Ever since Marco Polo, explorers have told stories about strange sounds they have heard in the desert. It is known that these sounds are produced by sand dunes when they avalanche, but the exact mechanism behind the phenomenon has remained a mystery. Now, Stéphane Douady from the French national research council and colleagues have proposed that the sounds come from the synchronisation of the motion of the sand grains as they pass over each other -- and not from the grains producing excitations on the surface of the dune, as was previously believed (Phys. Rev. Lett. 97 018002).
Singing dunes are one of the most puzzling and impressive natural phenomena. The sounds produced can be heard up to 10 kilometres away and resemble a drum, a low-flying jet or even an organ.. The sounds can be as loud as 105 decibels and have frequencies between about 65 and 110 Hertz depending on where the sand comes from. Using sand shipped from the Ghord Lahmar region in Morocco to their lab, Douady and colleagues found they could produce notes from the sand simply by pushing the sand grains together by hand or using a metal blade (figure 1). This means that the sounds are nothing to do with the dune itself but are produced by the motions of the sand grains — not from the entire dune resonating.
According to Douady, what happens when sand avalanches is that the grains bump over each other at different frequencies and set up standing waves in the flowing sand layer. These waves then reinforce one another and make the layer vibrate. Moreover, only a thin layer of between about two to three centimetres is needed to set up this resonance.
The CNRS team also found that the sand grains have to be moving at or above a speed of 0.45 metres per second before they can emit a sound. The researchers confirmed this finding by carrying out velocity measurements directly on a singing dune in the region of Morocco where their sand had come from. The team also found that the sound emitted depends on the surface state of the grains. Grains that sing are round with a smooth coating of silica gel, while grains that are wet or don’t have this layer do not emit a sound.
