IT MIGHT not knock Coldplay or Kanye West off the top of charts, but physicists who say they have cracked the riddle of 鈥渟inging鈥� sand dunes are compiling a CD of sand music. The team say their new theory allows them to predict the notes that different dunes will make.
Sand dunes in certain parts of the world are notorious for the noises they make as sand avalanches down their sides. Some emit low powerful booms, others sound like drum rolls or galloping horses, and some are even tuneful. These dune songs have been reported to last for up to 15 minutes and can sound as loud as a low-flying aeroplane. Physicists know it is the avalanches that set the grains humming, but the precise mechanism has remained controversial (快猫短视频, 18 December 2004, p 8).
St茅phane Douady of the French national research agency CNRS and his colleagues shipped sand from Moroccan singing dunes back to his lab to investigate. They found that they could play notes by pushing the sand by hand, or with a metal handle. That put to rest one theory that the noise was the result of the entire dune resonating.
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But after a month of singing, the sands seemed to lose their voice. 鈥淲e examined the size and shape of grains under the microscope to try and work out what happened,鈥� says Douady. The singing grains were round with a smooth coating of silicon, iron and manganese, which probably formed on the sand when the dunes once lay beneath an ancient ocean. But in the muted grains this coat had been worn away, which explains why only some dunes can sing, says Douady.
He admits he is unsure exactly what role the coating plays in producing the noise. But the broader phenomenon seems to be clear. When the sand avalanches, the grains jostle each other at different frequencies, setting up standing waves in the cascading layer, says Douady. These waves reinforce one another, making the layer vibrate like the surface of a loud speaker. 鈥淲hat鈥檚 funny is that in these massive dunes, only a thin layer of 2 or 3 centimetres is needed to set up the resonance,鈥� says Douady. 鈥淪oon all grains begin to vibrate in step.鈥�
Douady鈥檚 key discovery was that this synchronised frequency 鈥� which creates the note 鈥� is the result of the grain size. He has successfully predicted the notes emitted by dunes in Morocco, Chile and the US simply by measuring the size of the grains they contain ().
Back in the lab, Douady can manipulate these frequencies, by setting various quantities of the grains tumbling with a mechanical motor and changing their velocity. 鈥淲e make different notes that are expressive and emotional,鈥� he says. Douady is now putting together a CD of his own dune tunes.
It鈥檚 fantastic, really beautiful,鈥� says Hans Herrmann, a sand dune expert at Stuttgart University in Germany, who has listened to Douady鈥檚 early attempts at sand music. But he says there are still questions to be answered, such as the role of the coating on the grains. 鈥淭he riddle of the sand dunes has been with us for centuries and I鈥檓 sure there is still plenty more to learn.
