Singing sands find a new tune
[Here’s the unedited version of my latest article for news@nature, which has a few more comments from the researchers than the final piece does (published in print this week).]

A new theory adds to the controversy over why some desert dunes emit sonorous booms.

A new theory for why sand dunes emit eerie booming drones seems likely to stir up fresh controversy, as rival theories contend to answer this ancient puzzle.

Research on this striking natural phenomenon has become something of a battleground after two groups in France, previously collaborators, published their opposing theories. Now a team at the California Institute of Technology, led by mechanical engineer Melany Hunt, says that they’re both wrong [1].

“There are strong feelings in this field”, says physicist Michael Bretz at the University of Michigan, who has studied the ‘song of the sands’. “It’ll take a while longer to get it sorted out. But the explanations keep getting better.”

The ‘singing’ of sand dunes has been known for a very long time. Marco Polo described it on his journeys through the Gobi desert in the thirteenth century, attributing the sound to evil desert spirits. The noise can be very loud, audible for over a kilometre. “It’s really magnificent”, says physicist Stéphane Douady at the Ecole Normale Supérieure in Paris, who has proposed one of the competing theories to explain it.

The effect is clearly related to avalanches of sand, and can be triggered by people sliding down the slopes to get the sand moving – as was done at least since the ninth century during a festival on a sand-covered hill in northwestern China called Mingsha Shan (Sighing Sand Mountain). Charles Darwin heard the ‘song of the sands’ in Chile, saying that it was produced on a sandy hill “when people, by ascending it, put the sand in motion.”

In the twentieth century the doyen of dune science Ralph Bagnold, an army engineer who fell in love with the North African deserts during the Second World War, suggested that the noise was caused by collision of sand grains, the frequency being determined by the average time between collisions. This implies that the frequency of the boom depends on the size of the individual grains, increasing as the grains get smaller.

The previous explanations of the French researchers focused on these collisions during sand avalanches. Douady and his coworkers Bruno Andreotti and Pascal Hersen began to study ‘singing dunes’ during a research trip in Morocco in 2001.

Douady decided that in order for the moving grains to generate a single sound frequency, their motions must become synchronized. This synchronization, he argued, comes from standing waves set up in the sliding layer. The loudness of the noise results from the way that the dune surface acts like a giant loudspeaker membrane.

But Andreotti found a slightly different explanation.. The synchronization of grain motions, he said, comes from waves excited in the sand below the sliding layer itself, which then act back on the moving grains themselves, ‘locking’ their movements together and thus converting random collisions to synchronized ones.

It might seem like a small distinction, but Douady and Andreotti found that they could not resolve their differences, and in the end they published separate papers offering their explanations [2,3]. Andreotti now works at another lab in Paris.

But both explanations have serious problems, according to Hunt. For one thing, the measurements made by her team on several booming dunes in Nevada and California seem to show that the booming frequency doesn’t depend on the grain size at all, as Bagnold suggested and with which both Andreotti and Douady concurred.

What’s more, the previous theories imply that all dunes should be able to ‘sing’, since this is a general property of sand avalanches. But in fact some dunes sing while others don’t – that is, after all, why Mingsha Shan got its name. Why is that? Andreotti has proposed that ‘silent’ dunes aren’t dry enough, or have grains of the wrong shape. But Hunt and colleagues think that the answer lies literally deeper than this.

“Douady and Andreotti have focused on the grain sizes and the surface features of the grains, but did not take large-scale properties of the dunes into account”, says Hunt’s student Nathalie Vriend. “They have not found an explanation yet why the smaller dunes or dunes in the wintertime do not make this sound.”

The Caltech teams says that dunes have to be covered in distinct layers of sand in order to create a boom. Their careful measurements of vibrations in the sand – made with an array of ‘geophones’ on the dune slopes, like those used to monitor seismic waves in earthquake studies – showed that the speed of these seismic waves increases in abrupt steps the deeper the sand is.

In particular, the speed of the seismic waves increases suddenly by almost a factor of two at a depth of about 1.5 m below the dune surface.

The Caltech researchers think that this layered structure, caused by variations in moisture content and bonding of the grains to one another, enables the surface layer to act as a kind of waveguide for acoustic energy, rather like the way an optical fibre channels light. So while they agree that the boom is transmitted to the air by a loudspeaker effect of the dune surface, they think that the frequency is set by the width of the waveguide layer of sand.

Dunes that lack this layered structure – as smaller ones do, for example, won’t ‘sing’ at all: the vibrations simply get dispersed within the sliding sands. The researchers also find that more moisture condensed between the sand grains during the winter smears out the boundaries between the layers of singing dunes and silences them.

This is unlikely to be the last word on the matter, however. For one thing, the strange properties of the sand in ‘booming dunes’ don’t seem to rely on such large-scale influences. “You can take a cupful of this sand and excite it with your finger”, says Peter Haff, a geologist at Duke University in North Carolina who has studied it. “You can feel it vibrating, like running your finger over a washboard. But you can take sand from other parts of the dune, and there’s nothing you can do to make it boom.” Haff concludes that, while these theories may offer part of the answer, “there must be something else going on at a small scale.”

Douady agrees. “The problem for the Caltech theory is that we can recreate these sounds in the lab”, he says. He thinks that the sand layering might play a role in modifying the sound, but that it is “just a decoration” to the basic mechanism of booming. “It’s like the different between singing in a small room and singing in a cathedral,” he says.

Andreotti also finds several reasons to remain unconvinced. In particular, he says “They use sensors only at the surface of the dune. We have made measurements with buried sensors about 20 cm below the surface, and didn’t detect any vibration. This is a strong and direct contradiction of the paper.” So it seems that, with everyone sticking to their own theory, the riddle of the dunes is not yet solved.

References
1. Vriend, N. M. et al. Geophys. Res. Lett. 34, L16306 (2007).
2. Andreotti, B. Phys. Rev. Lett. 93, 238001 (2004).
3. Douady, S. et al. Phys. Rev. Lett. 97, 018002 (2006).

The history of singing dunes
It is asserted as a well-known fact that this desert is the abode of many evil spirits, which amuse travellers to their destruction with most extraordinary illusions. If, during the daytime, any persons remain behind on the road, either when overtaken by sleep or detained by their natural occasions, until the caravan has passed a hill and is no longer in sight, they unexpectedly hear themselves called to by their names, and in a tone of voice to which they are accustomed. Supposing the call to proceed from their companions, they are led away by it from the direct road, and not knowing in what direction to advance, are left to perish. In the night-time they are persuaded they hear the march of a large cavalcade on one side or the other of the road, and concluding the noise to be that of the footsteps of their party, they direct theirs to the quarter from whence it seems to proceed; but upon the breaking of day, find they have been misled and drawn into a situation of danger... Marvellous indeed and almost passing belief are the stories related of these spirits of the desert, which are said at times to fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms; obliging the travellers to close their line of march and to proceed in more compact order.
Marco Polo (1295)

Somewhere, close to us, in an undefined direction, a drum was beating, the mysterious drum of the dunes; it was beating distinctly, sometimes more vibrating, sometimes weakened, stopping, then taking again its fantastic bearing.
The Arabs, terrified, looked at themselves; and one said, in its language: "Death is on us." And here is that suddenly my companion, my friend, almost my brother, fell from horse on the head, struck down ahead by sunstroke.
And during two hours, while I was in vain trying to save it, always this imperceptible drum filled up me the ear of its monotonous, intermittent and incomprehensible noise; and I felt the fear slip into my bones, the true fear, the hideous fear, close to this liked body, in this hole charred by the sun between four mounts of sand, while the unknown echo was throwing us, two hundred miles away of any French village, the fast beat of the drum.
Maupassant (1883)

Whilst staying in the town I heard an account from several of the inhabitants, of a hill in the neighborhood which they called "El Bramador," - the roarer or bellower. I did not at the time pay sufficient attention to the account; but, 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. The same circumstances are described in detail on the authority of Seetzen and Ehrenberg, as the cause of the sounds which have been heard by many travellers on Mount Sinai near the Red Sea.
Charles Darwin (1889)

Update
Andreotti and his colleagues have submitted a comment on the paper by Vriend et al. to Geophys. Res. Lett., which is available here.