A recent article in Scientific American ("Booming Sands," by Franco Nori, Paul Sholtz and Michael Bretz, September 1997) reviews some explanations for sands that make sounds. Thomas S. Ahlbrandt of the U.S. Geological Survey in Denver, Colorado also has extensive personal familiarity with the phenomenon. He offers the following explanation:

Singing Dune in Mojave National Preserve.
For anyone who has experienced these phenomena as I have in several North America deserts, the mysticism or romance of the desert is amplified. Booming dunes has been discussed in Middle East literature for at least 1,500 years, in Chinese literature for 1,200 years. In 1941, British investigator R. A. Bagnold, who wrote a pioneering book in 1941, updated in 1954, entitled The Physics of Blown Sand and Desert Dunes, described them as the "song of the sands, the song of sirens who lure travelers to a waterless doom, the tolling of underground bells in sand engulfed monasteries"

Booming sands are a worldwide phenomenon with examples from North and South America, Africa, the Middle East, the Far East and Hawaii. The Hawaiian examples are interesting in that the sand composition is carbonate; most booming sands are composed dominantly of quartz sand. Most authors consider booming sands a feature of warm dry climates, however, I have heard booming dunes in two cold climate fields--the Killpecker Dunes in Wyoming and the Great Sand Dunes in Colorado.

Dunes are known to make two very different musiclike sounds when sand is sheared (commonly by the avalanche process). These sounds fall broadly into either the "whistling sand" or "booming sand" category. A higher pitched sound in the 800 to 1,200 Hz or 500 to 2,500 Hz range is known to occur in dune sand and beach sand; most commonly in dry sand but occasionally in wet sand conditions. Sands making such sounds are known as whistling, singing, squeaking, or barking sands. The sound usually lasts for a very short time, less than a second.

Sands of the requisite size, and surface smoothness can be easily induced to "whistle" by poking a stick or pencil into the sand, sweeping the sand surface quickly with a hand or stepping on the sand. The sand does not have to be on a steep surface; in fact many beach sands "whistle."

Squeaking or whistling sand is found where quartz sand is very well rounded and highly spherical. The frequency of the sound is related to the mean grain size and the amplitude is controlled by the surface texture of the grains. The loudest squeaking observed by researchers was produced in the middle of the day by hot dry sand in the tidal zone, although they observed that completely water-saturated sand could be induced to squeak as water was receding.

"Booming sands" are much more impressive in my opinion. One both hears them in a lower frequency band (50 to 264 Hz) than "whistling sand" and feels them--the ground trembles, the surface moves and ripples. Thus, booming sands have both acoustic and seismic components and the sound can last for much longer than do whistles or squeaks.

Bagnold captured the feelings of the experience in western Egypt, 300 miles from the nearest habitation in the following narrative:

"On two occasions it happened on a still night, suddenly--a vibrant booming so loud that I had to shout to be heard by my companion. Soon other sources, set going by the disturbance, joined their music to the first, with so close a note that a slow beat was clearly recognized. This weird chorus went on for more than five minutes continuously before silence returned and the ground ceased to tremble."
The explanation of the sound involves a brief discussion of physics. Bagnold commenced his work in the 1940s, but interrupted them with his efforts in support of the British Army in the deserts of North Africa, where he was a colonel and served with great distinction. In 1966, Bagnold published an article entitled The Stearing and Dilatation of Dry Sand and the "Singing Mechanism." This study was followed by a series of other publications by investigators who have gone into even more detailed explanations, which I will attempt to summarize briefly.

When dry sand accumulates to the point that it can avalanche, under certain conditions the shearing of sand and the resultant grain interactions of the sheared sand create both seismic and acoustic responses. To understand the shearing (avalanching) process, I need to briefly explain how sand is moved by wind. When wind reaches a certain threshold speed (generally at about 14 mph) as it flows over loose sand, the sand starts to move in a series of jerks and jumps called saltation.

As wind speed increases, saltation height increases to the point that one can truly have a miserable experience even at eye level. The grains bounce high due to the impact of one grain against another, like billiard balls, but sand grains also are lifted aerodynamically by the spin imparted to the grain by the striking grain.

Bagnold draws an analogy for this dispersive pressure (grains striking other grains) as snooker balls on an infinite table contained only by these shear boundaries. The moving sand starts accumulating into a sand dune and the dune builds to the point where the downwind side of the dune actually is sheltered from the wind. Sand is dropped from saltation (a process know as grainfall) and accumulates until it exceeds the angle of repose of sand, generally from 32 to 34 degrees. At this point, dry sand will start to avalanche.

The disruption (shearing) that occurs within the avalanching sand results in the conditions, given the right type of sand, that give rise to the acoustic and seismic responses known as "booming." My personal experience is that sands boom after a strong sand storm where avalanching, a continuous process during such storms, has not stabilized. Thus, there is an unstable condition where an equilibrium has yet to develop; any change, such as the sudden abatement of wind, may result in "booming" if the sand has the right characteristics.

Some researchers have carefully examined the textures and surfaces of booming sands and concluded that the sand grains needed to be highly polished and moderately well rounded. This high degree of polishing, sphericity and sorting is produced in certain dune settings as grains saltate into each other, each impact further rounding and polishing grains. Many sands do not "boom" due to surface texture, grain size ranges, or the occurrence of moisture and vegetation.

As a final thought, some authors conclude that although booming sand is relatively uncommon on Earth, it may be common in the waterless or near waterless environments of the Moon and Mars. One wonders if the Sojourner rover has felt or heard the "song of the sands."

FURTHER READING:

THE PHYSICS OF BLOWN SAND AND DESERT DUNES. R. A. Bagnold. Chapman and Hall, 1941; Methuen, 1954.

THE SHEARING AND DILATATION OF DRY SAND AND THE "SINGING" MECHANISM. R. A. Bagnold in Proceedings of the Royal Society, London, Series A., Vol. 195, pages 219-232, 1966.

SOUND-PRODUCING DUNE AND BEACH SANDS. J. F. Lindsay, D. R. Criswell, T. L. Criswell, B. S. Criswell in Geological Society of America Bulletin Vol. 87, pages 463-473; 1976.

MEASUREMENTS OF LONGITUDINAL AND SHEAR WAVE VELOCITIES IN "SINGING SAND." Masao Kimura, Masato Matsushita and Hirokazu Kawaguchi. Journal of the Faculty of Marine Science and Technology, Tokai University, Vol. 38, pages 29-41; 1994.

BOOMING SAND. Franco Nori, Paul Sholtz and Michael Bretz in Scientific American, Vol. 277, No. 3, September 1997