Park Mysteries
Rhythms of the DesertBy Scott Kirkwood Physcists are starting to unravel the mystery of dunesong produced by the Kelso Dunes in Mojave National Preserve.Some say it sounds like a freight train while others liken it to a low-flying airplane coursing through the sky, and still others compare the sound to foghorns, cannon fire, and drums. But nearly everyone who hears the low growling thunder of the Kelso Dunes for the first time has a hard time believing a few grains of sand can cause such a racket.
Most of us equate deserts with quiet desolation occasionally interrupted by the haunting sounds of wind whistling over a barren landscape, but in Mojave National Preserve and a handful of other deserts throughout the world, the right conditions prompt “dunesong” to fill the air. Scientists have spent years investigating the physics behind it all, and they’re finally beginning to make some headway.
It’s a mystery that’s been well preserved: Marco Polo wrote of evil spirits that fill the air with sound as long ago as 1295, and references can be found in Charles Darwin’s 1889 Voyages of the Beagle, and T. E. Lawrence’s Arabian Nights, published in 1915. At least 35 dunes have been identified for their lyrical abilities, stretching from the Americas to Africa, Asia, and the Hawaiian Islands. Dozens of scientific journals have published articles by physicists devoting a surprisingly serious level of attention to the matter—employing electron microscopes, schematic diagrams, and complex equations with strings of Greek symbols—all aimed at unlocking the secret of the dunes.
“I was part of a team of retired scientists that had been investigating dunesong since 1992, and it’s still an unexplained phenomenon as far as I’m concerned,” says Evan Evans, a former nuclear physicist. “We formed a club to solve some of the unsolved mysteries of science and that was the first one we tackled. Unfortunately I have to report it defeated us.”
“We’d climb to the highest ridge along Kelso Dunes and start sand avalanches down the steeper, softer face of the dune, where sand is toppled over the ridge by the wind,” says Evans. “These avalanches generated a loud 90-hertz tone, not unlike a low-flying B-17 bomber. We recorded the sounds using hydrophones buried in the sand [and eventually] discovered how to generate sound in the lab, using grains of sand that we’d gathered from various singing and silent dunes.”
Their work in the lab revealed that most finely sorted sands produce a noise of some sort, but it’s nearly impossible to detect that noise above the sound of the wind. In a quiet, controlled environment of a laboratory, microphones can record the sounds of friction produced by grains of sand taken from almost any locale. But the squeaking of sand under your feet at the local beach is quite different from the booming of the Kelso Dunes.
“The sand grains at Kelso Dunes have been blown across the desert from Soda Dry Lake, about 20 miles away,” says James Woolsey, chief of interpretation at Mojave National Preserve. “In the process of covering that distance, the really fine materials get blown into the atmosphere, and the bigger grains don’t make it all the way there, but the particles that are about the same size complete the journey, and get rounded off so they’re [nearly identical].” Consistent size and shape of the grains is one key ingredient in this musical score: Humidity levels and moisture content of each grain are also important. On days when the sky is cloudless and the wind is still, the sun-baked grains produce the loudest result.
“I once started a sand avalanche at the top of a dune, and the conditions were just right—it was like melted chocolate flowing down a hill in slow motion,” says Woolsey. “The sand kept picking up all the other sand around it, so by the time it got to the bottom it was a huge 40-foot wave of sand going down, and you could feel the whole dune vibrating.”
Those vibrations are the key part of the process, according to Bruno Andreotti, a scientist from the University of Paris. Andreotti recorded sounds from the Atlantic Sahara in Morocco, and determined that surface waves on the sand mimic the vibration of a huge loudspeaker. His article in a 2004 issue of Physical Review Letters correctly predicted the maximum volume of the singing to be 105 decibels (about as loud as a power mower or power saw) at which point the sand grains vibrate off the surface and become mute once again.
Although dune buggies and other vehicles can damage the dunes enough to silent the sands, visitors are welcome to slide down a dune’s face as much as they like, doing their best to start a tidal wave of noise. For those visitors who aren’t fortunate enough to visit on a day when conditions are ideal, the Park Service plays recordings of some of the dunes’ “greatest hits” in the nearby visitor center at Kelso Depot.
Not able to make it out to Mojave? Visit www.npca.org/sand_dunes to listen to 30 seconds of dunesong from your desktop.
Scott Kirkwood is senior editor for National Parks magazine.
