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Sand Dunes' Sounds Mystery (Booming Dunes)

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Why Sand Dunes Go Boom

Stefan Lovgren
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 bottoms—literally doing science by the seat of the pants—Hunt 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."

Age-Old Mystery

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, Hunt—with her research colleague, mechanical engineering professor Chris Brennen, and her undergraduate students—has 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."

Strong Vibrations

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," Brennan, Hunt's research colleague, said.

Landslide Flows

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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news.nationalgeographic.com/news/2004/10/1020_041020_sand_dunes.html
Link is dead. The MIA article (quoted in full above) can be accessed at the Wayback Machine:
https://web.archive.org/web/2005040....com/news/2004/10/1020_041020_sand_dunes.html
 
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CalTech professor Melany Hunt's webpage provides this overview of her sand dune sounds research.

Booming Sand Dunes
In approximately 30 known locations around the world with large sand dunes, an avalanching of sand is accompanied by a loud droning or booming sound, which is not a noise composed of many frequencies but instead contains a dominant audible frequency and several higher harmonics. The sound can be heard after a naturally occurring slumping event or triggered by forcing sand down the leeward face of a large dune. In the later case, the dune will continue to boom and vibrate even after the sand has visibly stopped moving. Field measurements show that the frequency ranges from 75 to 110 Hz depending on the desert location and time of the year. Our measurements suggest that the physical features (such as a moisture barrier) of the sand dune plus the characteristics of the shearing on the surface may contribute to a wave-guide phenomena that results in a resonate behavior at a characteristic frequency. We have made extensive measurements in several dune locations using seismic refraction techniques and ground penetrating radar.
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The webpage also lists this relevant publication:
M.L. Hunt & N.M. Vriend, Booming sand dunes, Annual Rev. Earth Planetary Sciences, 38, 281-301 (2010).
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SOURCE: http://www.hunt.caltech.edu
 
Here are two of the research reports Professor Hunt has co-authored on the phenomenon. More publications are listed at:
https://www.researchgate.net/scientific-contributions/M-L-Hunt-47815299
Booming Sand Dunes
Article in Annual Review of Earth and Planetary Sciences · May 2010

Abstract
“Booming” sand dunes have a remarkable capacity to produce sounds that are comparable with those from a stringed instrument. This phenomenon, in which sound is generated after an avalanching of sand along the slip face of a dune, has been known for centuries and occurs in at least 40 sites around the world. A spectral analysis of the sound shows a dominant frequency between 70 and 110 Hz, as well as higher harmonics. Depending on the location and time of year, the sound may continue for several minutes, even after the avalanching of sand has ceased. This review presents historical observations and explanations of the sound, many of which contain accurate and insightful descriptions of the phenomenon. In addition, the review describes recent work that provides a scientific explanation for this natural mystery, which is caused by sound resonating in a surface layer of the dune.

FULL ARTICLE DOWNLOAD:
https://www.researchgate.net/publication/45500727_Booming_Sand_Dunes/download


The waveguide theory for booming sand dunes
N.M. Vriend and M.L. Hunt

Abstract.
Sand on the leeward face of a large desert dune may create a persistent, low-frequency sound during a slumping event or natural avalanche. The sound may last for several minutes and be audible at far distances. The current manuscript describes the waveguide model and presents the mathematical derivation for the booming frequency from the waveguide model. The spatial and temporal variations of the waveguide influence the ability of a dune to produce the mysterious sounds of the desert.

FULL ARTICLE DOWNLOAD:
https://www.researchgate.net/public...eguide_theory_for_booming_sand_dunes/download
 
Would different mineral composition of sand in desert dunes vs sand on beach affect the propensity of desert dunes making the sounds? Just wondering (OP article) why the sand Hunt's students collected in jars made a burping noise while the beach sand did not. This would not be attributable to size of dune as the samples were in jars.
 
brownmane said:
Would different mineral composition of sand in desert dunes vs sand on beach affect the propensity of desert dunes making the sounds? Just wondering (OP article) why the sand Hunt's students collected in jars made a burping noise while the beach sand did not. This would not be attributable to size of dune as the samples were in jars.
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I don't know. I didn't find any explanation for the burping sound made by sand collected in a jar.
 
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