- If you've never heard a sand dune rumble, listen up.
Marco Polo in the 13th Century said the singing sands -- which he ascribed
to evil desert spirits -- "at times fill the air with the sounds of
all kinds of musical instruments, and also of drums and the clash of arms."
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- Yes, certain sand dunes will occasionally let out a loud,
low-pitch rumble that lasts up to 15 minutes and can be heard up to 6 miles
(10 kilometers) away. Some dunes are known to do it regularly, even daily.
But why?
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- To try and uncover the underlying nature of these mysterious
sounds, Bruno Andreotti from the University of Paris-7 took equipment out
to the Atlantic Sahara in Morocco, one of only 35 known places where the
mysterious natural music can be heard.
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- "Singing dunes constitute one of the most puzzling
and impressive natural phenomenon I have ever encountered," Andreotti
said.
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- Setting the stage
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- Andreotti and his team studied one of the large crescent-shaped
dunes, or barchans, which spontaneously sings all year long - sometimes
two or three times an afternoon, if windy enough.
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- Wind forces sand to accumulate at the top of the dune
until the angle of the slope reaches a tipping point of about 35 degrees.
The eventual avalanche of sand produces the bellowing noise. The sand must
be sufficiently dry for the singing to occur. For smaller barchans, the
sand must also be hot and the wind still.
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- "A small dune sings only the few days in which there
is no wind and no clouds so that the Sun can dry efficiently the slip face,"
Andreotti told LiveScience.
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- Although sand avalanches were known to be the cause of
the singing, the exact mechanism was still unclear. Not wanting to wait
for a spontaneous episode, Andreotti and his team induced avalanches in
the field by sliding down the dunes.
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- Nature's boom box
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- By measuring vibrations in the sand and air, Andreotti
was able to detect surface waves on the sand that emanated from the avalanche
at a relatively slow speed of about 130 feet per second (40 meters per
second). In this way, the face of the dune acts like a huge loudspeaker
- with the waves on the surface producing the sound in the air.
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- Andreotti explained these sand waves as resulting from
collisions that occur between grains at about 100 times per second, as
measured in the lab. In a kind of feedback loop, the waves synchronize
the collisions, so they are all on basically the same beat.
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- This model explains the low pitch - between 95 and 105
Hertz - of the sand song, which, according to Andreotti, resembles a drum
or a low-flying propeller aircraft.
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- The feedback mechanism, as outlined by the researchers
in the Dec. 1 issue of Physical Review Letters, also correctly predicts
the maximum loudness of the singing to be 105 decibels, at which point
the sand grains vibrate off the surface. This level of sound is comparable
to a snow blower or a Walkman at full volume.
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- The mystery is not completely solved. Recent research
has centered on a seemingly magical musical property of the singing grains.
It is not known, for instance, why the sliding of glass beads is silent,
while some rougher sand grains belt out a tune.
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