Certain sand dunes make their own desert music, singing, booming, or even "burping" -- a naturally occurring musical instrument. Scientists have discovered that these distinct sounds are each created by different types of waves moving through the dunes.
The 13th century explorer Marco Polo claimed in his writings that the sand dunes in the Gobi Desert filled the air "with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." He wasn't just being fanciful; reports of sand dunes squeaking, or booming, date back to 9th century China. There's even a famous "Singing Mountain" (Mount Ming-Sha-Shan) that booms like thunder when people slide down the sand. Even Charles Darwin noticed the phenomenon on his travels through Chile.
Now a research fellow at Cambridge University, Nathalie Vriend was a graduate student at Caltech when she first read about booming sand dunes in a science magazine, and she thought it would make an excellent PhD project, given her background in mechanical engineering and burgeoning interest in geophysics. She enlisted the aid of two professors, Melany Hunt and Rob Clayton, and together they trekked out to California's Mojave National Preserve and Death Valley National Park to study the phenomenon firsthand. They hoped their work would help explain certain discrepancies in past measurements of the singing sand phenomenon.
That's right: this isn't the first time intrepid scientists have ventured out into the desert to slide down sand dunes and record the resulting sound effects -- whether it be singing, booming or burping. Nearly 10 years ago, an international team led by Stephane Douady was studying the formation of crescent-shaped dunes in Morocco, and inadvertently set off an avalanche while scrambling up a dune. This produced a 100-decibel singing sound. They discovered that sliding down the dunes reproduced the same sound, which they duly recorded; they even managed to recreate it in doughnut-shaped sandbox back in the lab.
Sand is a form of granular media, meaning it can act as a solid, yet flow like a liquid when it avalanches. Douady suggested that the face of the dune acted as a kind of loudspeaker, amplifying the surface wave vibrations created by the avalanching sand. Individual grains collided about 100 times per second, creating a feedback loop of collisions tuned to a specific frequency. (Around 450 Hz is the most common frequency, although the dunes studied by Vriend et al. "sang" between 70 and 105 Hz.) The result was singing sand. But not just any sand will do. The grains must be round, and just the right size (between 0.1 and 0.5 mm in diameter), and must also contain silica. Nor can it be too humid.
That's why Vriend et al. conducted their field experiments over 25 days during the driest months: May through September. The mercury hit a scorching 47C on one of those days, which is why the team typically made their measurements between 3am and 4am. They used geophones to detect waves travelling through the dunes, similar to how microphones detect sound waves in the air.
Based on those measurements, they concluded that the different types of sounds the dunes produced are associated with different kinds of waves travelling through the sand dune. It's the so-called "P-waves" that are responsible for that booming sound; Vriend explained that they travel in volume and thus seem to spread throughout the entire dune. In contrast, "Rayleigh waves" seem to produce the burping sounds, and only spread near the surface of the dune. They are two distinct, though related, physical effects.
The team also discovered another surprising effect. They found they could trigger the dune's natural resonance by hitting a metal plate with a hammer, producing the telltale booming frequency.
Earlier this year, Vriend took a fresh batch of students to Qatar to study dune migration and avalanche dynamics, and they were thrilled to hear the booming effect firsthand. "It feels like your whole body starts to vibrate," Vriend told Gizmodo. "When you are standing away from the dune, it is really difficult to comprehend that such a small and thin avalanche creates such a loud sound that booms over the desert floor."
Dagois-Bohy, S., Courrech du Pont, S. and Douady, S. (2012) "Singing-sand avalanches without dunes," Geophysical Research Letters 39: L20310.
Douady, S. et al. (2006) "The song of the dunes as a self-synchronised instrument," Physical Review Letters 97: 018002.
Hunt, M.L. and Vriend, N.M. (2010) "Booming Sand Dunes," Annal Review of Earth and Planetary Sciences 38.
Vriend, N.M., et al. (2007) "Solving the mystery of booming sand dunes," Geophysical Research Letters 34: L16306.
Vriend, N.M., Hunt, M.L. and Clayton, R.W. (2015) "Linear and Nonlinear Wave Propagation in Booming Sand Dunes," Physics of Fluids, published online October 27, 2015.
Images courtesy of Vriend et al./Physics of Fluids.