[Geysers] Yellowstone Whispers

[Katherine]

Although this article is about sand dunes, its explanation for singing dunes
may somehow be extrapolated to the Yellowstone case.

regards
Katherine Luketina

Dunes are alive with the sand of music
18 December 2004
Exclusive from New Scientist Print Edition
Jenny Hogan

SOME roar, some boom, others squeak and a few even sing. They entranced
Marco Polo when he crossed the Gobi desert in the 13th century, and
references to their mysterious sounds can be found in 9th-century Chinese
literature. Now one physicist has put forward an explanation for why sand
dunes hit the right notes.

"Singing dunes constitute one of the most puzzling and impressive phenomena
I have ever encountered," says Bruno Andreotti of the University of Paris 7.
Andreotti has been studying the crescent-shaped sand dunes of the Sahara
desert in Morocco, one of around 30 locations in the world where dunes are
known to sing.

The Saharan dunes hum like a low-flying, twin-engined jet, and can be heard
kilometres away. Elsewhere, dune sounds have been likened to drums, foghorns
and trumpets, among other things. In all cases, the sound seems to be
triggered by sand avalanching down the sides of the dunes. But no one knew
why the tumbling sand produces a resonant note and not just a messy rumble.

To find out, Andreotti deliberately started avalanches in Saharan sand dunes
to set them singing. He then measured the vibrations in the sand bed and in
the air around the dune, and used the data to figure out how the sounds were
being produced. He argues that the cascading layer of sand behaves like the
membrane of a loudspeaker, moving up and down at a frequency that generates
audible sound. "The measurement clearly shows the existence of this motion,"
he says. The amplitude of the vibrations is about 0.07 millimetres - roughly
a quarter of the width of one grain of sand - so although they are not
visible to the naked eye, if you lie down on the dune you can feel them,
says Andreotti.

But there's still the matter of explaining why the cascading layer vibrates.
You would expect the sand grains in an avalanche to collide randomly,
creating a kind of "ssshhhrr" sound. According to Andreotti, however, as the
tumbling grains drum on the surface of the dune, the collisions set up a
surface wave that guides the motion of the falling sand, producing the
characteristic sound (Physical Review Letters, vol 93, p 238001).

Not everyone agrees. Stéphane Douady from L'Ecole Normale Supérieure in
Paris, who studied the Moroccan singing dunes with Andreotti, has developed
a different interpretation. He agrees that the sliding layer of sand creates
the sound as it vibrates, but says it is the sound of individual collisions
reflecting among the sand grains that synchronises their movement. Douady
has submitted his results for publication.

“The cascading sand behaves like the membrane of a loudspeaker moving at a
frequency that generates audible sound”Neither mechanism explains why only a
few dunes in each desert can sing. Melany Hunt of the California Institute
of Technology in Pasadena, who studies singing dunes in the US, suspects
that the internal structure of the dune could also play a part. Her team's
radar measurements reveal a hard layer about a metre or so beneath the
surface of the singing dunes. This layer might resonate like the soundbox of
a musical instrument, amplifying particular frequencies. "We have always
thought it is a lot more complicated than what's going on at the surface,"
Hunt told New Scientist. Her team is still collecting data to back up this
idea.

Until the rival theories are sorted out, Marco Polo's explanation will have
to suffice: in accounts of his travels through the Gobi, he ascribed the
strange noises to spirits of the desert.

>From issue 2478 of New Scientist magazine, 18 December 2004, page 8

> -----Original Message-----
> From: geysers-bounces at wwc.edu [mailto:geysers-bounces at wwc.edu]On Behalf
> Of Pat Snyder
> Sent: Monday, 7 February 2005 13:47
> To: geyser observation reports
> Subject: Re: [Geysers] Yellowstone Whispers
>
>
> Recently I bought a book called "Weird U.S.A." by Mark Moran and Mark
> Scheurman. It lists strange things seen and heard throughout the
> country, much of it stuff like hauntings, odd museums, strange
> collections and the like. However, one item is about Yellowstone, and
> is called the "Yellowstone Whispers".
>
> They are described as a "...strange natural phenomenon that takes place
> in the high country above Yellowstone Lake." It's a whistling sound
> that can last up to 30 seconds--usually between the hours of dawn and
> 10 a.m. The book claims early trappers and Native Americans also knew
> about it (no record of how this was documented listed).
>
> Has anyone heard of this legend/story before? Has anyone heard the
> whispers? Could it be something thermal causing this sound? Seems
> likely to me that it could be a fumarole or the like. Anyway, just
> curious.
> Thanks!
> Pat Snyder
>
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