Mystery Behind 'Burping' Sand Dunes Surfaces

Dunes residing in Death Valley National Park and the Mojave Desert have been known to act as instruments of sound, producing an array of loud, humming songs, and only recently have researchers uncovered the mystery behind the belch.

Maybe one of the planet’s most unusual natural instruments, dune facades create such sounds from the crashing sand sliding downward toward the foot of the hill. Depending on the varying amplitude and frequency, they can be classified accordingly.

“Booms” are loud, rumbling notes that range anywhere from 70-105 Hertz, while “burps” are much shorter bursts of noise that can reach a much wider range of frequencies. A Live Science report originally thought the different types and sizes of sand was the determining factor in whether the sand “burped” or “boomed,” and while some of that evidence still stands, a recent study by Nathalie Vriend finds that it’s the wave being sent through the sand that dictates the range of sound hit.

"During approximately 25 individual summer field days, on very hot and sandy dunes in California, we probed booming dunes," Vriend, then a Ph.D. student at California Institute of Technology, was reported stating in a ScienceDaily article. "They slowly revealed their underlying physics to us."

Vriend and her group put their focus on how the differing “booming” and “burping” waves traveled through the sand, measuring the morion of grains and the energy of the emitted sounds.

To do so, Vriend’s group used a geophone, an instrument that measures seismic vibrations within the ground, much like a microphone does with acoustical vibrations in the air.

During their study of the shifting dunes, Vriend’s team found that the “burping” sounds travel “radially along the surface of the dune in a nonlinear manner,” while the “booms” were “linear P-waves that travel volumetrically and are reflected from internal layers inside the actual dune."

Vriend’s study revealed that both “booms” and “burps” are two different types of acoustic sensations, which will leave physicists interested in learning more, but will also have those who study oil exploration and earthquakes looking into their studies. 

Understanding how paltry pieces, like sand, interact with sound waves could be a crucial contribution to advancing in fields such as revealing hidden oil reserves or forming a better concept of how seismic waves from an earthquake move through the Earth’s crust. 

http://www.weather.com/science/news/sand-dune-mystery-revealed