Research team from California Institute of Technology (Caltech) and the University of Cambridge has explained the science behind the loud, rumbling "booming" or short "burping" sounds produced by sand avalanches from dune faces in Death Valley National Park and the Mohave Desert. Basically, one can say the dunes "sing", behaving as a perfectly tuned musical instrument, American Institute of Physics (AIP) reported on October 27, 2015.
The noises coming out of sand dunes are apparently in agreement with the transmission of a class of different waves within the dune, according to the newest study. The dunes produce frequencies in range between 70 and 105 Hz, with higher harmonics. The shorter burping sounds occur over a broader frequency span before the booming sounds which are significantly louder and monotonous.
Nathalie Vriend has studied the "singing" dunes phenomenon in details, as a Ph.D. student at Caltech in collaboration with Melany Hunt, a professor of mechanical engineering and Rob Clayton, geophysics professor.
"During approximately 25 individual summer field days, on very hot and sandy dunes in California, we probed booming dunes, and they slowly revealed their underlying physics to us," Vriend said.
Experiments and analysis has been focused on how the sounds travel through sand, according to Vriend. The scientists have measured the characteristics of wave propagation, including the motion of sand grains and frequency and energy of the emitted sound, to discover that booming and burping are actually two quite different, yet related, phenomena.
Researchers have used geophones to measure seismic vibrations within the ground: "The waves traveling through the dune move individual grains of sand, which exert a force on the geophone that we use for measurements," Vriend explained.
The study results showed that burping sounds travel radially and nonlinearly along the surface of the dune, a motion which in fact corresponds to a surface Rayleigh wave: "This means that relations between these properties are complicated because of the influence of individual grains," Vriend added.
On the other hand, the booming sounds result from the linear P-waves which travel volumetrically and get reflected from internal layers inside the dune.
A surprising discovery was that for both types of sound, the surface and volumetric wave signals are present with their own characteristics, however, the dominant signals are responsible for distinguishing the sounds.
Scientists have also been able to produce the natural dune resonance on one occasion, an occurrence that was previously unknown: "A blow of a hammer on a plate triggered a natural resonance – around the booming frequency – inside the dune, which is something we've never seen described in literature," Vriend said.
New discoveries could help explain some differences in measurements and interpretations regarding singing sand dunes made during the past decade.
"More broadly, seismic surveys for oil field exploration or earthquake investigations tend to rely on length scales that are usually much larger than those used by our study. Even if the study is done on a sandy substrate, the 'effective medium' response is recorded and individual grain interactions aren't usually relevant. Our work illustrates the dual behavior of wave propagation when scales are reduced to a length where small- and larger-scale wave propagation converge," she added.
Vriend is now a Royal Society Research Fellow within the Department of Applied Mathematical and Theoretical Physics at the University of Cambridge. Her research group is working on a variety of projects to probe and solve other mysteries of granular dynamics.
One of these projects involves exploring "the granular dynamics during avalanching and its influence on the origin of structure in sand dunes in greater detail. Our recent work involves using field and laboratory techniques to probe natural avalanching and sorting on large desert dunes in Qatar," Vriend concluded.
An audio file of linear and nonlinear wave propagation in sand dunes can be listened here.
- "Linear and nonlinear wave propagation in booming sand dunes" – N. M. Vriend, M. L. Hunt and R. W. Clayton – Physics of Fluids (2015) – http://dx.doi.org/10.1063/1.4931971
Featured image: Sliding down on the seat of your pants and creating a large sand avalanche on the 200 m (656 feet) high Eureka Dune in Death Valley National Park, California. Image credit: Nathalie Vriend
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