Singing Sand Dunes: Some Deserts Play Eerie Music and Scientists Only Recently Started to Understand Why

In some deserts around the world, sand dunes emit a loud, low-pitched rumble that can last up to 15 minutes and be heard up to 6 miles (10 kilometers) away, sometimes occurring daily. But why?

Deserts aren’t always silent landscapes. Under the right conditions, some sand dunes come alive with an otherworldly music – deep booms, rumbles, or even a haunting hum. This phenomenon, known as singing sand dunes, has captivated explorers and scientists for centuries. But what causes these dunes to produce such sounds?

During his travels, 13th Century Venetian explorer Marco Polo encountered singing sand dunes, which he attributed to evil desert spirits, noting that they “at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms.”

Others have likened the sound, which occurs in at least 35 deserts from California and Africa to China and Qatar, to the deep hum of bees or some rumbling Gregorian chant. Well, decide for yourself.

One thing is sure, the moaning mountains haven’t only baffled desert explorers but also scientists for many years. Charles Darwin, for example, couldn’t explain the origin of the sound upon hearing it in the Chilean desert. But by today, we have gotten closer to solving the mystery.

The leading theory suggests a connection to avalanches. When dry, fine sand grains cascade down the steep lee slopes (downwind side) of a dune, they collide and vibrate, creating sound waves. This theory aligns with observations that singing dunes are often crescent-shaped (barchans) with steeper slopes that facilitate avalanches.

In a 2005 experiment conducted in the Sahara desert, Paris Diderot University professor Bruno Andreotti showed how the specific shape and size of a dune acts like a natural amplifier, or loudspeaker, shaping the vibrations into the low-pitched booms or hums we hear.

Andreotti used measurements of sand and air vibrations to detect surface waves traveling at around 130 feet per second (40 meters per second) generated by avalanches on sand dunes, akin to a colossal natural boombox. These waves stem from grain collisions occurring roughly 100 times per second, creating a synchronized rhythm.

The resulting sound, with a frequency of 95 to 105 Hertz, resembles that of a drum or a low-flying propeller aircraft. This feedback process accurately predicts the maximum volume of the phenomenon at 105 decibels, causing sand grains to vibrate off the dune surface, comparable to the noise level of a snow blower or the maximum volume level for personal listening devices, including very loud radios, stereos, and televisions, or loud entertainment venues like nightclubs and bars.

However, the mystery deepens when we consider that not all dunes sing, even under seemingly perfect conditions of wind speed, direction, and sand composition. There might be additional factors at play, or specific combinations of factors needed for the phenomenon to occur. Some researchers propose that a hidden layer of denser, dry sand beneath the loose surface layer might play a role in amplifying or shaping the sound waves.

But how can singing sand dunes produce multiple notes at the same time? To find out, a research team led by Simon Dagois-Bohy conducted a comparative study at two distinct locations: one in southwestern Morocco within the Sahara, and the other near Al-Askharah, a coastal town in southeastern Oman.

The sands in Morocco consistently emitted a note at 105 Hertz, resembling a G-sharp two octaves below middle C, while the Omani dunes produced a broader spectrum of frequencies ranging from 90 to 150 Hertz, spanning about nine notes from F-sharp to D. Notably, the sand grains in Morocco were relatively uniform in size, whereas those in Oman varied considerably.

To investigate further, the team isolated grains of different sizes and analyzed the sounds they generated when moving through the air in a laboratory environment. They concluded that the tones emitted by the sands are influenced by both the size of the grains and the velocity at which they travel through the air.

Despite this insight, the mechanism by which the irregular movement of flowing grains produces coherent musical notes remains elusive. The researchers speculate that the vibrations of the sand grains align, resonating at a shared frequency, causing the mass of grains to vibrate uniformly. This collective vibration generates thousands of small movements that converge to compress the surrounding air, akin to the action of a loudspeaker diaphragm.

“But why do they synchronize with each other?” Dagois-Bohy said in a statement. “That’s still not resolved.”

So, the quest to understand the singing dunes continues.

Sources: 1, 2, 3, 4, 5