Sand Dunes Sing
The dunes of the Sahara sometimes sing by emitting a low, haunting humming or booming sound. The still poorly explained sound puzzled some early explorers. Marco Polo in the the century blamed, “evil desert spirits [which] at times fill the air with the sounds of all kinds of musical instruments and also of drums and the clash of arms.” Modern researchers have recorded low-pitched rumbles audible for 0 miles ( 6 km), sometimes coming daily from certain dunes. Some researchers think that the sounds are caused by some kind of vibration that is picked up and amplified by the quartz crystals.
University of Paris researcher Bruno Andreotti recorded the 5-hertz rumble coming from one dune in Morocco, one of 5 known singing dunes. The best singers appear to be large, crescent-shaped dunes. Such dunes can belt out sounds two or three times in an afternoon when the wind kicks up. Andreotti concluded that the sounds are emitted when wind knocks loose an avalanche of extra dry sand on a slope of at least 5 degrees. On the other hand, some smaller dunes only sing when it is hot and there is no wind, according to research in the Physical Review of Letters. Clearly, something about the slide of sand down the face of the dune causes the humming, which Andreotti and his team demonstrated by starting sand slides that triggered the sound. The hum started during a slow-motion sand slide. Apparently, the curve of the dune acts as a loudspeaker to broadcast the sound of the sliding sand. The ringing of the quartz crystals in the sand smacking against one another 00 times per second somehow sets up a feedback loop, like the ear-splitting screech from an amplified speaker. In the case of the dune, the sounds Andreotti recorded have the pitch of a low-flying propeller plane or a drum, broadcast at about the volume of a snowblower. Still, no one knows why so few dunes sing and what constitutes the critical difference between a singing dune and the great mute majority.
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