The Dunes That Roar
Dumont Dunes, Calif.
In his 1923 book, Tales of Travel, George Nathaniel Curzon sought to determine why some large desert sand dunes made haunting, roaring sounds.
Chinese writers, he noted, had observed the bizarre phenomenon as early as the ninth century. Charles Darwin had witnessed it in Chile. Others had noted dunes in Nevada that made a sound "like that from telegraph wires fanned by a breeze," and similar dunes in Hawaii, Libya, and South Africa. The sounds had been objects of worship, mythmaking, and hopeful explanations. Some thought that they were made by sand coursing over hollow rocks, or that they emanated from submerged volcanoes, or even — according to some Arab desert dwellers — from the bells of a subterranean convent.
"The voice of the desert," Curzon reported, "speaking in notes now as of harp strings, anon as of trumpets and drums, and echoing down the ages, is invested with a mystic fascination to which none can turn a deaf ear."
To hear the sounds himself, he had slogged through intense heat and forbidding terrain. I understand something of the arduousness of his exertions as I rise at 4:30 a.m. with a research team from the California Institute of Technology, and hightail it 200 miles into the desert to scramble up immense dunes under a merciless sun so that we, too, can hear them.
Dumont Dunes, which border Death Valley National Park, extend for miles, and rise to more than 300 feet at an angle of 31 degrees. Any greater a pitch, and sand cascades down them. Wind draws sand to them from dry lake beds, through corridors between mountains in the basin-and-range formations that extend from Colorado to the Sierra Nevada. Music has been heard at these dunes, and the researchers are here today trying to create the sounds that nature normally makes, unaided. I am going to be, I have been informed, one of the sand movers.
"We don't walk straight up those, do we?" I ask Melany L. Hunt, a professor of mechanical engineering at Caltech whose dune research with Christopher E. Brennen, a professor of mechanical engineering there, has brought them and student assistants here during the last four summers.
"We have done," Ms. Hunt says, who appears impervious to the blazing heat, but she blessedly leads a slightly more circuitous route up one of the dunes. Footholds are still demanding; eventually we must clamber on all fours using our hands as grappling hooks.
"Just take your time," shouts Nathalie Vriend. The ebullient Dutch graduate student is a natural as manager of this research expedition. Nothing but a direct manner like hers could persuade anyone to scramble up such a hill umpteen times in one day.
Atop our dune, she and her colleagues break out long probes to collect sand samples and measure moisture content at various depths. Angel Ruiz-Angulo, a graduate student in mechanical engineering from Mexico, perches duneside and implants two seismic recording devices — geophones — beneath the sand.
"Sliders, get ready," shouts Ms. Vriend. She, Ms. Hunt, and I sit alongside Vala Hjorleifsdottir, a seismology doctoral student from Iceland, and Natalie Becerra, a mechanical-engineering undergraduate from Los Angeles.
And, we're off, hauling ourselves down the dune. (A word of warning: If ever called on to assist in research of this kind, do not wear short shorts or thong underwear.)
At first we hear halting sounds, as of a rubber boot rubbed along a thick, rubber sheet. But then the dune, for 20 or 30 yards in front of us, emits a deep, loud, sustained resonation, like a huge double bass, bowed; after we stop, the reverberation travels on ahead for several seconds.
Ms. Hunt and Mr. Brennen inherited their project from three Caltech colleagues whose research began in the 1970s. The booming interests researchers because it raises practical and theoretical issues. Ms. Hunt studies the flow of particulate materials saturated with liquid, such as slurries of mining debris.
"If we are able to model the dune phenomenon mathematically," says Ms. Hunt, "it may help in other circumstances, in understanding other flows," including the flow of pumped oil laden with solids that quickly erode pumps and pipes.
Knowing how dunes behave is also crucial in trying to stabilize or suppress the movement of sands that encroach on populated areas.
"But," says Ms. Hunt, "part of this is just a basic-science issue — wanting to understand what's happening."
So, what is?
The first clue, she says, is that sands that have been described as "singing," "sounding," "rumbling," "musical," "barking," and "moving" are rare. They have been found in only 30 locations.
Ms. Hunt and her colleagues have their theories. They have deduced that in dunes that boom, an upper layer of dry sand, about six feet thick, lies on top of a parallel, harder surface that is cemented by the little local rainfall. They have confirmed the presence of that harder surface using ground-penetrating radar as well as probes. The sand particles in booming dunes are "well sorted" — of roughly the same size due to erosion over time — so that they strike each other at roughly the same frequency. The sound of their collisions resonates against the harder surface, and is amplified to an extent determined by how fast the wave is propagated, and how thick the submerged hard surface is, says Ms. Hunt.
The relationship of loose particles to harder surface resembles that of, say, vibrating strings to the body of a cello that magnifies the vibrations.
Even at sites where the sound occurs, not all slopes produce it. High, steep dunes whose slopes often cascade are most likely to boom, says Ms. Hunt.
Recently, a team led by Stéphane Douady, of France's National Center for Scientific Research, announced a theory, not yet published, that dunes sing because particles of sand, coated with silicon, iron, and manganese while in an ancient ocean, resonate when they rub together while cascading down dunes.
Ms. Hunt objects that the thesis does not explain why only some dunes resonate, and why the resonance keeps spreading once the sand stops. Nor does it explain why only large dunes boom.
Three hours after mounting the dunes, Ms. Vriend signals to my great relief that it is time to head to Baker, population 390, and the Mad Greek, the research team's favorite watering hole.
There, I stick my head under the bathroom faucet, but get no immediate relief. It becomes apparent, though, that the ears may serve as catchments for extraordinary amounts of particulates, and that the nose really does act as a filter.
The dune sliders sit, weary and sand-caked, as Ms. Hunt treats them to swirling-brown-and-green fig shakes, volcanic with soft-serve ice-cream and Reddi-wip. As gyros arrive, the researchers fall to recounting dune lore.
They sing the praises of the classic, still-standard sand-dune text, The Physics of Blown Sand and Desert Dunes (1941) by R.A. Bagnold, a brigadier in the British army whose intimate familiarity with desert conditions enabled him to lead his British troops to crucial victories in North Africa during World War II.
We learn that in Death Valley National Park, at certain full moons, modern-day dune idolators gather at midnight to slide on their bottoms down huge dunes — naked. And Mr. Ruiz-Angulo tells of an industrial-racket classic by the band Nine Inch Nails — "The Wretched," — that incorporates, in its opening throbs, the Dumont Dunes' booming.
This is not, to be sure, what Marco Polo had in mind in the 13th century when he described dunes near the Caves of the Thousand Buddhas in Dunhuang, China. "Sometimes you shall hear the sound of musical instruments," he reported, "and still more commonly the sound of drums."
http://chronicle.comSection: Notes From Academe
Volume 52, Issue 11, Page A64
