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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?

Sources: [x] [x] [x] Image: [x]
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.