Enigma of the singing dunes

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.

Douady and Andreotti both came up with the idea that the sounds must be produced by sand grains becoming synchronised – moving in definite patterns as they move down the surface of the dune. Their hypotheses differed in that Douady believed the sounds, which after all are just vibrations of air molecules, were produced by air being squeezed out from between the synchronised grains. Andreotti proposed that the sound was due to the surface of the avalanche vibrating the air around it like a large hi-fi speaker. The pair began to follow very different lines of inquiry and ended up in complete disagreement. This, combined with a subsequent quarrel over how best to publish their findings, led to the two researchers falling out. So much so, in fact, that they now avoid each other, despite working in the same small field of physics. Their scientific adventures and disagreements were the subject of an award-winning article in the November 2006 edition of Physics World, the Institute of Physics members’ magazine.

There is still no consensus as to the exact explanation of the singing dunes. In fact, scientific papers published more recently suggest that the large-scale structure of the dune does play a part after all. The story of the differences between Douady and Andreotti, and the twists and turns in the wider investigation into the exact cause of the dunes’ song, highlight the fact that the progress of science can be affected by the vagaries of human nature. For example, why does a scientist choose a certain path of investigation? That’s something that your average physics textbook won’t ever answer, even if we do finally get to the bottom of why dunes sing.

www.physics.org