Booming Sand Dunes

Booming” sand dunes have a remarkable capacity to produce sounds that are comparable with those from a stringed instrument. This phenomenon, in which sound is generated after an avalanching of sand along the slip face of a dune, has been known for centuries and occurs in at least 40 sites around the world. A spectral analysis of the sound shows a dominant frequency between 70 and 110 Hz, as well as higher harmonics. Depending on the location and time of year, the sound may continue for several minutes, even after the avalanching of sand has ceased. This review presents historical observations and explanations of the sound, many of which contain accurate and insightful descriptions of the phenomenon. In addition, the review describes recent work that provides a scientific explanation for this natural mystery, which is caused by sound resonating in a surface layer of the dune. 281 A nn u. R ev . E ar th P la ne t. Sc i. 20 10 .3 8: 28 130 1. D ow nl oa de d fro m w w w .a nn ua lre vi ew s.o rg by C al ifo rn ia In sti tu te o f T ec hn ol og y on 0 7/ 17 /1 1. F or p er so na l u se o nl y. Click here for quick links to Annual Reviews content online, including: Other articles in this volume Top cited articles Top downloaded articles rehensive search Further ANNUAL REVIEWS EA38CH11-Hunt ARI 23 March 2010 17:22 OBSERVATIONS FROM HISTORICAL ACCOUNTS Desert travelers have referred to mysterious sounds originating from sand dunes for hundreds of years (Darwin 1835, Curzon 1923, Bagnold 1941). In 1923, the Marquess Curzon of Kedleston published Tales of Travel, which documented his own observations and reports of booming dunes from earlier world travelers. Although this work does not include scientific data or analysis, it does present an accurate and detailed description of the phenomenon as well as a list of locations where the booming sound can be heard. At the beginning of the 78-page chapter entitled “The Singing Sands,” Curzon wrote: I alluded to the phenomenon known in different places and parts of the world as singing sands, sounding sands, rumbling sands, musical sands, barking sands, moving sands, i.e. cases in which certain sands either when set in motion, or even in some cases when apparently quiescent, give forth sounds as of music, which are sometimes audible at great distance. In former days these tales, when they appeared in the pages of medieval travelers, were attributed to local superstition or to an excited imagination, and were not supposed to have any scientific basis. In the course of my travels I have made a study of these cases, about which I have found a good deal not only of literary inexactitude, but of scientific uncertainty, to prevail. . .The subject is one which, while severely scientific in one aspect, is in another full of a strange romance, since the voice of the desert, speaking in notes now as of harp strings, anon as of trumpets and drums, and echoing down the ages, is invested with a mystic fascination to which none can turn a deaf ear. As described by Curzon and shown in this review, the sounds generated by the dune have a musical quality because the sound occurs at discrete frequencies with higher harmonics (Lindsay et al. 1976, Haff 1979, Vriend et al. 2007). The sound results from an avalanching at the surface and can be transmitted sizable distances across the dune where no avalanching occurs. (To watch a video that demonstrates the sounds of booming sand, follow the Supplemental Materials link from the Annual Reviews home page at http://www.annualreviews.org.) As noted by Curzon, there has been considerable scientific disagreement about the cause of the sound. Although “booming” is not one of the adjectives used by Curzon, it is the term most commonly used in the current scientific literature (Bagnold 1941, Humphries 1966, Nori et al. 1997, Sholtz et al. 1997). Figure 1a shows a booming dune located in southern California. The earliest citation by Curzon is circa 800 A.D. from China, with later references to Marco Polo in the Gobi Desert (1295), the Afghan Emperor Baber outside Kabul (1519), and Charles Darwin in Chile (1835), as well as other reports from the Arabian Peninsula, the Western Sahara, the Libyan Desert, South Africa, the Hawaiian Islands, and North America. Other explorers or researchers produced documentation about many of the locations described by Curzon, including the extensive summary of locations provided by Lindsay et al. (1976). Carus-Wilson wrote in an 1890 letter to Nature that “only observers are rare—not the sands,” and now up to 40 locations with booming sand dunes have been identified. Table 1 summarizes the locations of the booming sand dunes, the approximate sizes of the dunes, and the sources of the references. In addition to identifying desert locations, Curzon’s accounts provide an accurate description of the circumstances in which the sounds are produced, a report of the nature and duration of the sounds, and some conjuncture about the scientific basis for the sounds. Although there were conflicting stories about the types of dunes that emit sound, Curzon concluded that although the dunes are of varying height and dimension (as demonstrated by the listing in Table 1), the sound was produced by avalanching sand along the face of the dune inclined at approximately 31◦ from the horizontal: sand’s angle of repose. Curzon also noted that the phenomenon became more likely 282 Hunt · Vriend Supplemental Material A nn u. R ev . E ar th P la ne t. Sc i. 20 10 .3 8: 28 130 1. D ow nl oa de d fro m w w w .a nn ua lre vi ew s.o rg by C al ifo rn ia In sti tu te o f T ec hn ol og y on 0 7/ 17 /1 1. F or p er so na l u se o nl y. EA38CH11-Hunt ARI 23 March 2010 17:22