Marco Polo’s fantastical tales of his travels were often so surreal that many discount his stories as fabrications. Yet kernels of truth lurk in these tales, like the story of the singing sands.
Polo writes of strange experiences in the "vast desert" near the town of Lop in the greater Gobi region. The nights were haunted with voices said to be from demons or spirits looking to lure people from the road. Even during daylight hours, these spirits "fill the air with the sounds of all kinds of musical instruments," writes Polo.
Though story may misattribute the source of the eerie song, many dunes around the world are known to sing, boom and even burp. Now, researchers based at the California Institute of Technology describe the seismic waves that ripple through dunes and give them a voice, reports Jennifer Ouellette for Gizmodo.
Many scientists have puzzled over the curious phenomena in the past, identifying various conditions important to produce the eerie tunes. The sand must be extremely dry and made up of round and silica-rich grains between 0.1 millimeters and 0.5 millimeters in diameter.
About a decade ago, after her team set off a sand avalanche in Morocco, researcher Stephane Douady also discovered that the size of the sand grains affects the tone.
But according to the researchers, the songs have been likened to a "hum," a "throaty booming," and even "burping." So how is it possible for the sands to make so many different noises?
The key is that different kinds of seismic waves create each of these various sounds, according the recent study published in the journal Physics of Fluids.
To measure these seismic waves, the team measured waves traveling through large dunes in California’s Mojave National Preserve and Death Valley National Park with instruments called geophones. They realized that the type of wave produced distinct sounds.
One type of wave, called a primary wave or P-wave, produces the booming sound. These waves are powerful and can travel through the entire dune. On the other hand, so-called "Rayleigh waves" only spread across the dune's surface. This movement causes the burping sounds.
Although sand is solid, a whole mass of tiny grains moving together acts a lot like a liquid, Ouellette writes. Douady suspected that the grains together act as a speaker that amplify vibrations during an avalanche.
While the work takes some of the mystery out of the singing dunes, it does nothing to lessen the wonder of experiencing it in person.
“It feels like your whole body starts to vibrate,” Vriend tells 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.”