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The Cambridge University geophysicist listening to tunes of the dunes

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Dr Nathalie Maria Vriend, Department of Applied Mathematics and Theoretical Physics, seen here with some of her experiments. Picture: Keith Heppell
Dr Nathalie Maria Vriend, Department of Applied Mathematics and Theoretical Physics, seen here with some of her experiments. Picture: Keith Heppell

At the Department of Applied Mathematics and Theoretical Physics, there is one research group that enjoys a performance of the desert song: not the Sigmund Romberg operetta but a very different and even more moving musical.

Dr Nathalie Vriend investigates some of the many riddles of the sands – among them the tunes of the dunes. What she started to learn a decade ago about strange noises in lonely places could also answer vital questions about the physics of avalanche, a problem that has taken her from Death Valley in California to the Alpine snows.

The music of shifting sands has been observed many times by travellers since Marco Polo first reported from the Central Asian wastes almost seven centuries ago, and more systematically by Nathalie Vriend, who worked her way to a PhD by sliding down sand dunes to precipitate her own avalanches and record the sound they made.

First in California and now in Cambridge, she and colleagues have established the precise conditions under which the shifting sands sing.

It is, she says, music in the sense that she could record a booming noise, and its harmonics, at the around 80 to 85 hertz, a frequency associated with the baritone voice. More musical eavesdroppers might even be able to assign a note, or nominate a key. The noise has also been likened to distant thunder, and to the drone of a didgeridoo.

She grew up in Andjik in the north-west Netherlands, graduated as a mechanical engineer at the University of Twente in Enschede, and then took a masters in aeronautics at the California Institute of Technology, known globally as Caltech. Then she read something by her adviser-to-be about the singing sand phenomenon.

“It sounded really interesting. I wasn’t experienced in desert travelling, but I love the outdoors, I was a mechanical engineering PhD student and I thought this sounds like something I can work on,” she says.

She learned the ropes by joining field trips. She ended up leading them. In a series of studies, she established the silica songbook rules for impromptu performance

“Let me start with the most basic one: avalanche. A unique motion, some shearing of the grain, but this can be initiated by scientists like us, sliding on the dunes with our bottoms, creating a sand avalanche between our legs, basically pushing it forward, or it could be created by the wind. If sand grains are blown over the crest, they accumulate at the top, and then once in a while they fall, they start this avalanche. That is the first pre-requisite, you need an avalanche.”

But it takes more than that: otherwise the beaches of Britain would hum in a low baritone. There are other conditions for the silica resonance, one of which is that the dune must have structure. The sand grains must be layered, in ways that amplify the good vibrations. Velocity, too, is important.

“This usually happens for very dry desert dunes in summer time, which made my work really hard as well. Usually we would do our fieldwork between May and September; we are talking about Death Valley here, so it would between 35-45° Celsius, which is difficult if you are carrying heavy equipment.”

There were great moments – the silence and the peace of a desert morning, and the beauty – but she wouldn’t necessarily call it a dream job.

“Let me take you out of this dream,” she laughs, and launches into a memory of one September day of great heat and high wind, shouting out instructions as her colleagues tried to plant sensors in the sands. They were all sandblasted. She was sandblasted with her mouth open. “Usually when you come back a field trip you would find sand everywhere in your hair, your ears, but this time my mouth was coated with sand: it was horrible.”

Dr Vriend is married, with two sons, aged three and six. She has her own PhD students, and she is a Royal Society Dorothy Hodgkin Research Fellow.

Now much of the research is done in laboratory simulations, and could be applied to a number of puzzles inherent in granular flow and the separation of grains, but she will, she says, “most likely” visit Qatar again, not for the sounds of the sands, but to study the ways dunes migrate. She moved Cambridge because she became interested in snow avalanches: not quite the same thing, but with some physics in common.

“It was the career opportunities. It doesn’t have anything to do with the prestige. I kind of hate that talk,” she says. “If you are at a conference and people realise you are from Caltech or Cambridge they suddenly find you very interesting.”

People from less famous universities don’t get the same response, and she hates that.

The sound of shifting sands

In the desert of Lop, travellers at night might suddenly hear “a noise like the clatter of a great cavalcade of riders away from the road,” the 13th-century Venetian traveller Marco Polo reports. “Even by daylight men hear these spirit voices and often you fancy that you are listening to the strains of many instruments, especially drums, and the clash of arms.”

Lord Curzon of Kedleston, once Viceroy of India, writes in Tales of Travel (1923) that he quizzed the Amir of Afghanistan about supernatural sounds that supposedly emerged from a steep slope at Reg-i-Ruwan: it was the wind, he decided, humming in the rocks. A young Cambridge graduate who visited Chile in 1835 also collected reports of musical emptiness, from “a hill in the neighbourhood which they called ‘El Branador’ – the roarer or bellower.”

But, writes Charles Darwin in his Voyage of the Beagle (1839) “as far as I understood, the hill was covered by sand, and the noise was produced only when people, by ascending it, put the sand in motion.”

And as he travelled through the desert to Cairo, another Cambridge graduate, Alexander Kinglake, records in Eothen (1844) hearing the sound of bells “not ringing for joy but properly, prosily, steadily, merrily ringing ‘for church’.”

He did wonder also if he might have been dreaming.

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