaker Road. No vehicles are allowed on the dunes, only hikers. Eureka Dunes in Death Valley and Sand Mountain in Nevada are also known boomers.
A few tips from people who have done it:
- Go in the dry season, after a long stretch without rain. Plan for desert heat and carry plenty of water.
- Climb to the crest and find the steep side, the one facing away from the wind.
- Sit down and slide slowly, or push sand ahead of you with your arms. Lewis described doing it “in slow jerks.”
- Listen and feel. The first sounds are short burps. If the dune is in the mood, they settle into a steady hum that you feel through the sand.
- Follow park rules, stay off fenced or plant-covered areas, and expect silence on a damp day.
Even if a dune stays quiet, it is a striking place to stand. And when one does hum, it is easy to see why Marco Polo’s companions thought something was out there.
Further reading
The Physics of Blown Sand and Desert Dunes — Ralph Bagnold’s 1941 classic on how wind builds and moves dunes, written by the desert explorer who made one of the first scientific attempts to explain booming sand.
Sand: The Never-Ending Story — Geologist Michael Welland’s readable biography of sand, from how a single grain forms to how deserts, beaches, and dunes shift over time.