Why can some deserts make strange sounds?

In some deserts around the world, sand dunes emit a loud, deep rumbling sound that can last up to 15 minutes and can be heard up to 6 miles (10 km) away, this sometimes happens every few hours. day.

Deserts are not always silent landscapes. Under the right conditions, some dunes will come alive with the eerie sounds they produce - deep explosions, rumbles or even haunting hums. This phenomenon, known as the singing sand dunes, has fascinated explorers and scientists for centuries. But what makes these dunes make such sounds?

During his travels, the 13th-century Venetian explorer Marco Polo encountered singing dunes that he attributed to malevolent desert spirits. Marco Polo noted that they " sometimes filled by the sounds of all kinds of musical instruments, as well as the sounds of drums and the sound of the instruments clashing with the arms."

There are also reports of strange noises occurring in at least 35 deserts from California and Africa to China and Qatar, with deep buzzing of bees or rumbling sounds. hard to understand.

Deserts are not always silent landscapes.

Marco Polo's fantastical accounts of his travels were often so surreal that many people considered his stories to be fabrications. However, the core of truth is still hidden in these stories, like the story of the singing sands.

One thing is for sure, these mysterious noises have not only baffled desert explorers but also scientists for years. For example, Charles Darwin could not explain the origin of sound when he heard it in the Chilean desert. But today, we are closer to solving this mystery.

The theory most widely agreed upon by the scientific community is: When dry, fine sand grains fall down the leeward slopes (downwind side) of the dunes, they collide and vibrate, creating sound waves. This theory is consistent with observations that sand dunes are often crescent-shaped (barchans) with steeper slopes creating favorable conditions for sand slides.

In a 2005 experiment conducted in the Sahara Desert, professor Bruno Andreotti of the University of Paris Diderot showed that the specific shape and size of sand dunes can act like a natural amplifier , determining the shape and size of sand dunes. shape the vibrations into the booming or bass sounds we hear.

The shape and size of sand dunes can act like a natural amplifier.

Andreotti used vibration measurements of sand and air to detect surface waves moving at about 130 feet per second (40 meters per second) created by sand avalanches on dunes. These waves originate from collisions of particles that occur about 100 times per second, creating a synchronized rhythm.

The resulting sound has a frequency between 95 and 105 Hertz, similar to the sound of drums or low-flying propeller aircraft. This feedback process accurately predicts the phenomenon's maximum volume at 105 decibels, causing sand grains to vibrate off the dune surface, comparable to the noise level of a snow blower or maximum volume level. for personal listening devices, including extremely loud radios, stereos and televisions, or noisy entertainment venues such as nightclubs and bars.

However, the most mysterious thing is that not all sand dunes can "sing" , even under seemingly perfect conditions in terms of wind speed, wind direction and sand composition. There may be additional factors at play or a specific combination of factors required for this phenomenon to occur. Some researchers propose that a layer of dry, denser sand hidden beneath a loose surface layer may serve to amplify or shape sound waves.

Scientists also recorded the 'singing' sound of sand in the laboratory.

Scientists also recorded the ' singing' sound of sand in the laboratory. Sand can produce many notes at once, which comes from the different velocities of different sized particles.

But how can singing dunes produce so many notes at once? To find out, a research team led by Simon Dagois-Bohy conducted a comparative study at two separate locations: one in southwestern Morocco in the Sahara desert and one near Al-Askharah, a town coastal town in southeastern Oman.

Sand dunes in Morocco continuously emit a note at a frequency of 105 Hertz, like a G note two octaves below middle C, while sand dunes in Oman produce a broader frequency spectrum from 90 to 150 Hertz, spanning about nine notes long from F-shap to D. Notably, sand grains in Morocco are relatively uniform in size, while in Oman there are significant differences.

To investigate further, the team separated particles of different sizes and analyzed the sounds they made as they moved through the air in a laboratory environment. They concluded that the sound emitted by sand is influenced by both the size of the sand grains and the speed at which they move through the air.

Not all sand dunes can "sing".

Despite this insight, the mechanism by which the irregular motion of sand grains produces coherent musical notes remains poorly understood. The researchers speculate that the vibrations of aligned sand grains, resonating at a common frequency, cause the grain mass to oscillate uniformly. This collective vibration creates thousands of tiny movements that converge to compress the surrounding air, similar to the action of a speaker diaphragm.

"But why are they in sync with each other?" Dagois-Bohy said in a statement, "it remains to be discovered and explained."