Dersert of Mingsha Mountain in Duhuang, summer, China. On it can the people see the Dunhuang City Singing sand, also called whistling sand or barking sand, is sand that produces sound. The sound emission may be caused by wind passing over dunes or by walking on the sand. 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 at a certain humidity. The most common frequency emitted seems to be close to 450 Hz. There are various theories about the singing sand mechanism. It has been proposed that the sound frequency is controlled by the shear rate. Others have suggested that the frequency of vibration is related to the thickness of the dry surface layer of sand. The sound waves bounce back and forth between the surface of the dune and the surface of the moist layer, creating a resonance that increases the sound's volume. The noise may be generated by friction between the grains or by the compression of air between them.[1] Other sounds that can be emitted by sand have been described as "roaring" or "booming"
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