It starts as a hum, barely audible above the howl of the winds shimmying between sand dunes. Then it builds. Within seconds, the sound resembles a World War II–era prop plane flying low overhead. But there’s nothing there. “It’s very mystical and eerie,” says Nathalie Vriend, a mechanical engineer and geophysicist at the University of Cambridge.
This resounding roar, called a boom, has intrigued desert explorers for centuries. Yet only recently have scientists demystified the source of booms and softer, shorter-lived burps that emanate from deserts’ sandy slopes. Much of the new insight has come from Vriend. Her extremely hands-on research, which includes sliding down towering dunes, has revealed that tumbling sand induces pressure pulses that course above and below the surface to produce a cacophony of peculiar noises.
Vriend wasn’t exactly born to be a desert explorer. She grew up in a Dutch village with a climate closer to Seattle than to the Sahara. At Caltech in 2004, she read a magazine article on desert booms and burps — and was inspired to do her Ph.D. research on the topic.
Between 2005 and 2009, Vriend and her colleagues made sporadic summer visits to California’s Death Valley and the Mojave Desert, departing campus with a couple of student volunteers at 4 a.m. to make the several-hours trek and complete fieldwork before the heat of the day. Vriend’s team induced booms by sliding down the dunes and triggering miniature avalanches. Microphones along the slope captured audible sound, while a network of 48 geophones — essentially microphones placed a few centimeters under the surface of the sand — tracked sound waves propagating underground.
In the October Physics of Fluids, Vriend’s team reports that booms and burps result from different types of sound waves. The large-scale tumbling of sand down a dune creates primary waves, or P waves, the pulses that travel fastest during an earthquake. The P waves get amplified as they spread through a dune’s interior and emerge as a loud boom. (Earthquake P waves have a frequency too low for human hearing.)
Burps are much easier to trigger. Just moving one’s hand through the sand shifts the grains enough to generate lower-frequency waves that crawl along the surface, the team found.
“It may not directly save the world,” Vriend says, “but it captures people’s imaginations.” Now based in England, Vriend is studying the physics of avalanches, both the sand and snow variety.