Booming Sands

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.