Science by the seat of the pants

Sliding down a sand dune on your derriere might seem a bit undignified for a Caltech professor. But for Melany Hunt, it’s all in the name of science.

The professor of mechanical engineering wants to know why certain large desert sand dunes give off a loud, droning sound when the dune avalanches, a scientist skims down it, or a strong wind blows. While “booming dunes” have been known for centuries, their cause remains a mystery.

Most believe it’s friction—grains of very dry sand rubbing together. But that’s only part of the story, Hunt suspects, noting that the noise continues after the movement has stopped. Further, the sound is different in winter than in summer.

These intriguing questions tie into Hunt’s research on the flow of particulates and granular materials, including the natural environment of sand and debris flows. Thus, she has spent the past few summers investigating booming dunes as a mentor with Caltech’s SURF (Summer Undergraduate Research Fellowships) program, which brings college students from various schools to campus to conduct research with faculty members.

Several times each summer, Hunt, her students, and Professor of Mechanical Engineering Chris Brennen trek to Death Valley or Mojave, California. There they slog up to a dune’s crest, carting a radar unit, geophones (a type of microphone), and lots of water to combat the 100-
degree-plus weather.

The group works to confirm Hunt’s theory that the sound is a resonance effect, like when a string is plucked on a musical instrument. Over time, the dune absorbs rain and a crust of hard, moist sand forms some two meters (3.3 feet) below its face. When the dune’s surface is disturbed, friction between the dry sand grains creates noise that reverberates back and forth with the hard 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, and the loudest dunes are the tallest and steepest. Hunt also believes the noise differs by season due to varying amounts of moisture in the sand.

Using radar, the group probes for wet sand below the dune’s surface; the geophone captures the sound as students skid down its slopes. To view (and hear) a QuickTime movie of students sliding down a dune, visit www.prettypixel.net/Dunes/index.html.
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