Shortlisted
The Song of the Gobi
St. Paul’s Co-educational College Primary School, Laetitia Cheung, 10
H
ave you ever heard an acapella symphony of whistles? Have you ever listened to a
chorus of barking puppies? Or walked one step and set off a booming avalanche? Out of
the barren heat of the desert, out of the bumping and rubbing of fine smooth grains of
sand, springs a mysterious song – the song of the Gobi Desert.
Over 1,200 years ago, the ancient Chinese first described singing dunes at the Hill of
Sounding Sand, located in a line of sand dunes leading to Dunhuang:
‘This hill has strange supernatural qualities. Its peaks taper up to a point, and between them
there is a mysterious hole which the sand has not been able to cover up. In the height of summer
the sand gives out sounds of itself, and if trodden by men or horses, the noise is heard many tens
of li (Chinese kilometer) away ...’ Tun Huang Lu, a manuscript, recorded.
Like curious boys and girls, the ancients climbed up this hill to experience a new song on
festival days:
‘On Tuan Wu Day (Dragon Boat Festival), it is customary for men and women to clamber up
to some of the highest points and rush down again … which causes the sand to give forth a loud
rumbling sound like thunder. Yet when you come to look at it the next morning the hill is found
to be just as steep as before.’
Over the centuries, singing sand has aroused the curiosity of explorers, emperors and scientists
– from Marco Polo and Afghan Emperor Baber to Charles Darwin. Yet this phenomenon remains a
mystery, with scientists unable to agree on an explanation for the strange whistling or booming.
What makes a sand dune sing? If you scoot down the slopes of certain sand dunes, you may
hear the sands roar or bellow like a tuba blowing a deep groaning hum. Others describe it as the
sound of a propeller plane flying right over your head. Do you know how the motion of these
sandy oceans affects the pitch of the singing sands?
There are several different theories about how sand produces sound. Scientists have observed
that certain conditions must come together. The sand grains are round and small, with a diameter
of 0.1 mm to 0.5 mm. The sand is rich in silica. And a certain wetness or humidity surrounds the
grain. In other words, singing or booming doesn’t occur on all desert dunes. Booming only happens
when conditions are hot and dry, and when the sand grains are clean, round and polished.
One of the first explanations came from a British commanding army officer, Ralph
Bagnold, based with his troops in North Africa during World War II. Bagnold proposed a simple
mathematical model to describe the dunes’ songs: the high or low pitch of sound was proportional
to the speed of the sand grains sliding down the dune, just like what happens in an avalanche. He
also observed that the higher the note produced, the smaller the size of the sand grain.
Between 2001 and 2004, two French scientists, Bruno Andreotti and Stephane Douady studied
crescent-shaped dunes known as barchans in Morocco. They accidentally set off avalanches that
triggered a booming sound. They observed that sound waves were produced when two grains of
sand bumped over each other.
They soon disagreed over the source of the booming. According to Douady, booming was
caused by friction – the synchronized rubbing between similar sized grains. But Andreotti believed
it was caused by sound waves bouncing back and forth between the moving upper surface of the
