On his travels through the Gobi Desert in the 13th century, Marco Polo encountered a curious sonic phenomenon in the dunes that he described as "the sounds of all kinds of musical instruments", and also "of drums and the clash of arms". A local legend had it that evil spirits were trying to disorient travellers and draw them deeper into peril. Today, scientists understand that this spooky desert noise - a low, sustained groan that sounds like a cross between a low-flying aeroplane and a didgeridoo - is caused by the slippage of a certain kind of sand down the face of a certain type of dune. But they have only recently begun to comprehend just how these sand movements make the dunes sing. The science of singing sand is so new, in fact, that there is still controversy among physicists over which model most accurately describes the mechanism.
The earliest discussion of singing sand dates back to 880AD in the Ton-Fan region of China. A manuscript talks of a "Singing Mountain" where "the sound of stepping on the sand will reach some dozen miles or so". Over the centuries, the phenomenon drew poetic descriptions from travellers and writers such as Guy de Maupassant, who called it "the mysterious drum of the dunes". But starting in the 19th century, it was scientists who were increasingly drawn to the sounds in search of an explanation.
One early scientific treatise said that singing sand was caused by "periodic oscillations of air pockets between the grains". Another scientist said it was the product of "underground volcanoes". The current wave of research began in 2001, when Stephane Douady, a physics professor at the Ecole Normale Superieure in Paris, and Bruno Andreotti, his former PhD student, encountered a singing dune by accident while researching the shape and motion of sand dunes in the Western Sahara region of Morocco.
Prof Douady had read about the phenomenon before, but had expected the sound to be a squeaking of solid-against-solid, "like a door that's not well oiled". Instead, he was surprised to find the sand moved more like a liquid. Moreover, he thought that perhaps the loud and sonorous nature of the sound was created by the dune resonating like the body of a cello, but soon disproved this theory by finding that dunes of varying size produced the same note.
"We were very excited then, because it was clear that it had to be some sort of original way of producing sound," he said. The team dug deeper upon their return to Paris, coming up with the theory that the sound was caused by a synchronisation of grains that caused the surface of the falling sand to behave like the cone of a speaker. But Prof Douady and Dr Andreotti had different ideas about what caused the synchronisation.
Dr Andreotti, who co-authored the most recent scientific paper on the phenomenon in the January 2007 issue of Physical Review, argued that "the song of the dunes originates from a wave particle mode locking". The sound "is mostly independent of the avalanche thickness and of the dune size but depends on the sand grain's diameter", Dr Andreotti said. Meanwhile, Prof Douady visited dunes around the world and brought sand back to his laboratory to reproduce their songs. Now, he says: "We are making singing avalanches on a plate in the laboratory, so we are finding that the dune itself is not necessary."
A third strain of research being conducted at the California Institute of Technology argues that, to the contrary, the song is in the resonance of the dune. What is clear from empirical observation is that not just any dune sings. There are about 30 documented singing sand locations in the world, including the Southwestern United States, Chile, China, South Africa, North Africa and the Rub Al Khali, or Empty Quarter, shared by Saudi Arabia, Oman, Yemen and the UAE.
In Arabic, the sound is called za'eeq al raml, or "shouting sand", a name that aptly describes the noise's startling loudness. Volumes can reach up to 115 decibels and be heard up to 10km away. Their pitch can range from 65 to 100 Hz, depending on the size and quality of the grains. Steep dunes work best, as do dry ones, because "humidity acts like a glue that sticks the grains together", Prof Douady said. As a result, the best songs are usually heard at high noon, and many dunes stop singing at sunset.
Grains that have travelled a long distance and are covered by a "varnish" also seem more likely to sing, Prof Douady said. Part of his research has suggested that salt acts as a catalyst for this varnish, so dunes near salt flats, like those in Morocco and Abu Dhabi's southern Liwa region, are likely to sing well. Although Prof Douady admits that there are few applications for his research beyond tourism, the mystery of the singing sands has provided rich fodder for artists. Birkbeck and Duffy, the Manchester-based art duo, recently mounted an installation at the Manchester Science Festival based on Andreotti's audio and video recordings from Morocco.
"We were interested in it as a kind of sensory experience, the idea of this soundscape enveloping you," said Joe Duffy, a film artist who worked with Eimer Birkbeck, a sound artist, on the project. The viewers walking among the multiple speakers and screens were intrigued, he said, because they had never heard anything like it before. "I think the children were quite scared by the sounds," he said.
"It's a natural response. When you go back in history to earlier writings about singing sand, particularly in North Africa, the singing sands were described as djinn, or evil spirits. It was described like that because the sound was so loud and it surrounded people. It sounds like an aeroplane, and I guess if you were surrounded by that hundreds of years ago, before there were any aeroplanes, you would think it was more malevolent too."