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The song of dunes / Le chant des dunes / Booming dunes / Roaring dunes

Many desert sand dunes emit a loud and harmonious sound as they avalanche, with large amounts of sand sliding down the dune. An avalanche spontaneously occurs when the surface angle is locally larger than a threshold called the static angle (usually 32 to 35°). The flow freezes when the slope is relaxed below a second angle called the dynamical angle or the angle of repose (30 to 32°). The phenomenon was first documented in a Chinese manuscript dating back to the ninth century. In 1298, Marco Polo described it as the sounds of all kinds of musical instruments--and more ordinary, drums--filling the air. This loud sound, which can be heard up to 10 km away, resembles the sound of a foghorn or of a low-flying twin-engined jet.

Most dunes can sing in particular weather conditions. The largest ones can sing almost all the time as their surface becomes very hot in the afternoon and thus very dry. Others (for instance, the 10,000 barchan dunes that compose the Atlantic Sahara desert around Laayoune, Morocco) may sing only if the sun has been able to dry the dune surface, which is more and more difficult to achieve for smaller and smaller dunes. Finally, some dunes never sing (such as the Pyla dune in France). They are usually not composed of eolian (windblown) sand that is characterized by a narrow particle-size distribution, but by a smooth rounded shape and by an unpolished surface due to the hammering in the collisions. Infrared spectroscopy has suggested that sound-producing grains could be covered by a characteristic silica-gel layer, formed during cycles of humidification and dry-out. Still, the role of this layer has not been found so far. Moreover, nobody has succeeded in producing artificial singing sand, starting for instance from industrial glass beads or river sand.

The sound is not related to the wind around the dune or to a resonance inside the dune but only to the grain motion inside avalanches, only. The same sound is produced by a naturally triggered avalanche or by a person sliding down the slip face of the dune to push the avalanche. The sound emitted has a well-selected frequency, ranging from 60 Hz (for large grains of 350 µm) to 100 Hz (for small grains of 100 µm), independently of the location of the avalanche. The phenomenon can be reproduced at small scale, in a controlled way, by bulldozing the sand with the hand--it is the usual way to check whether the sand sings or not. The frequency is observed to increase with the pushing velocity and can reach 300 Hz. The granular motion inside the avalanche is thus responsible for the phenomenon.

The emission of sound in the air is due to seismic-like surface waves in the avalanche, and but also in the surrounding area where the grains are at rest. Like a loudspeaker, the oscillation of the sand bed surface emits the sound. These surface waves are themselves excited by the moving grains in the avalanche, at the frequency of collision of the grains. In turn, the bed oscillation tends to synchronize the motion of the grains. This mechanism allows understanding the origin of the frequency--the shear rate in the avalanche--but also the amplitude that usually reaches 100 to 105 dB. This incredibly loud sound is produced by a vibration of the soil of amplitude one-fourth of grain size. This is just sufficient to balance gravity and form a fluidized layer of grains at the surface.

 

The Song of Dunes as a Wave-Particle Mode Locking,
B. Andreotti, Phys. Rev. Lett. 93, 238001 (2004).  Article in PDF format.

In this article, it is shown:
(i) that the sound emitted in the air is not due directly to the avalanche but by the vibration of the free surface (surface elastic waves)
(ii) that these surface waves exists outside the avalanche and are indeed localized at few centimeters below the surface.
(iii) that the frequency is controlled by the shear rate inside the avalanche (the collision rate of grains inside the avalanche), which scales in the gravity case on the square root of gravity divided by the grain size
(iv) that the amplitude (105 dB) saturates precisely when the acceleration of the free surface just balances gravity (when the grains at the surface start taking of)
(v) that the elastic surface waves tend to synchronize the collisions; the phenomenon is thus an interaction between the collisions inside the avalanche, which excites surface elastic waves, and in turn, these elastic waves which synchronize the collisions of grains.
(vi) that the elastic surface waves propagate at a speed around 40-50 m/s

 

Surface elastic waves in granular media under gravity and their relation to booming
avalanches

L. Bonneau, B. Andreotti and E. Clément, Phys. Rev. E 75, 016602 (2007). Article in PDF format.

Acoustics of singing dunes, comment on ''Solving the mystery of booming dunes''
B. Andreotti, L. Bonneau, and E. Clément, Geophys. Res. Lett., 35, doi :10.1029/2007GL032509R. (2008).Article in PDF format.

 

Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media
Bonneau L., Andreotti B. & Clément E. 2008 , submitted to Phys. Rev. Lett.Article in PDF format.

 

Take care: you need a loud speaker which restitutes a 100 Hz bass. It is probably not the case of the internal loud speakers of your computer. Use good headphones in that case.

If you wish to ear the song of dunes , click on one of the following images:

Sound heard by the person sliding down the dune (1)

Microphone (left ear) and fixed accelerometer initially at the surface (right)

Sound heard by the person sliding down the dune (2)

Same as above but low-pass filtered

Microphone (left ear) and accelerometer floating at the surface (right ear)

Same as left but low-pass filtered

Click on the <play> button to see a movie showing an avalanche view in side with a comment explaing the booming mechanism (in french).

Picture of singing/booming sand


 

When you ride a horse, or go on a walking tour, the sound of stepping on the sand will reach some dozens miles or so. On the boy's festival day, as was the custom long ago, towns folk within the castle wall used to climb Mt. Ming-sha-shan and slide down on the sand in union.   The sliding sound was almost like the rolling of thunder.

Chinese manuscript (Ton-Fan 880 A.D.)

 

CHAPTER XXXVI Of the town of Lop - of the desert in its vicinity - and of the strange noises heard by those who pass over the latter.

The town of Lop is situated towards the north-east, near the commencement of the great desert, which is called the Desert of Lop. It belongs to the dominions of the grand khan, and its inhabitants are of the Mahometan religion. Travellers who intend to cross the desert usually halt for a considerable time at this place, as well to repose from their fatigues as to make the necessary preparations for their further journey. For this purpose they load a number of stout asses and camels with provisions and with their merchandise. Should the former be consumed before they have completed the passage, they kill and eat the cattle of both kinds; but camels are commonly here employed in preference to asses, because they carry heavy burdens and are fed with a small quantity of provender. The stock of provisions should be laid in for a month, that time being required for crossing the desert in the narrowest part. To travel it in the direction of its length would prove a vain attempt, as little less than a year must be consumed and to convey stores for such a period would be found impracticable. During these. thirty days the journey is invariably over either sandy plains or barren mountains; but at the end of each day's march you stop at a place where water is procurable; not indeed in sufficient quantity for large numbers, but enough to supply a hundred persons, together with their beasts of burthen. At three or four of these halting-places the water is salt and bitter, but at the others, amounting to about twenty, it is sweet and good. In this tract neither beasts nor birds are met with, because there is no kind of food for them. 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. Sometimes likewise during the day these spirits assume the appearance of their travelling companions, who address them by name and endeavour to conduct them out of the proper road. It is said also that some persons, in their course across the desert have seen what appeared to them to he a body of armed men advancing towards them, and apprehensive of being attacked and plundered have taken to flight. Losing by this means the right path, and ignorant of the direction they should take to regain it, they have perished miserably of hunger. 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. They find it necessary also to take the precaution before they repose for the night, to fix an advanced signal, pointing out the course they are afterwards to hold, as well as to attach a bell to each of the beasts of burden for the purpose of their being more easily kept from straggling. Such are the excessive troubles and dangers that must unavoidably be encountered in the passage of this desert.

Marco Polo , “The travels of 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 a 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)

 

On July 1st we reached the valley of Copiapo. The smell of the fresh clover was quite delightful, after the scentless air of the dry, sterile Despoblado. 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.

One person with whom I conversed had himself heard the noise: he described it as very surprising; and he distinctly stated that, although he could not understand how it was caused, yet it was necessary to set the sand rolling down the acclivity. A horse walking over dry coarse sand, causes a peculiar chirping noise from the friction of the particles; a circumstance which I several times noticed on the coast of Brazil.

Charles Darwin (1889)

 

Près de la côte de l'une des îles Hawai, il y a un vieux cimetière balayé par le vent. Les débris de corail s'accumulent sur cette étendue stérile. Les barques de pêches Hawai, quand le vent le leur permet, préfèrent passee au large de cette côte: il provient de l'étendue blanche un gémissement étrange, une plainte, comme le hurlement d'un chien, que les indigènes attribuent aux esprits sans repos des disparus.

Ledoux (1920)

 

On remonta sur le pont après dîner. Devant nous, la Méditerranée n'avait pas un frisson sur toute sa surface, qu'une grande lune calme moirait. Le vaste bateau glissait, jetant sur le ciel, qui semblait ensemencé d'étoiles, un gros serpent de fumée noire ; et, derrière nous, l'eau toute blanche, agitée par le passage rapide du lourd bâtiment, battue par l'hélice, moussait, semblait se tordre, remuait tant de clartés qu'on eût dit de la lumière de lune bouillonnant.

Nous étions là, six ou huit, silencieux, admirant, l'oeil tourné vers l'Afrique lointaine où nous allions. Le commandant, qui fumait un cigare au milieu de nous, reprit soudain la conversation du dîner.

"Oui, j'ai eu peur ce jour-là. Mon navire est resté six heures avec ce rocher dans le ventre, battu par la mer. Heureusement que nous avons été recueillis, vers le soir, par un charbonnier anglais qui nous aperçut."

Alors un grand homme à figure brûlée, à l'aspect grave, un de ces hommes qu'on sent avoir traversé de longs pays inconnus, au milieu de dangers incessants, et dont l'oeil tranquille semble garder, dans sa profondeur, quelque chose des paysages étranges qu'il a vus ; un de ces hommes qu'on devine trempés dans le courage, parla pour la première fois : "Vous dites, commandant, que vous avez eu peur : je n'en crois rien. Vous vous trompez sur le mot et sur la sensation que vous avez éprouvée. Un homme énergique n'a jamais peur en face du danger pressant. Il est ému, agité, anxieux ; mais la peur, c'est autre chose."

Le commandant reprit en riant :

"Fichtre ! je vous réponds bien que j'ai eu peur, moi."

Alors l'homme au teint bronzé prononça d'une voix lente : "Permettez-moi de m'expliquer ! La peur (et les hommes les plus hardis peuvent avoir peur), c'est quelque chose d'effroyable, une sensation atroce, comme une décomposition de l'âme, un spasme affreux de la pensée et du coeur, dont le souvenir seul donne des frissons d'angoisse. Mais cela n'a lieu, quand on est brave, ni devant une attaque, ni devant la mort inévitable, ni devant toutes les formes connues du péril : cela a lieu dans certaines circonstances anormales, sous certaines influences mystérieuses, en face de risques vagues. La vraie peur, c'est quelque chose comme une réminiscence des terreurs fantastiques d'autrefois. Un homme qui croit aux revenants, et qui s'imagine apercevoir un spectre dans la nuit, doit éprouver la peur en toute son épouvantable horreur."

"Moi, j'ai deviné la peur en plein jour, il y a dix ans environ. Je l'ai ressentie, l'hiver dernier, par une nuit de décembre."

"Et, pourtant, j'ai traversé bien des hasards, bien des aventures qui semblaient mortelles. Je me suis battu souvent. J'ai été laissé pour mort par des voleurs. J'ai été condamné, comme insurgé, à être pendu, en Amérique, et jeté à la mer du pont d'un bâtiment sur les côtes de Chine. Chaque fois je me suis cru perdu, j'en ai pris immédiatement mon parti, sans attendrissement et même sans regrets."

"Mais la peur, ce n'est pas cela."

"Je l'ai pressentie en Afrique. Et pourtant elle est fille du Nord : le soleil la dissipe comme un brouillard. Remarquez bien ceci, messieurs. Chez les Orientaux, la vie ne compte pour rien : on est résigné tout de suite : les nuits sont claires et vides de légendes, les âmes aussi vides des inquiétudes sombres qui hantent les cerveaux dans les pays froids. En Orient, on peut connaître la panique, on ignore la peur."

"Eh bien ! voici ce qui m'est arrivé sur cette terre d'Afrique :"

"Je traversais les grandes dunes au sud de Ouargla. C'est là un des plus étranges pays du monde. Vous connaissez le sable uni, le sable droit des interminables plages de l'Océan. Eh bien ! figurez-vous l'Océan lui-même devenu sable au milieu d'un ouragan : imaginez une tempête silencieuse de vagues immobiles en poussière jaune. Elles sont hautes comme des montagnes, ces vagues inégales, différentes, soulevées tout à fait comme des flots déchaînés, mais plus grandes encore, et striées comme de la moire. Sur cette mer furieuse, muette et sans mouvement, le dévorant soleil du sud verse sa flamme implacable et directe."

"Il faut gravir ces lames de cendre d'or, redescendre, gravir encore, gravir sans cesse, sans repos et sans ombre. Les chevaux râlent, enfoncent jusqu'aux genoux, et glissent en dévalant l'autre versant des surprenantes collines."

"Nous étions deux amis suivis de huit spahis et de quatre chameaux avec leurs chameliers. Nous ne parlions plus, accablés de chaleur, de fatigue, et desséchés de soif comme ce désert ardent. Soudain un de ces hommes poussa une sorte de cri ; tous s'arrêtèrent, et nous demeurâmes immobiles, surpris par un inexplicable phénomène connu des voyageurs en ces contrées perdues."

"Quelque part, près de nous, dans une direction indéterminée, un tambour battait, le mystérieux tambour des dunes ; il battait distinctement, tantôt plus vibrant, tantôt affaibli, arrêtant, puis reprenant son roulement fantastique."

"Les Arabes, épouvantés, se regardaient ; et l'un dit, en sa langue : "La mort est sur nous." Et voilà que tout à coup mon compagnon, mon ami, presque mon frère, tomba de cheval, la tête en avant, foudroyé par une insolation."

"Et pendant deux heures, pendant que j'essayais en vain de le sauver, toujours ce tambour insaisissable m'emplissait l'oreille de son bruit monotone, intermittent et incompréhensible ; et je sentais se glisser dans mes os la peur, la vraie peur, la hideuse peur, en face de ce cadavre aimé, dans ce trou incendié par le soleil entre quatre monts de sable, tandis que l'écho inconnu nous jetait, à deux cents lieues de tout village français, le battement rapide du tambour."

"Ce jour-là, je compris ce que c'était que d'avoir peur ; je l'ai su mieux encore une autre fois..."

Le commandant interrompit le conteur :

"Pardon, monsieur, mais ce tambour ? Qu'était-ce ?"

Le voyageur répondit :

"Je n'en sais rien. Personne ne sait. Les officiers, surpris souvent par ce bruit singulier, l'attribuent généralement à l'écho grossi, multiplié, démesurément enflé par les vallonnements des dunes, d'une grêle de grains de sable emportés dans le vent et heurtant une touffe d'herbes sèches ; car on a toujours remarqué que le phénomène se produit dans le voisinage de petites plantes brûlées par le soleil, et dures comme du parchemin."

"Ce tambour ne serait donc qu'une sorte de mirage du son. Voilà tout. Mais je n'appris cela que plus tard."

Maupassant (1883)

   Last Updated: April 1, 2008

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