FROM THE ARCHIVES: singing sands

Perplexedjenluc_7In honor of Comic-Con, here's an early post from 2006 inspired by an episode of Dr. Who, about the phenomenon of singing sands. Sand: a marvelous substance!

here's a classic story line in Dr. Who, in which the time-traveling TARDIS crew crash-lands in the middle of the Gobi Desert and conveniently bumps into famed explorer/trader Marco Polo and his caravan. They are on their way to the palace of Kublai Khan. There's a great deal of convoluted plot, including a young Chinese woman on her way to an arranged marriage with an elderly man, and a devious Mongol warlord; also, Marco Polo becomes enchanted by the TARDIS (he calls it a "magic caravan") and confiscates the key, forcing Dr. Who and his entourage to travel with him to the Kublai Khan's stately pleasure dome. Along the way, they witness a strange desert phenomenon: singing sands.

One doesn't expect a high level of historical accuracy in a campy sci-fi series, but Marco Polo did claim to have journeyed across the Gobi Desert, and said the dunes filled the air "with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." It's a fairly rare phenomenon. Only about 30 dunes worldwide are musically inclined — the rest, apparently, are tone deaf — although astronomers and geologists suspect that the sandy hills on Mars might also be alive with the sound of music.

Marco Polo's storied adventures took place in the 13th century, but accounts of singing, or booming (and sometimes squeaking) sand dunes date as far back as 9th century China. According to an ancient manuscript, people used to climb Mount Ming-Sha-Shan (now known as the "Singing Mountain") on a special festival day, and slide down the sand, producing the sound of rolling thunder. Centuries later, Charles Darwin commented on singing sands while traveling in Chile.

Popular folklore has blamed underground rivers or even genies; Marco Polo blamed evil spirits.  Scientists have also puzzled over the phenomenon. There have been numerous hypotheses as to the cause, most centered on the notion that it comes from vibrations of the dune as a whole, not the individual grains of sand acting in concert (pun intended). For example, some surmised the effect was similar to how a violin bow moving across the strings produce a musical tone, with the sandy sounds coming from blocks of sand stick-slipping across the body of a dune. Another instrumental hypothesis likened the effect to how a flute produces a pure tone via resonating air in the hollow tube.

It turns out that neither of these hypotheses are right. A team of scientists — hailing form the University of Paris, the CNRS Lab in Paris, Harvard University, and the Universite Ibn Zohr in Morocco — have conducted field studies all around the world, supplemented with controlled lab experiments, and published their findings in Physical Review Letters. They found that the sounds come from the sliding motion of large-ish grains of dry sand acting together — a very active field of study known as avalanche dynamics. (You can listen to the nifty sounds they recorded from sand dunes in China, Oman, Morocco and Chile here. And if you want to tour the globe and visit those melodic dunes yourself, check out this map.)

Apparently, the team stumbled on their insight a few years ago by accident while studying the formation of crescent-shaped sand dunes in Morocco. As they scrambled up a particularly steep dune, they set off an avalanche, producing a 100-decibel singing sound. And they found they could reproduce the sound by sliding down the dunes with their legs swung out. (Yes! Just like those 9th century Chinese!) They took recordings back to the lab in France and managed to replicate the same sound in a donut-shaped sandbox.

Specifically, they were able to measure vibrations in the sand and air, thereby detecting surface waves emanating from the avalanche. According to head researcher Stephane Douady, the face of the dune seems to serve as a kind of loudspeaker and the surface waves produce the sound in the air. The waves result from collisions between individual grains — about 100 times per second, according to the lab measurements — that create a kind of feedback loop of synchronized collisions at a specific frequency, and voila! The dune begins warbling a sandy song. It just so happens that if sand is packed tightly, it can't move without expanding its volume. Douady thinks that a dune avalanche serves to compress and decompress air among individual sand grains, and this causes the singing sound. In fact, the grains themselves are unique: round, with a coating of silicon, iron and manganese.

That doesn't solve all the outstanding questions about sand, which is pretty fascinating stuff, from a physics standpoint. It's a type of substance known as granular material, since it acts both like a liquid and a solid: dry sand collected in a bucket pours like a fluid, yet it can support the weight of a rock placed on top of it, like a solid, even though the rock is technically denser than the sand. So sand defies all those tidy equations describing various phases of matter, and the transition from flowing "liquid" to a rigid "solid" happens quite rapidly. It's as if the grains act as individuals in the fluid form, but are capable of suddenly banding together when solidarity is needed, achieving a weird kind of "strength in numbers" effect. (Jen-Luc finds this metaphor rather inspiring, but she's feeling a bit emotional at present.)

Nor can physicists precisely predict an avalanche. That's partly because of the sheer number of grains of sand in even a small pile, each of which will interact with several of its immediate neighboring grains simultaneously — and those neighbors shift from one moment to the next. Not even a supercomputer can track the movements of individual grains over time, so the physics of flow in granular media remains a vital area of research. Among other agencies, NASA is funding work in this area.

The frivolous summer pastime of building sand castles might also be considered interesting physics. Everyone knows sand has to be wet to build sand castles — just enough to cause the dampened grains of sand to stick together via surface tension. (Jen-Luc Piquant has experimented extensively and found that the perfect ratio for an optimum castle is one pail of water for every eight pails of sand.) The water forms "liquid bridges" between the contact points of the grains, and the resulting tension creates an attractive force between them.

A few years ago scientists at MIT and Clark University studied sand in a rotating drum, observing that the sand would build up into a pile and reach a steep slope before collapsing. (I believe physicists call this self-organized criticality, which I've always thought to be a pretty nifty moniker.) They did this with both dry and damp sand, and found that adding even a tiny bit of water made the grains stick to each other more effectively, so that the pile could reach steeper angles and collapse less drastically than when the sand was dry. And just like sand castles collapsing on a beach in clumps, some of the "bridges" in the experiment stayed intact.

Quicksand is another special case, since it's a mixture of fine sand, clay and saltwater. At rest, it's little more than loosely packed grains of sand resting on top of water. But if some unfortunate soul happens to fall into quicksand and start flailing about, the movement transforms that delicate balance, turning it into a dense liquid soup. This means the victim sinks deeper, at which point the the water and sand starts to separate again into water-rich and sand-rich levels. The wet sand sediment becomes densely packed, trapping the victim. Fortunately, the average density of a human body is about 62 pounds per cubic foot, much less than the 125 pounds per cubic foot of most quicksand. So the trick to survival, apparently, is not to panic.  Instead, relax, stay still and stretch out on your back to increase your surface area, and then just wait until your legs pop free.

Studying quicksand could lead to insights into the shifting of wet soil underground during earthquakes that causes buildings to sink into the ground (soil liquefaction). The water-soaked soil "liquefies" because of the vibrations of the quake. The shock waves compress the soil faster than the water can escape, raising the pressure so that the water bears more of the load than the sand. And the buildings start to sink. For safety reasons, obviously scientists would like to understand this process better, perhaps developing better building foundations to prevent liquefaction.

My point is that you think you know a substance, it's so familiar, you see it every day, but somehow it keeps surprising you with unexpected or inexplicable behavior. That's how scientists feel about their ongoing relationship with sand and other forms of granular materials, which is looking like a pretty long-term commitment, thanks to this constant element of surprise. So while you're hanging out at the beach this summer, attempting to escape the sobering news of the day, take a moment to appreciate the intricate complexity that can be found in that collection of millions of grains of sand.

7 thoughts on “FROM THE ARCHIVES: singing sands”

  1. Hello, let’s not panic at the latest media report. All men have testosterone, and many men have way too much. The winner of the Tour De France should automatically be made President of France; it would give France true leaders. As you can see from today’s photos, I’ve been drinking vodka too.

  2. Joel may be a slacker, but he does pick up the tab most of the time, so he’s allright by me.
    Louise, that idea about making the winner of le Tour President of France is great.
    Please note that I have avoided adding any of the 1000+ jokes/comments regarding “excessive testosterone” that have come to mind over the past couple of days. This *is* a PG blog, isn’t it?
    Sand is fascinating stuff, and I speak to this from the perspective of a long-time sandcastler, including some this past June on the Jersey shore (there’s excellent sand at Wildwood Crest and Cape May). I’ve learned a lot from it.
    I can even make sand castles without the condiments of little green army men and popsicle sticks.
    bc

  3. “you know a substance, it’s so familiar, you see it every day, but somehow it keeps surprising you with unexpected or inexplicable behavior. That’s how scientists feel about their ongoing relationship with sand and other forms of granular materials, which is looking like a pretty long-term commitment, thanks to this constant element of surprise.”
    This is a nice sentiment Jennifer, and applies to almost everything: it’s how physicists feel about air, light, the vacuum, … We learn so much more every year, but there is always so much more to know.

  4. Calvin Goodrich

    When I was a kid, I lived for a couple of years with my grandparents who lived in a beach house near Santa Barbara, CA (MAN, was that a great time in my life.) During my time there, I noticed a curious thing: while walking on flat dry sand, if you scuffed your bare feet in the sand you could make noises not unlike the sound made by quickly rubbing two pieces of denim or corduroy together. (Being a somewhat largish kid at the time, my jeans made that kind of noise just walking around, whether on sand or not.) The noise obviously wasn’t nearly as loud as the 100 Db the UoP scientists observed in Morroco, but it was definitely audible to people around me. When you’re a kid, making funny noises is kinda your job.
    Never really thought anything of it at the time, but somehow this post reminded me of it.

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