In certain deserts around the world, sand dunes emit a deep, rumbling sound that can last up to 15 minutes and be heard from as far as 6 miles (10 km) away, a phenomenon that sometimes occurs daily.
Deserts are not always silent landscapes. Under the right conditions, some sand dunes become alive with strange sounds they produce – deep booms, rumblings, or even haunting buzzing. This phenomenon, known as singing sand dunes, has fascinated explorers and scientists for centuries. But what causes these dunes to produce such sounds?
During his travels, the 13th-century Venetian explorer Marco Polo encountered singing sand dunes, which he believed were the work of malevolent desert spirits. Marco Polo noted that they “sometimes fill the air with the sound of all kinds of instruments, as well as drums and the clashing of instruments with arms.”
Additionally, there have been reports of strange sounds heard in at least 35 deserts from California and Africa to China and Qatar, including the deep buzzing of bees or incomprehensible rumblings.
Deserts are not always silent landscapes.
The fantastical tales of Marco Polo’s travels are often so surreal that many consider his stories to be fabrications. However, the core of truth is still embedded in these tales, much like the story of the singing sands.
One thing is certain: these mysterious noises have puzzled desert explorers as well as scientists for many years. For example, Charles Darwin could not explain the origin of the sound he heard in the Chilean desert. Yet today, we are closer to unraveling this mystery.
The theory most widely accepted by the scientific community is that when fine, dry sand cascades down the sheltered (leeward) slopes of a sand dune, it collides and vibrates, generating sound waves. This theory aligns with observations that singing dunes often have a crescent shape (barchans) with steeper slopes that facilitate sand avalanches.
In a 2005 experiment conducted in the Sahara Desert, Professor Bruno Andreotti of the University of Paris Diderot demonstrated that the specific shape and size of the sand dune can act like a natural amplifier, shaping the vibrations into the explosive sounds or bass tones we hear.
The shape and size of the sand dune can act like a natural amplifier.
Andreotti measured the vibrations of sand and air to detect surface waves traveling at approximately 130 feet per second (40 meters per second) generated by sand avalanches on the dune. These waves arise from collisions of particles occurring about 100 times per second, creating a synchronized rhythm.
The resulting sound frequencies range from 95 to 105 Hertz, similar to the sound 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 particles to vibrate off the surface of the dune, comparable to the noise level of a snow blower or the maximum volume for personal listening devices, including large radios, stereo systems, and televisions, or noisy entertainment venues like nightclubs and bars.
However, the most mysterious aspect is that not all sand dunes can “sing”, even under seemingly perfect conditions of wind speed, direction, and sand composition. There may be additional factors at play or a specific combination of necessary elements for this phenomenon to occur. Some researchers suggest that a layer of denser dry sand hidden beneath the loose surface layer may amplify or shape sound waves.
Scientists have even recorded the ‘singing’ of sand in the laboratory.
Scientists have recorded the ‘singing’ of sand in the laboratory. Sand can produce multiple notes simultaneously due to the different velocities of particles of various sizes.
But how can singing sand dunes produce multiple notes at once? To investigate, 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 another near Al-Askharah, a coastal town in southeastern Oman.
The dunes in Morocco consistently emitted a note at a frequency of 105 Hertz, similar to a G two octaves below middle C, while the dunes in Oman produced a broader frequency spectrum 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, while there was significant variation in Oman.
To delve deeper, the research team separated grains of different sizes and analyzed the sounds they produced when moving through the air in a laboratory setting. 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 profound understanding, the mechanism by which the uneven movement of sand grains creates coherent musical notes remains unclear. Researchers speculate that the vibrations of aligned sand grains resonate at a common frequency, causing the mass of grains to oscillate uniformly. This collective vibration generates thousands of small movements that converge to compress the surrounding air, similar to the operation of a loudspeaker membrane.
“But why do they synchronize with each other?” Dagois-Bohy stated in a press release, “that remains unexplored and unexplained.”
