singing sand dunes


Subject: singing sand dunes
From: Innes A. Park (oceanicverse@postmaster.co.uk)
Date: Wed Sep 21 2005 - 20:59:13 EDT


('binary' encoding is not supported, stored as-is) .. two messages from the World Forum for Acoustic Ecology list

Singing Croissandwich
For centuries, explorers have reported that crescent-shaped sand dunes can emit low sounds like that of a turbo-prop airplane (giving rise to a legend involving the ghost of a buried dinosaur). Apparently the best way to experience the effect is to climb to the top of a dune and then slide down the steep slip face (causing a small avalanche). A news item in the 4 April issue of Science (page 47), describes how a team headed by Stephane Douady was able to replicate this sound by slowly turning 72 kg of Moroccan sand in a 2 meter doughnut shaped container. Douady believes that this phenomenon can be explained by something called the Reynolds dilatency—a vibration created by the dilation and compression of air as grains separate and come together. (http://www.lorentz.leidenuniv.nl/~mvhecke/Workshop/granular_workshop.html).

For more links and sound recordings of "singing sand" see
http://www-personal.engin.umich.edu/~nori/booming_sand.html
http://www.desertusa.com/magjan98/dunes/jan_dune1.html
http://www.bigai.ne.jp/~miwa/sand/index.html
http://www.schweich.com/sbdA.html

*Sound dunes: Desert resonance / Sonic harddrive*

"Sand dunes in certain parts of the world are notorious for the noises
they make," the New Scientist <1>
reports, "as sand avalanches down their sides. Some [dunes] 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."

To test for the causes, properties, and other effects of these sand
dune
booms, "Stéphane Douady of the
French national research agency CNRS <2> and his colleagues shipped
sand
from Moroccan singing dunes back
to his lab to investigate." There, Douady's team "found that they could
play notes by pushing the sand by
hand, or with a metal handle."

They performed, in other words, the transformation of a sand dune –
and,
by extension, the entire Sahara
desert, indeed any desert – even, by extension, the rust deserts of
Mars
– into a musical instrument.

Music of the spheres, indeed.

"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. 'What's funny is that in these
massive dunes, only a thin layer of 2
or 3 centimetres is needed to set up the resonance,' says Douady. 'Soon
all grains begin to vibrate in
step.'"

Douady has so perfected his technique of dune resonance that he has now
"successfully predicted the notes
emitted by dunes in Morocco, Chile and the US simply by measuring the
size of the grains they contain."
The music of the dunes, in other words, was determined entirely by the
size, shape, and roughness of the
sand grains involved, where excessive smoothness dampened the dunes'
sound.

I'm reminded of the coast of Inishowen, a peninsula south of Malin Head
in the north of Ireland, where the
rocks endlessly grind across one another in the backwash of heaving,
metallic, grey Atlantic waves. Under
this constant pressure of the oceanic, the rocks carve into themselves
and each other, chipping down over
decades into perfectly polished and rounded spheres, columns, and eggs

as if ideal, Archimedean solids
<3> or the nested orbits of Kepler could be discovered on the Irish
ocean foreshore – all glittering. The
rocks, I later learned, were actually semi-precious stones, and I had a
kind of weird epiphany, standing
there above the hush and clatter of bejewelled rocks, rubbing and
rubbed
one to the other in the
depopulated void of a coastal November. It was not a sound easy to
forget.

Because the earth itself is already a musical instrument: there is "a
deep, low-frequency rumble that is
present in the ground even when there are no earthquakes happening.
Dubbed the 'Earth's hum', the signal
had gone unnoticed in previous studies because it looked like noise in
the data." <4>

"Competing with the natural emissions from stars and other celestial
objects, our Earth sings like a
canary – it drones on in a constant hum of a gazillion notes. If it
were
several octaves higher, and
hence, audible to the human ear," <5> it could probably get recorded by
the unpredictably omnidirectional
antennas of ShortWaveMusic <6> and... you could download the sound of
the earth.

*Free Radio Interterrestrial*.

Which, finally, brings us to Ernst Chladni and his Chladni figures <7>,
or: architectonic structures
appearing in sand due to patterns of acoustic resonance.

Architecture through sound, involving sand. Silicon assuming structure.
Desert harddrive, humming.

The gist of Ernst Chladni's experiments involved spreading a thin layer
of sand across a vibrating plate,
changing the frequency at which the plate vibrated, and then watching
the sand as it shivered round,
forming regular, highly geometric patterns. Those patterns depended
upon, and were formed in response to,
whatever vibration frequency it was that Chladni chose.

So you've got sand, dune music, terrestrial vibration, some Chladni
figures – one could be excused for
wondering whether the earth, apparently a kind of carbon-ironic bell
made of continental plates and
oceanic resonators, is really a vast Chladni plate, vibrating every
little mineral, every pebble, every
grain of sand, perhaps every organic molecule, into complex,
three-dimensional, time-persistent patterns
for which we have no standard or even technique of measurement. Or
maybe
William Blake knew how to do it,
or Pythagoras, or perhaps even Nicola Tesla, but...

The sound dunes continue to boom and shiver. The deserts roar. The
continents hum. <8>

(Geoff Manaugh)

+ + + + + + + + + +

<1> http://www.newscientist.com/article.ns?id=dn8014
<2> http://www.cnrs.fr/index.html
<3> For an illustration of the 13 Archimedean solids see
http://www.math.dartmouth.edu/~matc/math5.geometry/unit6/unit6.html,
and
scroll down.
<4> http://www.newscientist.com/article.ns?id=dn6464; see also
http://news.bbc.co.uk/2/hi/science/nature/3701944.stm
<5>
http://www.space.com/scienceastronomy/planetearth/space_symphony_000323.html
[Note:
this link offers a contrary, but no less interesting, theory (published
in 2000) about
the origins of these planetary soundwaves]
<6> Here I’m referring to the excellent transglobal sonic resource,
ShortWaveMusic,
found at http://shortwavemusic.blogspot.com/
<7> For more on Ernst Chladni, click through his Wikipedia entry:
http://en.wikipedia.org/wiki/Ernst_Chladni; and for more on Chladni
figures themselves
see http://www.phy.davidson.edu/StuHome/jimn/Java/modes.html
<8> An illustrated version of this post appears on BLDGBLOG:
http://bldgblog.blogspot.com/2005/09/sound-dunes.html

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