Sand dunes have long been discovered to create haunting whistling sounds. These sand “songs” are so iconic that they feature prominently in films that show vast expanses of sandy desert. For years, the sound phenomenon has baffled scientists, but it seems that whether these singing dunes will whistle or not are largely determined by the size of their sand grains.
These strange sand songs have long been attributed to ghosts. In fact, Marco Polo wrote about these ghostly voices when he heard the sand dunes whistling all the way from China. The residents of Copiapo in Chile on the other hand, called their whistling sand dunes El Bramador because of its loud howls and bellows.
Scientists have determined that these “singing dunes” are just caused by sand grains shuffling down the slopes of dunes whenever they are disturbed by the wind. The displaced sand creates a deep, heavy groan that could be heard for miles around.
These sounds are widely debated, especially since different types of dunes produce different types of sound, and other dunes can even produce multiple “voices” or tunes.
The theory that the sound is produced by different sand grain sizes is originally suggested by a small group of Parisian biophysicists. They came upon the answer by experimenting with two singing dunes: the first one was located in Morocco, and the second one is located in Oman. In both cases, they triggered the sounds by literally sliding down the dunes with their legs, to mimic the effect of wind displacing the sand grains.
They then recorded the sounds these mini avalanches produced, and found that the Moroccan hill produced a distinct G sharp note — roughly 105 Hz. On the other hand, the hill fond in Oman produced nine distinct notes that ranged from 90 to 150 Hz.
After they recorded the sounds, the scientists took samples from both dunes: they hauled 110 pounds of Moroccan sand back to their laboratory, and 220 pounds of the Omani sand. The reason they brought back so much sand to the Paris Diderot University is because they wanted to recreate the conditions in the lab on a small scale, forcing the sand to “sing”.
They started recreating these conditions and recorded the sounds that the dunes make. They controlled the speed and the cascade of these lab created mini dunes, and found something surprising: the one note Moroccan sand had grains that had a uniform size. All of them measure 160 millionths of a meter, or microns in diameter. On the other hand, the Omani sand, which created a whole orchestra of sounds had grains between 150 to 300 microns.
They then started separating the Omani sound by size, and repeated the procedure from the beginning. The uniform sand then started to create single notes instead of multiple notes, but produced multiple notes if they are mixed together once more.
The scientists believe that the sounds are created by the constant collision of the sand particles against each other as they fall, creating strong reverberations that can be heard for miles.