ContemporaryPhysics, 1997,volume 38, number 5, pages 329 ± 342 Sound-producingsand avalanch es PAUL SHOLTZ, MICHAEL BRETZ and FRANCO NORI* Sound-producingsand grains constitute one of nature’ smore puzzlingand least understood physicalphenomena. They occur naturally in two distinct types: booming and squeaking sands.Although both varieties ofsand produce unexpectedly pure acoustic emissions when sheared,they diŒ er intheir frequencyrange and duration of emission, as well as the environmentin which they tend to be found. Large-scale slumping events ondry booming dunescan produce acoustic emissions that can be heard up to 10 km awayand which resemble hums,moans, drums, thunder, foghorns or the drone of low-¯ ying propeller aircraft.These analogies emphasize the uniqueness of the phenomenon and the clarity of the producedsound. Although reports ofthese sandshave existed inthe literature for overone thousandyears, a satisfactoryexplanation for either typeof acoustic emission is still unavailable. 1.Introduction sands haveled to aconsensus that although both types of There existtwo distinct types of sand that areknown to sand produce manifest acoustic emissions, their respective produce manifest acoustic emissions when sheared. The sounding mechanisms must be substantially diŒerent. More more common of the two, known colloquiallyas ` squeak- recent laboratory production of `squeaks’ in booming sand ing’or `whistling’sand, produces ashort ( <1/4 s), high- (HaŒ1979) has nonetheless suggested acloser connection frequency (500± 2500Hz) `squeak’ when sheared or com- between the two mechanisms. Asatisfactory explanation pressed. It is fairlycommon in occurrence, and canbe for either type of acoustic activityis still unavailable. found atnumerous beaches, lakeshores and riverbeds This brief reviewis not aclosed chapter in awell- around the world. The other, rarer type of sound- understood research area,but is asummary of the producing sand occurs principallyin large,isolated dunes incomplete and unsatisfactoryproposals put forward to deep in the desert (Nori et al.1997,Criswell et al. 1975). explainsound production in squeaking and, especially, The loud, low-frequency(typically50 ± 300Hz) acoustic booming sand. Our reviewpoints out the serious problems, output of this `booming’ sand, resultant upon avalanching, incomplete theories, and the severe limitations of diŒerent has been the subject of desert folklore and legend for approaches published so far. centuries. Marco Polo (1295)wrote of evildesert spirits Booming and squeaking sands eachshow amarkedly which `attimes ®llthe airwith the sounds of allkinds of diŒerent response to waterexposure. Booming occurs best musical instruments,and alsoof drums and the clash of when the grainsare very dry, preferably severalweeks after arms’.References canbe found datingas far back asthe the lastrain. Small amounts of atmospheric humidity, ArabianNights (Carus-Wilson 1915),and asrecently asthe which creates a¯uid surface coatingon the grains, science ®ction classic Dune (Herbert 1984).Charles Darwin eŒectively preclude booming emissions in these desert (1889)also makes mention of it in his classic Voyagesof the sands (Lewis1936). Even mixing as little as® vedrops of Beagle.At least31 desert and back-beach booming dunes waterinto aone litre bagfull of booming sand cansilence havebeen located in North and South America, Africa, the acoustic emissions (HaŒ1979). Similarly, squeaking Asia,the Arabian Peninsula and the HawaiianIslands sand that is visiblymoist is not acousticallyactive either. (Lindsay et al.1976,Miwa and Okazaki1995). Sharply However,sound is most easilyproduced from squeaking contrasting diŒerences between squeaking and booming sand immediatelyafter the grainshave been `washed’in waterand subsequently dried. It is not clearwhether this is due to the washingaway of ®ne impurities in the sample *Authorto whom correspondence should be addressed (Brown et al.1961)or to the creation of alooser, more Authors’address: Departmentof Physics, The University of Michigan, AnnArbor, MI 48109-1120,USA natural grainpacking (Clarke 1973), although it may http://www-personal.engin.umich.edu /~nori/ explainwhy squeaking sand typicallydoes not extend 0010-7514/97 $12.00 Ó1997 Taylor &FrancisLtd 330 P. Sholtz et al. inland more than 30m from the shore (Richardson 1919). located between the grains(Bolton 1889a).Also, a This process of cleaningcan also ` revive’squeaking sand preliminary explanation of booming relied on observed that has lost its abilityto squeak, acondition that often electricalcharging of the grains(Lewis 1936). Such air- occurs after repeated compression (Hashimoto 1951). cushion and electricalcharge models were,however, based Finally,squeaking sand canemit sound evenwhen on misguided evidence and havebeen discredited bymore completely submerged in water(Lindsay et al.1976,Brown recent experiments. Most modern theories stress the et al.1961),suggesting that intergranular cohesion in moist importance of intergranular friction. It has been suggested sand precludes acoustic output. that the unusually smooth and polished surfaces of The mean grainsize (diameter) of most sand, whether or booming grainsmay allow for exaggeratedvibration at not it is acousticallyactive, is roughly 300 l m. The the natural resonant frequency of sand (Criswell et al. 1975, frequency of emission generated bysqueaking sand is Lindsay et al.1976).DiŒ erences in grainpacking have also thought to varyas the inverse square root of the mean grain been considered (Humphries 1966).The most complete size (Bagnold1954a, 1966), although mean grainsize does development is givenby Bagnold (1954a, 1966), who argues not byitself determine the abilityof sand to sound (Lindsay that both types of emission result from nonlinear oscilla- et al.1976).It is unlikely that booming frequencies depend tions of dispersive stress in the layerof movinggrains, or similarlyon grainsize alone, asa fairlywide range of shear layer.Even this analysis,however, fails to address key fundamental frequencies is often generated in large-scale aspects of the booming mechanism. slumping events. Particle-size distributions (Folk and Ward 1957)of sound-producing grainsusually only extend over a narrow range,a condition that is called` being well-sorted’. 2.Acoustic and seismic outputof a boomingevent Also, booming and squeaking grainstend to be both Booming emissions produce awidevariety of sounds that spherical and lackabrupt surface asperities. The latter havebeen compared to moans, hums, roars, drums, condition is called` being well-rounded’(Powers 1953),a tambourines,thunder, cannon ®re, the rumble of distant term not to be confused here byour use of the word carts, foghorns, the buzzing of telegraph wiresand the `polished’,which wetake to mean granularsurfaces which drone of low-¯ying propeller aircraft (Lindsay et al. 1976, aresmooth on the 1 l mlength scale.Both types of sand Curzon 1923).These analogiesall emphasize the distinct further exhibit anunusually high shear strength, and in the qualityof the booming sand phenomenonand the clarity caseof squeaking sand, adecrease in shear strength has with which these sounds areproduced. The comparisons to been shown to correspond to adecrease in sounding ability drums and tambourines moreover illustrate how the (Hashimoto 1951,Humphries 1966).Experiments in which booming mechanism canproduce characteristic beat spherical glassbeads produce acoustic emissions similarto frequencies (1to 10Hz), though to result from periodic those of genuine squeaking sand when compressed, albeit amplitude modulation of the acoustic emissions which are under somewhat contrived experimental conditions (Brown often observed in prolonged, large-scale¯ ows.Perhaps et al.1965),provide further support for the notion that most illuminating,however, are those analogieswhich squeaking is caused primarilyby frictional eŒects. The compare booming emissions to musical instruments, such combination of these four grainproperties (ahigh degree astrumpets, bells or low-stringed instruments.Such clear of: size-sorting, sphericity, roundedness and resistance to emissions usuallyoccur only in small-scale¯ ows,whereby shear) is thought to be criticalfor the onset of squeaking. only one fundamental frequency of vibration is produced. Booming, on the other hand, is substantially less sensitive Criswell et al.(1975)point out the similarities in the to diŒerences in grainshape and sorting, and is likely acoustic amplitude trace of asmall-scale, in situ booming governed bythe unusually smooth and polished surface emission and that of apipe organ.It is remarkable that an texture present in allbooming grains(Lindsay et al. 1976). avalancheof granularmaterial could produce anacoustic Exactlywhat governs either sounding mechanism is still oscillation comparable in purity to a®nely-crafted musical anopen question. Research has been hindered both bythe instrument. Our 1994observations of small,induced rarityof the phenomena and the diculty in reproducing avalanchesat Sand Moutain, Nevada,during the second the sounds in alaboratory environment. Moreover, the driest summer on record, revealemissions similarto a increasingtra c of vehicleson dunes appears to suppress didjeridoo (an aboriginalinstrument from Australia) with the natural sounds of sands. Furthermore,manyresearch- its low,droning cadence. ers had problems diŒerentiating between booming and In general,several fundamental frequencies of vibration squeaking sands, and the earlyliterature on the