![]() The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. These regions are known as compressions and rarefactions respectively. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.īecause of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. As the tines of the fork vibrate back and forth, they push on neighboring air particles. This is what characterizes sound waves in air as longitudinal waves.Ī vibrating tuning fork is capable of creating such a longitudinal wave. The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The singing sands of the Gobi and other deserts aren’t caused by spirits, but they are beautiful enough to uplift the spirits of those lucky enough to hear them.Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. The sound waves get trapped in what is something like a giant sound box, reflecting and amplifying the sound of the tumbling grains of sand. You’re not likely to hear the dunes sing after a rain.īut it’s the structure of the dune itself that makes the singing loud enough to “startle the silence of the desert.” Hunt explains that at some depth within the dune there is a very hard layer, and this layer reflects and amplifies the sound. Whether or not this is enough to create a song depends on several things. ![]() When innumerable grains of sand tumble down a dune in an avalanche, or even when sand is displaced by humans, such as Marco Polo and his companions or Hunt and hers, the grains slide along the face of the dune and jostle against one another. It really is quite remarkable.” Dune Structure ![]() You can feel the vibrations with your hands. “We would have a group of people, and we'd all start at the top and slide down together on our backsides, pushing sand down with our hands and feet to create an avalanche,” she explains. ![]() When Hunt’s team wanted to do some studies, they goosed the dunes a bit. But in all the times we went out there, that was pretty unusual.”īut if you’re there to study it, you can’t wait around to get lucky. “We've been out there a couple times when maybe the wind kicks up and starts a small avalanche on the side of the dune,” says Hunt. “If we can explain why it happens, we should also be able to explain why it doesn't happen in other places, or other times,” says Hunt, a professor of mechanical engineering at Caltech.Ĭatching a dune mid-melody is tricky, though. That’s why Melany Hunt and her team set out to discover what makes sand dunes sing. Musical dunes can be found in the Sahara, the Mojave, and several other deserts around the world. But it wasn’t until the 21st century that the mystery was (mostly) solved. Theories of restless dead beating drums were replaced by the movement of underground water or breezes blowing in just the right way across the sands. In the years following Polo’s historic trek across the Gobi, various theories about what caused the sounds were put forward. (It’s worth pointing out that these “singing sands” are doing something quite different - and far more awe-inspiring - than the squeaky sound you sometimes hear when you walk barefoot on the beach.) Though singing sands often sound like the cello section of an orchestra warming up, they can make a variety of pitches and patterns. Scientists call it singing or booming, or less often whistling. A Bagnold, an English explorer who led many desert expeditions in the early 20th century, poetically described the phenomenon as, “the great sound which in some places startles the silence of the desert.” The sands of the Gobi desert do make some bizarre and hauntingly beautiful sounds. Polo wasn’t hearing anything supernatural, but he wasn’t imagining it, either. And sometimes you shall hear the sound of a variety of musical instruments, and still more commonly the sound of drums." … Even in the day-time one hears those spirits talking. "… when travellers are on the move by night, and one of them chances to lag behind or to fall asleep or the like, when he tries to gain his company again he will hear spirits talking. When Marco Polo crossed the Gobi Desert in the 13th century, he thought he heard the voices of restless spirits.
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