Abstract

Field and laboratory investigations have confirmed differences between the acoustic and seismic emissions of “singing” and “booming” sands and revealed that booming grains possess extremely smooth surfaces. Singing sand is the most common of the two types of sound-producing sands. It occurs widely as a beach sand and consists of well-rounded highly spherical grains that have a well-sorted highly symmetric grain-size distribution. Sound is produced when the sand is mechanically sheared, possibly causing the closely packed grain array to dilate in a coherent manner. Frequency (>500 Hz) is controlled by grain size, and amplitude may in part relate to grain morphology. Booming sand is a relatively rare phenomenon that occurs in some desert regions. This sand produces a low-frequency (fo ≃ 80 Hz) sound during avalanching. The process efficiently (≈ 0.1 to 1 percent) produces very narrow band seismic energy in the 50- to 80-Hz range. Simultaneously produced audio signals are broader band but are composed of signals that peak at the same fundamental frequencies as the seismic emissions. In addition, the acoustic emissions display first and second harmonics. Acoustic production is ≈ 400 times less efficient than seismic energy production. Booming occurs in quartz and carbonate sand grains that are well sorted, fine skewed, and mesokurtic. The individual quartz sand grains are only moderately well rounded. When compared to normal eolian grains, however, they have highly polished surfaces that are smooth on the 1-µm scale. The exceptional smoothness of the grains may facilitate booming. The effective Q (magnification factor) and compressibility (k) of the grain system may be the key physical quantities involved in booming. Thus, whereas booming is rare in the terrestrial environment, it may be common in the high-Q soils of the Moon and the near waterless dune environment of Mars.

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