Talk about strange terrain. Sand dunes in Death Valley National Park and the Mojave Desert are known to make bizarre sounds that resemble human burps, and also to emit loud rumbling booming noises as well.
The sounds are triggered by sand avalanches, but for a long time, scientists didn't know exactly how the dunes produced the sounds. And they didn't understand why the booming sounds, which generally follow a series of burps, and always occur within a specific frequency range of 70 to 150 Hertz.
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But now, a trio of researchers have an explanation. In an article for the journal Physics of Fluids, they report that the booming and burping sounds correspond to the transmission of a class of different waves within the dune.
Nathalie Vriend, a former graduate student at California Institute of Technology who is now a research fellow at the University of Cambridge, and Caltech professors Melany Hunt and Rob Clayton, spent 25 days in the desert studying the mystery. They focused upon figuring out how the burping and booming sounds traveled through the sand, by using geophones, which are similar to microphones, to pick up acoustic vibrations. The waves actually move individual grains of sand and exert force, in the same way that sound can move molecules of air.
By measuring the wave propagation characteristics of the two sounds, which include the motion of grains and frequency and energy of the emitted sound, they discovered that the two sounds are different but related phenomena.
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As Vriend explained in a press release, the burping sounds correspond to a surface Rayleigh wave, travelling radially along the surface of the dune in a nonlinear manner. "This means that relations between these properties are complicated because of the influence of individual grains," she said.
The loud booming sounds, in contrast, originate from linear P-waves that travel volumetrically and bounce off internal layers inside the dune.
Even more surprisingly, the researchers figured out how to cause the booming sound by triggering an impulse on the dune surface.
"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.