* Astronomy

For centuries, world travellers have known of sand dunes that issue loud sounds, sometimes of great tonal quality. In the 12th century Marco Polo heard singing sand in China and Charles Darwin described the clear sounds coming from a sand deposit up against a mountain in Chile. Now, a team of scientists has disproved the long held belief that the sound comes from vibrations of the dune as a whole and proven, through field studies and through controlled experiments in a lab, that the sounds come from the synchronized motions of the grains in avalanches of a certain size.

Small avalanches don't produce any detectable sound, while large avalanches produce sound at lots of frequencies (leading to cacophonous noise). But sand slides of just the right size and velocity result in sounds of a pure frequency, with just enough overtones to give the sound "colour," as if the dunes were musical instruments. In this case, however, the tuning isn't produced by any outside influence but by critically self-organizing tendencies of the dune itself. The researchers thus rule out various “musical” explanations.

For example, the dune sound does not come from the stick-slip motion of blocks of sand across the body of the dune (much as violin sounds are made by the somewhat-periodic stick-slip motion of a bow across a string attached to the body of the violin). Nor does the dune song arise from a resonance effect (much as resonating air inside a flute produces a pure tone) since it is observed that the dune sound level can be recorded at many locations around the dune.

Instead, the sand sound comes from the synchronized, free sliding motion of dry larger-grained sand producing lower frequency sound. The scientists -- from the University of Paris, Harvard University in Cambridge, Mass., the CNRS lab in Paris, and Ibn Zohr University, in Morocco -- have set up a Web site where one can listen to sounds from different dunes in China, Oman, Morocco, and Chile.

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