- UID
- 1333346
- 在线时间
- 小时
- 注册时间
- 2018-3-14
- 最后登录
- 1970-1-1
- 主题
- 帖子
- 性别
- 保密
|
第一篇巨难，讲的是sand dune 的形成，怎么成堆的，怎么移动的，然后还有一些研究，生词太多没太读懂，建议考古，篇幅也很长。
Squeaking sand produces sounds with very high frequencies—between 500 and 2,500 hertz, lasting less than a quarter of a second. The peals are musically pure, often containing four or five harmonic overtones. Booming sand makes louder, low-frequency sounds of 50 to 300 hertz, which may last as long as 15 minutes in larger dunes (although typically they last for seconds or less). In addition, they are rather noisy, containing a multitude of nearby frequencies. Booms have never been observed to contain more than one harmonic of the fundamental tone.
These dramatic differences once led to a consensus that although both types of sand produce acoustic emissions, the ways in which they do so must be substantially different. In the late 1970s, however, Peter K. Haff, then at the California Institute of Technology, produced squeaks in booming sand, suggesting a closer connection between the two.
Both kinds of sand must be displaced to make sounds. Walking on some sand, for example, forces the sand underfoot to move down and out, producing squeaks. In the case of booming sand, displacement occurs during avalanches.
It is within the avalanche that sound begins and where the answers must be hiding. Before an avalanche can occur, winds must build a dune up to a certain angle, usually about 35 degrees for dry desert sand. Once the angle is achieved, the sand on the leeward side of the dune begins to slump. Intact layers of sand slip over the layers below, like a sheared deck of cards. At the same time, the individual grains in the upper layers tumble over the grains underneath, momentarily falling into the spaces between them and bouncing out again to continue their downward journey. Their concerted up-and-down motion is believed to be the secret source of sound. Fully developed avalanches, in which sliding plates of sand remain intact for most of their motion, have the greatest acoustic output. In some places, where large amounts of sand are involved, booming can be heard up to 10 kilometers away.
The mysteries of the vibrations are many. To begin with, the multiple frequencies of booming sand are not well understood. In the 1970s David R. Criswell and his collaborators at the University of Houston found that each frequency seems to exhibit its own rise-and-fall time, independent of the others. Taken together, these frequencies can cover a fairly broad range, the width of which is determined by various factors. For example, Sand Mountain booms at roughly 50 to 80 hertz; sands at Korizo, Libya, drone at between 50 and 100 hertz; and in the Kalahari Desert of South Africa, the frequencies range from 130 to 300 hertz. Such output—presumably caused by multiple modes of vibration within the shearing plates—is often unmusical and jarring.
Because it is caused by large volumes of shearing sand, the roaring is also loud. In fact, sounds made by booming sand can be nearly deafening, and the vibrations causing them can be so intense that standing in their midst is nearly impossible.
A good place to start in exploring the vibrational properties of sand is with the grains themselves. The mean diameter of most sand grains, whether acoustically active or not, is about 300 microns. Usually the grains in a booming dune are very similar in size, especially near the leeward crest, where the sound most often originates; such uniformity allows for more efficient shearing. Otherwise, the smaller grains impede the smooth motion of the larger ones.
Similar sizes do not alone allow sand to boom. On the contrary, the booming sands of Korizo and Gelf Kebib, also in Libya, feature an uncharacteristically broad range of particle sizes. Moreover, silent dune sand often contains grains somewhat similar to nearby booming sand.
Grains of booming sand also tend to have uncommonly smooth surfaces, with protrusions on the scale of mere microns. Booming dunes are often found at the downwind end of large sand sources; having bounced and rolled across the desert for long distances, the sand grains in these dunes are usually highly polished. Over time a grain can also be polished by repeated shifts within a moving dune. And squeaking sand as well tends to be exceptionally smooth.
Close inspection of Sand Mountain and Kalahari booming sand, however, reveals that not all grains are highly spherical or rounded. And in 1936 A. D. Lewis in Pretoria, South Africa, even claimed to have produced booming in the cubed grains of ordinary table salt. Conversely, spherical glass beads cannot be made to boom. These findings show that although smoothness and roundness are essential to producing sound, so is some degree of roughness.
Another important factor is humidity, because moisture can modify the friction between the grains or cause sand to clump together, thus precluding shearing. Sounds occur in those parts of the dune that dry the fastest. Precipitation may be rare in the desert, but dunes retain water with remarkable efficiency. Sand near the surface dries quickly, however, and sand around a dune’s crest tends to dry the fastest.
Near the leeward crest, the combination of smooth, well-sorted grains and lack of moisture leads to conditions more likely to produce sounds during shearing. And because wind usually deposits more sand closer to the top of the lee face, sand accumulates there faster than in lower regions, thereby slowly increasing the dune’s incline to where avalanches occur.
Typically, large platelike slabs of sand break off near the crest. In booming sand, these plates tend not to slow into loose flows as they encounter gentler slopes. Instead their upper parts collapse or telescope violently into the lower parts. The plates’ eventual breakup is unusually turbulent.
Learning more about sound-producing sand has not been easy. Research has been hindered by the rarity of the phenomenon—especially booming sand —and the difficulty in reproducing sounds in laboratories. In addition, for years researchers did not clearly differentiate between booming and squeaking sands, making the early literature on the topic less than reliable.
|
|
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2019-10-13 10:42:06
收藏
收藏
楼主