Science by the Seat of the Pants
PASADENA, Calif. — Sliding down a sand dune on your derriere might at first take seem a bit undignified for a professor from the California Institute of Technology. But for mechanical engineering professor Melany Hunt, it's all in the name of science.
Hunt wants to know why many desert sand dunes give off sound--and a loud, droning sound to boot--whenever the dune avalanches, or a strong wind blows, or a scientist slides down its side. While the phenomenon has been known about for centuries (Hunt has a book, Tales of Travel, circa 1923, that mentions Marco Polo knew about it), what causes the sound remains a mystery. Most believe the answer is friction--tiny grains of sand rubbing together. But that's only part of the story, Hunt believes, noting that the sound continues even after the movement has stopped. And further, the sound a sand dune makes in winter differs from the sound it makes in summer.
Intriguing questions, says Hunt, and it ties into her research concerning the flow of particulates and granular materials, including the natural environment of both sand and debris flows. Which is why she has spent the last few summers investigating the phenomenon of sand dune sound as a mentor with Caltech's Summer Undergraduate Research Fellowships (SURF) program. (Every summer, the SURF program brings undergraduate students from various schools to Caltech to conduct independent research with faculty members.)
So several times each summer, Hunt, her research colleague, mechanical engineering professor Chris Brennen, and her students make the long drive to the Eureka Dunes in Death Valley, California, or the Dumont Dunes nearby, or to the Kelso Dunes in the Mojave National Preserve, CA. Once there, they slog up to the dune's crest line, carting a radar unit, geophones (a type of microphone), and lots of water to combat the common 100-plus degree temperatures.
The equipment is being used to confirm Hunt's theory about the loud sound that's generated--she believes it's a resonance effect, much like a string being plucked on a musical instrument. Over a long period of time, whatever rain that falls in this desert environment percolates into the sand dune, eventually forming a band of moisture some two meters (6.6 feet) down. In time this sand hardens, says Hunt, forming a hard, cement-like crust. When the sand on the surface is disturbed, friction between sand grains creates a noise that reverberates, back and forth, between the dry sand on the surface and the wet sand below.
"That may be why smaller dunes don't make sound," says Hunt, "because they haven't been around long enough to form that hard layer of sand." The minimum needed is about two meters of thickness, she says. The loudest dunes are the tallest and the steepest, those with a maximum 30-degree angle of repose; that is, the steepest the dune's face can be without collapsing. It's also the reason she believes the sound varies by the season, which affects how much moisture is in the sand.
Hunt and her students dragged the radar to the top of the dune and used it to confirm the existence of the band of wet, hard sand down below. The geophone was used to record the noise as the students slid down the dune.
Hunt plans at least one more trip to a sand dune sometime in September; intrepid reporters are invited to attend. Meanwhile, for a QuickTime movie, complete with sound, of students sliding down a dune, please see the website of Kathy Brantley, one of Hunt's former students, at http://www.prettypixel.net/Dunes/index.html.