Surface texture and sound-producing mechanism of booming quartz sands

Abstract

BOOMING sand as a miraculous natural phenomenon has long attracted great interest of scholars at home and abroad and evoked a variety of guesses and hypotheses on the sound-producing mechanism'']. Over the past 100 years, although numerous studies on this subject were carried out, almost no convincing explanation concerning the sound-producing mechanism of booming sands has so far been put forward"]. Recently, Goldsack et a l . suggested that the sound production of booming sand could be attributed to the Si02 gel deposited on the sand grain surface. The reason for this is that booming sands not only have Si02 gel deposited on their surface but also are similar to industrial gel in chemical composition and sonorous frequency spectrum[31. However, they did not explain the sound-producing nature of booming sands in detail. It is important to mention here the fact that the SiOz gel deposited on the sand grain surface is similar to the industrial gel not only in chemical composition but also in physically porous surface textureL4]. To further confirm the contribution of the surface texture of quartz grains to the sound production of booming sands['], the authors conducted the following experiments. ( i ) Washing experiment of sand grain surface. Desert sand and beach sand in the grain-size range of 0.10-0.25 mm collected from Japan, Australia, Kuwait and China were first treated with hydrochloric acid (HC1) for 10 min, then washed with distilled water and dried in oven. Under the impact excitation of glass rod, the clean sands emit a sound with the same frequency spectrum (400-900 Hz) as that of booming sands. Examined under the scanning electron microscope (SEM), the treated sands showed a better porous surface texture than that of the untreated sands (figure l ( a ) and ( b ) ) . ( ii ) Simulated chemical cornsion experiment of glass microball surface. Industrial glass microballs, 0.08-0.10 mrn in diameter, were selected for this experiment. First, the glass microballs were soaked in the hydrofluoric acid solution (HF), then washed with distilled water and oven-dried. Under the impact excitation of glass rod, the treated glass microball without SiQ gel on surface unexpectedly emitted a pleasant sound with the same frequency spectrum as that of booming sands. To our astonishment, as examined under the SEM the glass microballs treated with HF solution exhibited a porous appearance, whereas the untreated glass microbalk which emit no noise were devoid of such surface textures (fig. l (c) and (d)) . Going back to the 1930s, when Bagnold used the wcalled "frozen water beads", or known as man-made glass balls, to conduct sonorous experiments and faded to hear the booming due to lack of such surface textures. Above-mentiond experimental results showed that the sound-producing mechanism of booming sands is unrelated to the chemical composition of sand grain surface but related to their physically porous (pit) surface texture. From this it follows that under the actions of wind erosion, water erosion, chemical corrosion and chemical precipitation the surface of natural sand grains formed the resonance chambers consisting of porous (pit) texture. This is in fact the internal cause for the