SOME roar, some boom, others squeak and a few even sing. They entranced Marco Polo when he crossed the Gobi desert in the 13th century, and references to their mysterious sounds can be found in 9th-century Chinese literature. Now one physicist has put forward an explanation for why sand dunes hit the right notes.
鈥淪inging dunes constitute one of the most puzzling and impressive phenomena I have ever encountered,鈥� says Bruno Andreotti of the University of Paris 7. Andreotti has been studying the crescent-shaped sand dunes of the Sahara desert in Morocco, one of around 30 locations in the world where dunes are known to sing.
The Saharan dunes hum like a low-flying, twin-engined jet, and can be heard kilometres away. Elsewhere, dune sounds have been likened to drums, foghorns and trumpets, among other things. In all cases, the sound seems to be triggered by sand avalanching down the sides of the dunes. But no one knew why the tumbling sand produces a resonant note and not just a messy rumble.
Advertisement
To find out, Andreotti deliberately started avalanches in Saharan sand dunes to set them singing. He then measured the vibrations in the sand bed and in the air around the dune, and used the data to figure out how the sounds were being produced. He argues that the cascading layer of sand behaves like the membrane of a loudspeaker, moving up and down at a frequency that generates audible sound. 鈥淭he measurement clearly shows the existence of this motion,鈥� he says. The amplitude of the vibrations is about 0.07 millimetres 鈥� roughly a quarter of the width of one grain of sand 鈥� so although they are not visible to the naked eye, if you lie down on the dune you can feel them, says Andreotti.
But there鈥檚 still the matter of explaining why the cascading layer vibrates. You would expect the sand grains in an avalanche to collide randomly, creating a kind of 鈥渟sshhhrr鈥� sound. According to Andreotti, however, as the tumbling grains drum on the surface of the dune, the collisions set up a surface wave that guides the motion of the falling sand, producing the characteristic sound (Physical Review Letters, vol 93, p 238001).
Not everyone agrees. St茅phane Douady from L鈥橢cole Normale Sup茅rieure in Paris, who studied the Moroccan singing dunes with Andreotti, has developed a different interpretation. He agrees that the sliding layer of sand creates the sound as it vibrates, but says it is the sound of individual collisions reflecting among the sand grains that synchronises their movement. Douady has submitted his results for publication.
鈥淭he cascading sand behaves like the membrane of a loudspeaker moving at a frequency that generates audible sound鈥�
Neither mechanism explains why only a few dunes in each desert can sing. Melany Hunt of the California Institute of Technology in Pasadena, who studies singing dunes in the US, suspects that the internal structure of the dune could also play a part. Her team鈥檚 radar measurements reveal a hard layer about a metre or so beneath the surface of the singing dunes. This layer might resonate like the soundbox of a musical instrument, amplifying particular frequencies. 鈥淲e have always thought it is a lot more complicated than what鈥檚 going on at the surface,鈥� Hunt told 快猫短视频. Her team is still collecting data to back up this idea.
Until the rival theories are sorted out, Marco Polo鈥檚 explanation will have to suffice: in accounts of his travels through the Gobi, he ascribed the strange noises to spirits of the desert.
