Singing Sand Dunes
But there is a marvellous thing related of this Desert, which is that when travellers are on the move by night, and one of them chances to lag behind or to fall asleep or the like, when he tries to gain his company again he will hear spirits talking, and will suppose them to be his comrades. Sometimes the spirits will call him by name; and thus shall a traveller ofttimes be led astray so that he never finds his party. And in this way many have perished. [Sometimes the stray travellers will hear as it were the tramp and hum of a great cavalcade of people away from the real line of road, and taking this to be their own company they will follow the sound; and when day breaks they find that a cheat has been put on them and that they are in an ill plight. Even in the day-time one hears those spirits talking. And sometimes you shall hear the sound of a variety of musical instruments, and still more commonly the sound of drums.
The Travels of Marco Polo
Travellers in the desert have long known that shifting sand can make an eerie noise, ranging from a bass boom to a baritone bark and a soprano whistle. The sound occurs when the ridge of a sand dune builds up and eventually topples. This shear effect causes a mini-avalanche of sand in which millions of grains rub against each other as they fall. But different materials and different conditions make different songs.
Lab experiments show that synchronicity plays a vital role. Put simply, enough grains have to be flowing at the same rate in order to create and amplify the oscillation. In turn, the factors behind synchronicity are wind speed, humidity, the size of the sand grain and the smoothness of its coating, too.
Much of the scientific fascination surrounding booming dunes stems from the fact that their properties are so hard to pin down. Booming doesn’t occur on all desert dunes. And on those that do boom, the phenomenon doesn’t occur throughout the entire year or everywhere across the dune. The frequency can vary too – from roughly 65 to 120 Hertz – while the volume can reach 110 decibels — just 20 dB short of the pain threshold.
The sound is not related to the type of dune or its location. And while it’s mostly at a pitch akin to the drone of a low-flying aeroplane, its timbre ranges from a rough brass-like clamour of Oman’s dunes, on the Arabian Peninsula, to the pure vocal sound of Morocco’s. Scientists agree that the noise only arises from a dune’s upper slip face (the leeward side), never from the shallow, windward face. What’s more, booming only happens when conditions are hot and dry and when the sand grains are clean, round and polished.
Despite these clues, the most fundamental question remains: what does make the dunes sing?
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Audio: Physicist Simon Dagois-Bohy and his fellow researchers at Paris Diderot University in France recorded two different dunes: one near Al-Askharah, a coastal town in southeastern Oman, and one near Tarfaya, a port town in southwestern Morocco..