Singing Sands—Nature’s Boom Boxes

[Singing sands] at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms—Marco Polo, 13th Century Explorer

Certain sand dunes, particularly when disturbed, will occasionally emit a loud, low-pitch rumble that lasts up to 15 minutes and can be heard up to 6 miles (10 kilometers) away. Some dunes are known to do it regularly, even daily. The sounds have been variously described as roaring or booming, singing and musical (e.g., kettle drum, zither, tambourine, bass violin, and a trumpet). They have also been compared with the sound of a foghorn or low-flying propeller-driven aircraft . You can listen to them here, or here.

This phenomenon was reported from the Middle East for more than 1500 years and appears in Chinese literature from as early as the ninth century. Marco Polo was convinced that evil spirits caused them.

According to Kenneth Chang of the N.Y. Times, “The dunes at Sand Mountain in Nevada sing a note of low C, two octaves below middle C. In the desert of Mar de Dunas in Chile, the dunes sing slightly higher, an F, while the sands of Ghord Lahmar in Morocco are higher yet, a G sharp.”

"Singing dunes constitute one of the most puzzling and impressive natural phenomenon I have ever encountered," Bruno Andreotti, scientist at the University of Paris, told LiveScience writer Michael Schirber. To try and uncover the underlying nature of these mysterious sounds, Andreotti took equipment out to the Atlantic Sahara in Morocco, one of only 35 known places where the mysterious natural music can be heard. Andreotti and his team studied one of the large crescent-shaped dunes, or barchans, which spontaneously sings all year long - sometimes two or three times an afternoon, if windy enough.

Singing sand, whistling sand or barking sand is sand that produces sounds of either high or low frequency under pressure. The sound emission is usually triggered by wind passing over dunes or by walking on the sand. The sound is generated by shear stress.
Certain conditions have to come together to create singing sand:

• The sand grains have to be round and between 0.1 and 0.5 mm in diameter
• The sand has to contain silica
• The sand needs to be a certain humidity

The most common frequency emitted seems to be close to 450 Hz.
All sound-producing desert dune sands are medium grained, and very well sorted to well sorted, but so are many non-musical desert sands. The sand must be dry, because as little as 0.1% moisture makes a marked difference in sound producing capability, and 1% moisture permits only feeble sound production.

It was earlier suggested that the sound is produced by a piezo-electric effect: the production of electrical currents by quartz grains under mechanical stress. It was more recently shown that a booming sound is produced by the internal shearing of sand grains during avalanche down a dune slope. The overriding of superjacent layers of sand grains provides the energy of sound production. Others have since suggested "that two independent mechanisms must be at work ...squeaking sand suggests a simple mechanical explanation of sound production ...in both wet and dry sand. Sound production of booming sand appears to be related to mechanical coupling between grains".

Whatever internal mechanisms exist, if sand of an appropriate grain-size and smoothness is naturally or artificially put into motion, sound will emanate and will in some instances provide an observer with vibrations which can be felt through the body.

The relative rarity of booming desert sands throughout the world indicates that they form or exist under special and limited conditions. The main requirement for the production of sound by desert sands is likely an ability for the wind to produce well-rounded, extremely smooth, frosted sand grains. Check here for a list of singing dunes in North America, with good descriptions.

Apparantly, a dune isn’t even necessary to cause the “singing”, according to a team of scientists from the United States, France and Morocco who say that “collisions between sand grains cause the motions of the grains to become synchronized. The outer layer of the dune vibrates like the cone of a loudspeaker. The particular note depends primarily on the size of the grains.” (N.Y. Times, 2006). The scientists shipped sand from a Moroccan desert to a Paris laboratory and reproduced the singing by pushing the sand around with a metal blade.

The most beautiful dune tune comes from the sands of Oman according to Dr. Stéphane Douady of the National Center for Scientific Research in Paris. “Very pure sound,” Dr. Douady said. “This one is really singing.”


References:

Fairbridge, R. W. 1968. Encyclopedia of geomorphology. New York: Reinhold Book Corporation.

Haff, D. K. 1979. Booming sands of the Mojave Desert and the Basin and Range Province, California. Caltech Internal Report NSF PHY7683685. 26 p.

Humphries, D. W. 1966. The booming sand of Korizo, Sahara, and the squeaking sand of Gower, South Wales -- A comparison of the fundamental characteristics of two musical sands. Sedimentology. 6: 135-152.

F. Nori, P. Sholtz, and M. Bretz, “Sound Producing Sand Avalanches” in Scientific American Vol. 277, No. 3, p. 84 (September 1997): http://www-personal.umich.edu/~nori/booming_sand.html

Lindsay, J. F., D. R. Criswell, T. L. Criswell, and B. S. Criswell. 1976. Sound-producing dune and beach sands. Geological Society of America Bulletin. 87(3):463-473. Boulder, CO: Geological Society of America (GSA), March 1976.

Nina Munteanu is an ecologist and internationally published author of novels, short stories and essays. She coaches writers and teaches writing at George Brown College and the University of Toronto. For more about Nina’s coaching & workshops visit www.ninamunteanu.me. Visit www.ninamunteanu.ca for more about her writing.