Singing Dunes : KAZAKHSTANThe Singing Dunes are in the Altyn-Emel State National Park, Kazakhstan's largest national park. The crescent-shaped sand dunes, called barchans, produce a sound described as roaring, booming, squeaking, or simply as the "Song of the Dunes." This low-pitch rumble, a natural sound phenomenon, can be as loud as 105 decibels and can last several minutes. The sound accompanies a slumping or avalanching movement of the sand, usually triggered by wind passing over the dune. Collisions between grains of sand emit surface elastic waves that, in turn, cause the motions of the grains to become synchronized. The surface of the sand acts as a loudspeaker, transmitting it through the air. f you've never heard a sand dune rumble, listen up. Marco Polo in the 13th Century said the singing sands -- which he ascribed to evil desert spirits -- "at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." Yes, certain sand dunes will occasionally let out 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. But why? To try and uncover the underlying nature of these mysterious sounds, Bruno Andreotti from the University of Paris-7 took equipment out to the Atlantic Sahara in Morocco, one of only 35 known places where the mysterious natural music can be heard. "Singing dunes constitute one of the most puzzling and impressive natural phenomenon I have ever encountered," Andreotti said. 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. Wind forces sand to accumulate at the top of the dune until the angle of the slope reaches a tipping point of about 35 degrees. The eventual avalanche of sand produces the bellowing noise. The sand must be sufficiently dry for the singing to occur. For smaller barchans, the sand must also be hot and the wind still A small dune sings only the few days in which there is no wind and no clouds so that the Sun can dry efficiently the slip face," Andreotti told LiveScience.
By measuring vibrations in the sand and air, Andreotti was able to detect surface waves on the sand that emanated from the avalanche at a relatively slow speed of about 130 feet per second (40 meters per second). In this way, the face of the dune acts like a huge loudspeaker - with the waves on the surface producing the sound in the air. Andreotti explained these sand waves as resulting from collisions that occur between grains at about 100 times per second, as measured in the lab. In a kind of feedback loop, the waves synchronize the collisions, so they are all on basically the same beat. This model explains the low pitch - between 95 and 105 Hertz - of the sand song, which, according to Andreotti, resembles a drum or a low-flying propeller aircraft. The feedback mechanism, as outlined by the researchers in the Dec. 1 issue of Physical Review Letters, also correctly predicts the maximum loudness of the singing to be 105 decibels, at which point the sand grains vibrate off the surface. This level of sound is comparable to a snow blower or a Walkman at full volume. The mystery is not completely solved. Recent research has centered on a seemingly magical musical property of the singing grains.
When the thirteenth century explorer Marco Polo encountered the weird and wonderful noises made by desert sand dunes, he attributed them to evil spirits. But 700 years later, scientists still don’t completely understand the causes of this eerie phenomenon.Sand dunes can be heard ‘singing’ in more than 30 locations worldwide, and in each place the sounds have their own characteristic frequency, or note. In reality the sounds produced are less like singing and more like a low-frequency drone (low frequency corresponds to low notes; bass as opposed to treble). The sounds are emitted when sand cascades down the face of a dune in an avalanche, the cause of which can be the wind, people walking on the top of the dune or even sliding down it. In 2001 a team of French physicists, including Stéphane Douady and Bruno Andreotti, went to Morocco to study the shape and motion of sand dunes. They became fascinated by the singing of the dunes and began to investigate it in addition to their other research. They found that avalanches they triggered manually produced the same sound as those that occurred naturally, which suggests that the wind doesn’t play a part. They also concluded that the sound is not produced by the dune resonating, as happens in the case of a musical instrument for example, because the frequency of the sound produced is the same for different sizes of dune. Thus the team focused their investigation on the motion of the sand grains, rather than on the properties of the entire dune.
