There are about 30 locations around the world where sand dunes emanate a booming tone that lasts up to several minutes. The science behind these booming sand dunes is not fully understood, but researchers have noted several characteristics of the dunes that could explain this natural sound phenomenon.

Sand on a dune possesses energy in the form of gravitational potential energy. The greater the height of the sand dune, the more gravitational potential energy the sand on top has. When the sand on top of a steep dune is disturbed, it slides down the slope, and gravitational potential energy is transformed into kinetic energy. It is this kinetic energy of the falling sand that produces the booming sound. As the sand particles bounce against each other, some of their kinetic energy is transformed into acoustic energy -- or sound, waves of vibrations transmitted through a medium.

However, not all avalanching sand dunes produce booming sounds. In order to produce a musical tone, the dune needs to have sand grains that are particularly round and smooth and almost uniform in size. In addition, the dune needs to have a top layer of dry sand over a layer of wet sand. When rainfall seeps into the sand, the sand becomes damp and holds together due to the surface tension of water. (Surface tension is an effect of cohesion -- the tendency of water molecules to be attracted to other water molecules.) Over time, the top layer of sand is heated by the sun, causing it to dry out. When there is less water in the sand, there is less cohesion and therefore the sand is more prone to movement. When the dry top layer is disturbed, the loose sand avalanches while the wet sand beneath it stays in place. The interaction of the dry sand particles hitting against each other produces the vibrations for the booming sound.

The tone of the sound is determined by the frequency of the vibrations. Frequency, the number of vibrations per second, is related to pitch: A low-frequency vibration produces a low-pitched sound. The low-pitched rumble of the sand is influenced by the size and shape of the grains, the height of the dune, and the depth of the layer of dry sand. Because the grains of sand are fairly uniform, they tend to vibrate at roughly the same frequency. This synchronization of vibrations, or resonance, produces larger amplitudes of vibration. Amplitude is a measure of the amount of energy in the wave. The greater the amplitude, the louder the sound. In addition, the sound waves reverberate against the surface of the damp layer of sand below, contributing to the loudness of the sound.

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