Why Sand Dunes Go Boom
for National Geographic News
|October 20, 2004|
It sounds like a low-flying propeller plane or maybe the deep humming of an electric wire. The booming sound made by some sand dunes has been a mystery for centuries.
The sound is produced when sand on the surface of a dune avalanches. Scientists have long believed the friction between grains creates the strange noise.
But Melany Hunt, a mechanical-engineering professor at the California Institute of Technology in Pasadena, thinks there is something else at play too.
For the past three years she has been taking her students out to the desert of eastern California and Nevada. By sliding down the dunes on their bottomsliterally doing science by the seat of the pantsHunt and her researchers set the sand in motion to measure the sound waves it makes.
They have found that the sound continues even after the movement has stopped. Also, the sound that a sand dune makes in winter differs from the sound it makes in summer.
Hunt believes that when the sand on the surface is disturbed, the friction between sand grains creates a noise that reverberates back and forth between dry sand on the surface and wet sand below.
"That may be why smaller dunes don't make sound," Hunt explained. "They haven't been around long enough to form that hard layer of [wet] sand."
Records of the sound are centuries old. Hunt has a book, Tales of Travel, published in 1923, which mentions that explorer Marco Polo knew about it. The tribes of the Sahara in Africa are said to have thought God was speaking to them through the sand.
Hunt became interested in the phenomenon when a colleague played her recordings of the sound he had made 25 years ago.
Then a group of students went out to the Kelso Dunes near California's Mojave Desert. They brought back a jar of the booming sand. Shaking the jar, they found, produced a burping sound. This did not happen with sand taken from the beach, for example.
"I already had a sense that something was different about this sand than other materials we have studied," Hunt said.
Several times each summer, Huntwith her research colleague, mechanical engineering professor Chris Brennen, and her undergraduate studentshas made the long drive to one of four sand dunes in the California and Nevada desert.
The most recent trip took them to Dumont Dunes, north of Baker, California. There, the primary dune extends for a mile (1.6 kilometers), with smaller dunes scattered around it. The main dune is up to 400 feet (120 meters) high.
The team uses a combination of regular microphones and "geophones" (microphones placed underground) to record the vibrations both acoustically and seismically.
Then, Hunt and her students slide down the sand dune.
"We know from the geophone measurements that we can pick up the vibrations a good ways from where we have made the avalanching of the sand," she said. "It seems to be able to travel through the dune, and persist for up to a minute after we slide."
Hunt says the sound has a really pure frequency that one would associate with a musical instrument. "The question is, what sets up that frequency?" she said.
Until now the only explanation has been the rubbing of grains over each other. At the Kelso Dunes there is even a sign that informs people that the rubbing of the grain causes the booming sound they hear from the dunes.
But that didn't seem to fit Hunt's observations.
"You can feel these vibrations even where there is no shearing of the sand," she said. "And the frequency [of the vibrations] seem to change with the time of the year."
So what else is happening within the dune?
Using radar, Hunt and her students have confirmed the existence of a band of wet, hard sand some two meters (6.6 feet) below the surface. This is caused by rainfall percolating into the sand dune over a period.
"If you have some water at depth within this dune that reflects the radar signal, maybe it also reflects the acoustics signal," Hunt said. "Maybe what you're really having is this reverberation within the dune that's affected by a harder or wetter layer below that causes a reflection in the dry sand in the upper part of the dune."
Since the seasons affect how much moisture is in the sand, the wet-layer theory would also explain why the sound varies by the season.
"The sound is low and very discreet in frequency, and it's that discreetness that can really only come about as a result of some kind of resonance within the dune," Brennen, Hunt's research colleague, said.
The testing of the dunes may sound a little offbeat, but it ties into Hunt's research on the flow of particulates and granular materials, including the natural environment of both sand and debris flows.
A better understanding of the way granular material moves could lead to advances in industrial uses. It could also help scientists in predicting the paths of landslide flows, for example.
To Hunt, it's also a chance to solve an age-old mystery.
"People have reported this phenomenon for hundreds of years," she said. "This is about understanding better what happens in nature."
Brennan says the beauty of the sand dunes is part of the allure.
"It's a special experience to go out there on a summer morning and climb to the top of [a] 400-foot [120-meter] sand dune and start the sand moving and hear this incredible sound," he said. "It has a kind of ethereal wonder to it."
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