Bedouin, desert dwellers, sand surfers and those just passing through –like Marco Polo– have heard the eerie phenomenon of shifting sands. In National Geographic’s Why Sand Dunes Go Boom, Stefan Lovgren writes:
"Tribes of the Sahara in Africa are said to have thought God was speaking to them through the sand."
As it happens, there are several theories revolving the source of the sound, God simply being the earliest documented conjecture. A 1954 hypothesis suggests the effect is achieved by the production of electrical currents by quartz grains under mechanical stress. –Sort of like a pick up on a classical guitar, I'd like to imagine, whether true or false.
Modern theories were aggregated in a September 1997 Scientific American article Sound-Producing Sand Avalanches. In that article Paul Sholtz, Michael Bretz and Franco Nori ( Department of Physics, The University of Michigan, Ann Arbor) provide an ‘Analysis of the theories and experiments done so far on sound-producing (e.g., roaring, booming) sand avalanches’. The authors begin with an encapsulated history of the topic:
"There exist two distinct types of sand that are known to produce manifest acoustic emissions when sheared. The more common of the two, known colloquially as "squeaking" or "whistling" sand, produces a short (< 1/4 sec), high-frequency (500-2500 Hz) "squeak" when sheared or compressed. It is fairly common in occurrence, and can be found at numerous beaches, lake shores and riverbeds around the world. The other, rarer type of sound-producing sand occurs principally in large, isolated dunes deep in the desert (Nori et al.,1996; Criswell et al., 1975). The loud, low-frequency (typically 50-300 Hz) acoustic output of this "booming" sand, resultant upon avalanching, has been the subject of desert folklore and legend for centuries."
Even more esoteric:
"Marco Polo (1295) wrote of evil desert spirits which "at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." References can be found dating as far back as the Arabian Nights (Carus-Wilson, 1915), and as recently as the science fiction classic Dune (Herbert, 1984). Charles Darwin (1889) also makes mention of it in his classic Voyages of the Beagle . At least 31 desert and back-beach booming dunes have been located in North and South America, Africa, Asia, the Arabian Peninsula and the Hawaiian Islands (Lindsay et al., 1976; Miwa and Okazaki, 1995).
Sharply contrasting differences between squeaking and booming sands have led to a consensus that although both types of sand produce manifest acoustic emissions, their respective sounding mechanisms must be substantially different. More recent laboratory production of "squeaks" in booming sand (Haff, 1979) has nonetheless suggested a closer connection between the two mechanisms. A satisfactory explanation for either type of acoustic activity is still unavailable."
Even a decade later, in Matthew Chalmers' November 2006 Physics World article, The Troubled Song of the Sand Dunes, we learn the basic question of what makes the sand sing still hasn’t been answered. Chalmers writes of a researcher, Stéphane Douady, having to ‘trawl’ through the scientific literature in order to come up with a meager 10 papers relating the acoustic emission of sand dunes:
"But despite some articles also predicting that the sound comes from the relative motion of sand grains, none offered a convincing explanation of how this happens…Singing dunes, he thought, were the result of air being pushed in and out between the synchronized grains… effectively turning it into the membrane of a powerful loudspeaker."
Douady’s idea was supported by an observation that sand only sings when sand layers above a certain thickness slide over one another.
"This, he reasoned, means that the sound must arise from a resonance within the shear layer itself, whereby grains bump over each other at the same frequency and set up standing waves that, in turn, synchronize the grains."
But Douady’s equally passionate colleague, Bruno Andreotti, looked at the same evidence and arrived at quite different conclusion:
"...the collisions between grains excite waves outside the shear layer on the dune surface that then synchronize the collisions via a mechanism called wave–particle locking."
And lets just say Andreotti backed up his argument with a bit o’ math.
Returning to National Geographic’s Lovgren, that author offers us yet another take on subject: While scientists have long believed the friction between surface grains of sand avalanching produces the sonic phenomenon, Melany Hunt, a mechanical-engineering professor at the California Institute of Technology in Pasadena, noticed that dunes continue to sound even after surface movement has stopped, and that such sounds differs depending on the season.
So what gives?
Hunt postulates that the sometimes musical frequencies of moving sand are created when "the friction between sand grains creates a noise that reverberates back and forth between dry sand on the surface and wet sand below."
Knowing this, it is difficult not to feel a bit of irony when rereading the 1997 Scientific American article. Note of the authors' early observation:
"Booming and squeaking sands each show a markedly different response to water exposure. Booming occurs best when the grains are very dry, preferably several weeks after the last rain. Small amounts of atmospheric humidity, which creates a fluid surface coating on the grains, effectively preclude booming emissions in these desert sands (Lewis, 1936). Even mixing as little as five drops of water into a 1-liter bag full of booming sand can silence the acoustic emissions (Haff, 1979). Similarly, squeaking sand that is visibly moist is not acoustically active either.
However, sound is most easily produced from squeaking sand immediately after the grains have been "washed" in water and subsequently dried. It is not clear whether this is due to the washing away of fine impurities in the sample (Brown et al., 1961) or to the creation of a looser, more natural grain packing (Clarke, 1973), although it may explain why squeaking sand typically does not extend inland more than 30 m from the shore (Richardson, 1919). This process of cleaning can also "revive" squeaking sand that has lost its ability to squeak, a condition that often occurs after repeated compression (Hashimoto, 1951). Finally, squeaking sand can emit sound even when completely submerged in water (Lindsay et al., 1976; Brown et al., 1961), suggesting that intergranular cohesion in moist sand precludes acoustic output."
Ah, so close and yet so far: They must be kicking themselves! If Hunt is correct, it’s not that the sound is connected to the relative cleanliness of the sand, but rather to the water that sits beneath it.
I am not a scientist, of course, but as this discussion appears to be an ongoing one, I won't rule out the possibility that some combination of all the above theories is at play. Wouldn't many like think that IT IS God's voice heard in the sand?
Click the link to listen to an impressive sample of Seismic Sounds, recorded by John N. Louie, Associate Director, Nevada Seismological Laboratory.
Stéphane Douady and Bruno Andreotti, both, have also created their own respective great pages to hear dune sounds–
Stéphane Douady: Chant des Dunes
Bruno Andreotti: Morphodynamics Lab The Song of Dunes