During the 13th century, when Marco Polo made his journey towards China, he heard humming and chants emanating from the sandy hills of the Gobi desert. The Venetian explorer described them as various musical instruments which he ascribed to evil desert spirits.
Even today, whirring melodies that sound like musical notes can be heard in several deserts worldwide. But unlike the legend, these sounds are not caused by spirits but by a phenomenon known as "singing sands."
Singing Sand Dunes
Around 20% of the Earth is covered by deserts, and many have dunes. Sand dunes are created when sand piles up on a large scale after being carried by the wind. These dunes come in various types depending on the shape of the pile.
Certain sand dunes produce a distinct low-frequency noise described as 'singing,' 'humming,' or 'booming.' These sounds hum at a frequency resembling a cello's musical notes or an aircraft's droning sound. They can last up to 15 minutes and be heard up to 6.2 miles (10 kilometers) away.
Catching the melody of a dune is a bit tricky, though, since it does not always happen. Some dunes produce sounds over 105 decibels, louder than a motorcycle, rock concert, car horn, or lawn mower.
The singing sand dunes can only be observed in about three dozen global areas, such as the Sahara, the Mojave, the Great Sand Dunes National Park & Preserve, and Mongolia's Badain Jaran Desert. This phenomenon was also reported in Nevada's Sand Mountain, UAE's Sand Dunes of Liwa, and California's Eureka Dunes.
Different sand dunes hum various tunes in the lower part of the frequency range of the cello. For instance, Nevada's Sand Mountain hum tones are between 60 and 70 Hertz (low B to C sharp). Meanwhile, the sands at Chile's Mar de Dunas sing at 87 Hertz (low F).
How Do Sand Dunes Produce Sounds?
Not all sand dunes produce good quality sounds, since certain conditions must be met for them to do so. First, the sand must be very dry, which is easy to achieve in a desert. However, even small amounts of moisture or humidity can prevent the dunes from producing sounds.
The sand particles must also have a uniform size, with an average diameter of 200 micrometers. The wind meets this condition, which lifts the particles within a certain size range, acting as a sieve for the sand.
The size and texture also affect the production of desert music. In particular, the sand particles must be spherical and smooth, which makes booming predominantly observed in dunes that contain a large amount of quartz.
During an avalanche, the sand grains rub against each other. The shearing of grains produces frictional energy, similar to the violin's bows. Meanwhile, the waveguide affects the sound behavior, which prevents the sound waves from escaping to the external environment and amplifies them through internal reflection. Finally, the dune contains a solid sublayer that reflects the sound waves and prevents them from scattering. As a result, the sound gets amplified on a large scale within the sand and finally moves into the atmosphere as a 'song.'
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RELATED ARTICLE: NASA Captures Unusual Circular Sand Dunes on Mars, Sparking Interest Among Astronomers Worldwide
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