[PD-ot] Dune tunes

[Chris McCormick]

----- Forwarded message from physnews at aip.org -----

DUNE TUNES.  For centuries, world travelers 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 “color,” 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 (France), Harvard (US), the CNRS lab in Paris,
and the Universite Ibn Zohr (Morocco)---have set up a website
(http://www.lps.ens.fr/~douady/SongofDunesIndex.html ) where one can
listen to sounds from different dunes in China, Oman, Morocco, and
Chile. (Douady et al., Physical Review Letters, upcoming article;
contact Stephane Douady at douady at lps.ens.fr)

----- End forwarded message -----
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chris at mccormick.cx
http://mccormick.cx