Sound-producing sand avalanches
Sound-producing sand grains constitute one of nature’s more puzzling and least understood physical phenomena. They occur naturally in two distinct types: booming and squeaking sands. Although both varieties of sand produce unexpectedly pure acoustic emissions when sheared, they differ in their frequency range and duration of emission, as well as the environment in which they tend to be found. Large-scale slumping events on dry booming dunes can produce acoustic emissions that can be heard up to 10 km away and which resemble hums, moans, drums, thunder, foghorns or the drone of low-flying propeller aircraft. These analogies emphasize the uniqueness of the phenomenon and the clarity of the produced sound. Although reports of these sands have existed in the literature for over one thousand years, a satisfactory explanation for either type of acoustic emission is still unavailable
Types of sand that are known to produce manifest acoustic emissions
There exist two distinct types of sand that are known to produce manifest acoustic emissions when sheared. The more common of the two, known colloquially as `squeaking’ or `whistling’ sand, produces a short, high frequency `squeak’ when sheared or compressed. It is fairly common in occurrence, and can be found at numerous beaches, lake shores and riverbeds around the world. The other, rarer type of soundproducing sand occurs principally in large, isolated dunes deep in the desert. The loud, low-frequency acoustic output of this `booming’ sand, resultant upon avalanching, has been the subject of desert folklore and legend for centuries.
Written accounts of this phenomenon
Marco Polo (1295) wrote of evil desert spirits which `at times ®ll the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms’. References can be found dating as far back as the Arabian Nights , and as recently as the science action classic Dune . Charles Darwin (1889) also makes mention of it in his classic Voyages of the Beagle. At least 31 desert and back-beach booming dunes have been located in North and South America, Africa, Asia, the Arabian Peninsula and the Hawaiian Islands . Sharply contrasting differences between squeaking and booming sands have led to a consensus that although both types of sand produce manifest acoustic emissions, their respective sounding mechanisms must be substantially different. More recent laboratory production of `squeaks’ in booming sand has nonetheless suggested a closer connection between the two mechanisms. A satisfactory explanation for either type of acoustic activity is still unavailable.
Singing of the Beach
On some beaches around the world, dry sand will make a singing, squeaking, whistling, or barking sound if a person scuffs or shuffles their feet with sufficient force.The phenomenon is not completely understood scientifically, but it has been found that quartz sand will do this if the grains are very well-rounded and highly spherical. It is believed by some that the sand grains must be of similar size, so the sand must be well sorted by the actions of wind and waves, and that the grains should be close to spherical and have dust, pollution, and organic matter free surfaces. The “singing” sound is then believed to be produced by shear as each layer of sand grains slides over the layer beneath it. The similarity in size, the uniformity, and the cleanness mean that grains move up and down in unison over the layer of grains below them. Even small amounts of pollution on the sand grains reduces the friction enough to silence the sand.