Name: Booming Sands
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<div class="rich-text">(This video is no longer available for streaming.) At roughly 30 locations around the world, sand dunes seem to sing, producing haunting and baffling sounds. Marco Polo noticed the phenomenon in the Gobi Desert. Ancient travelers have heard it in the Sahara. Even Charles Darwin puzzled over it in the Chilean desert. For centuries, no one was able to explain why it happens. Now, engineers Melany Hunt and Christopher Brennen of the California Institute of Technology are on the case. Their theory is that the booming sand dunes act like enormous musical instruments.
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How and why do certain sand dunes produce mysterious noises—croaks, whistles, barks, burps, and booms?

<div class="rich-text">How and why do certain sand dunes produce mysterious noises—croaks, whistles, barks, burps, and booms?</div>

How and why do certain sand dunes produce mysterious noises?

Booming Sands

Planet Earth

<div class="rich-text"><h3>
 Booming Sands
</h3>

 PBS air date: January 25, 2005


 So James collects ants and Dara collects worms, and we know a lot of people
who collect sand.


 Don't ask why, but, like all collectors, sand fanciers have stories they
tell. And this one's a mystery, and it's an ancient mystery. For a long, long
time, people have noticed that in certain places, under certain conditions,
sand sings.


 Well, no, no, no "sings" isn't quite right. There are beaches...


 Can I go to the beach? Thanks.


 ...all over the world, where if you walk along, sand underfoot, like a
frog, it croaks. It's a very distinctive sound. No one is quite sure what
produces that sound. But far more mysterious and even rarer—there are
only about 30 places on earth where this can happen—sand will...well
you'll see.


 Here's our report from correspondent Chad Cohen.



 CHAD COHEN
 
 (scienceNOW Correspondent): Listen, you hear
that sound? You might think it's the wind, but it's not. It sounds like a
sustained musical note coming from the sand dunes themselves. This is the sound
that has mystified generations. Marco Polo noticed it in the Gobi Desert.
Ancient travelers have heard it in the Sahara. Even Charles Darwin puzzled over
it in the Chilean desert.


 So far no one has been able to explain what it is, or why it happens.
That's why Cal Tech engineers Melany Hunt and Christopher Brennen are here at
the Dumont Dunes in Death Valley, California.



 MELANY HUNT
 
 (California Institute of Technology): This location
is one of about 30 places around the world that have what are described as
booming dunes.



 CHAD COHEN:
 
 For the last two years the Cal Tech team has been
trying to figure out just what makes these 30 locations so unique. What about
them causes the sand to sing? Their theory is that the booming dunes are really
like an enormous musical instrument.


 Imagine the loose, moving sand particles on the surface are like the
vibrating strings on a cello. And underneath the surface you'll find a damper,
harder layer of sand which reflects and magnifies the vibrations, just like the
body of a cello does.



 CHRISTOPHER BRENNEN
 
 (California Institute of Technology): Better
get your goggles on.



 CHAD COHEN:
 
 To test their theory, the Cal Tech team has to
trek up the 300-foot-plus dunes. When they reach the peak, they have to
simulate an avalanche to put the loose sand in motion and make the surface
particles vibrate, a task they've learned is best accomplished on the seat of
their pants.



 CHRISTOPHER BRENNEN:
 
 Ready, set, go. Wait a minute, don't go too
fast.



 CHAD COHEN:
 
 But on this, their first run of the day, the dunes
are silent.



 CHRISTOPHER BRENNEN:
 
 That's too bad, hardly booms at all.



 CHAD COHEN:
 
 What could have gone wrong?



 CHRISTOPHER BRENNEN:
 
 Today the wind is not blowing in the normal
direction. Usually the wind blows from that direction over this way, leaving a
load of loose sand on this side of the dune.



 CHAD COHEN:
 
 That missing loose sand has an unusual
characteristic, the sand grains are almost all the same size, so that they
resonate together.



 CHRISTOPHER BRENNEN:
 
 We believe that that's part of the explanation for
this booming sound: that all the grains are more or less the same size, so when
they flow over one another, they hit each other at roughly the same frequency,
just as a bow rubbing over a string has a characteristic frequency.



 CHAD COHEN:
 
 Fifty feet down the dune they find what they think
they've been looking for. A patch of loose sand that looks promising.



 CHRISTOPHER BRENNEN:
 
 Go.



 CHAD COHEN:
 
 As they slide down the dune, at first, they hear
nothing...



 CHRISTOPHER BRENNEN:
 
 Come on, keep going guys. I got a little bit of a
boom,...



 CHAD COHEN:
 
 And then suddenly...



 CHRISTOPHER BRENNEN:
 
 ...a little bit of a boom. You hear it there? We
got it. Down here, Mel.



 CHAD COHEN:
 
 It is the music of the dune.



 MELANY HUNT:
 
 Not nearly as loud as it has been at other times, but we
still did, we still certainly heard it down there.



 CHAD COHEN:
 
 To record and measure this booming sound, a
seismic device called a geophone is buried just below the surface.


 Later, back at Cal Tech, this data gets fed into a computer.



 CHRISTOPHER BRENNEN:
 
 And this shows you that the sound that we are
hearing is predominantly a single frequency.



 CHAD COHEN:
 
 A frequency equal to the musical note of G.


 Over the last two years the Cal Tech team has observed and documented that
the sound of the booming dunes actually changes from day to day and from dune
to dune.


 What ultimately determines the musical range, they think, is not just the
size of the sand particles, but also how much space there is between the top
where the sand is loose and that hard surface down below.



 CHRISTOPHER BRENNEN:
 
 Another four inches, three inches...



 CHAD COHEN:
 
 The greater the distance between the two, the
lower the frequency and pitch of the booming dune.



 CHRISTOPHER BRENNEN:
 
 It's got a good consistency to it, doesn't it?



 CHAD COHEN:
 
 So after two years of research and frequent visits
to three of 30 booming dune sites worldwide, here is what the Cal Tech data has
shown: for a dune to boom it must be at least 150 feet high; there must be
loose, dry sand of similar particle size at the top and a hard layer below; and
the sounds recorded fall into the musical range of either an E, an F or a
G.


 So is the mystery solved? Not quite yet; there is much more
documentation to be done, more readings needed to confirm their theories. But
this resourceful team of scientists is hopeful that one day they will be able
to prove their premise, and when they do they can proudly claim that they
accomplished it by the seat of their pants.
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