By Elise Kleeman Staff Writer
San Gabriel Valley Tribune [West Covina CA]
PASADENA - In about 30 of the world's deserts, the shifting sands create a booming noise that has baffled scientists for decades.
Early explorers imagined the strange rumbling sounds - roughly an octave and a half below middle C - as the cries of a buried horseman, or the bells of an underground convent.
Others have described it as the sound of musical instruments, or the drone of an airplane. Exactly how it happens, though, has long been a mystery.
"It's a really remarkable and weird phenomenon," said Christopher Brennen, a Caltech mechanical engineer. "When sand squeaks, we call that chirping: for example, when you walk along dry sand at the beach. But the booming of these dunes is different."
With ground-penetrating technology, cobbled- together sampling tools and some help from the seat of their pants, Brennen and his colleagues believe they have found the key to the sand's deep voice.
They have been studying the rare singing sands at two nearby dune fields: Kelso Dunes in Mojave National Preserve, and Dumont Dunes 30 miles north of Baker.
One requirement for their music, the researchers have long known, is for the sand to be on the move.
This can happen naturally as winds pile sand up one face of the dune until it avalanches down the other.
But to make the desert boom on command, the researchers have adopted a decidedly unprofessional-looking technique: climbing to the dune peak hundreds of feet above the desert floor and scooting down on their behinds.
If conditions are right, the result is the same.
"You can feel it vibrate through your fingers and your toes when you stand," Brennen said. "The whole dune vibrates."
French scientists had theorized that the booming was caused by scores of similarly sized sand grains rubbing together as they rolled. The bigger the sand grains, they believed, the lower the sound.
But samples that the Caltech team collected showed that that hypothesis "didn't really make sense," said mechanical engineer Melany Hunt.
"That's not what we think happens on the sand dune," she said. "The frequency that we hear ... really is determined by the characteristics of the dune itself, not just by the grain sizes."
Hunt compared the dunes to her daughter's cello.
"In the cello, you're strumming the string but it's the whole instrument that's vibrating," she said. "We think it's similar in the dune."
The breakthrough for the Caltech team came when they used ground-penetrating radar and other imaging techniques to spy on what was happening beneath the desert surface.
They found that although sound travels slowly through the top layer of sand in the dune, "when you go down to a depth of six feet, you find there is kind of a hard layer that has a much higher speed sound," Brennen said.
That layer works to reflect sound waves back toward the surface, he said.
As the noise of the tumbling sand grains bounces around within the top-most layer of sand, Brennen said, certain low frequencies appear to become amplified, creating the mysterious boom.
This speaker effect can only happen, Hunt said, if the dunes are enormous and bone-dry.
The deserts therefore boom their best in the scorchingly hot summer months, making the research hot, sweaty work - perhaps the one drawback to sliding down sand dunes in the name of science.