What Makes Sand Dunes Sing

鸣沙之因

Engineers at Caltech discovered that for sand dunes to produce sound they need a dry layer on top that amplifies internal frequencies during sand movement. Christopher Intagliata reports

加利福尼亚理工学院的工程师发现，为了让沙丘产生声音，需要在沙丘表面有一层干沙层，以来放大内部沙子运动时产生的声波频率。

撰文/播音 克里斯托弗·因塔利亚塔（Christopher Intagliata）

翻译 惠丹丹

审核 M

<<Dune sounds>> The sound of supernatural spirits talking? Well, no. But that’s what it sounded like to Marco Polo as he travelled through China's Lop Desert in the 13th century. He also described the sound as “a variety of musical instruments.” And in reality, the mysterious noises are the product of what could be considered to be an unusual sort of instrument—the desert’s sand dunes. <<dune sound>> 

<<沙丘发出的声音>> 这是幽灵在说话吗？不是的。但马可波罗在13世纪穿越中国的罗布泊沙漠时，就是这样认为的。他还把这种声音描述为是各种各样的乐器演奏出来的。实际上，这种神秘的声音可以认为是由一类不平常的乐器发出的——沙丘。<<沙丘发出的声音>>

"And it's a sound that's very similar to kind of the tones you'd get on a cello." Melany Hunt, a mechanical engineer at Caltech. The dunes sing when sand avalanches down the side, "which would then be somewhat equivalent to using the bow on the strings of the cello."

加利福尼亚理工学院的机械工程师梅兰妮·亨特（Melany Hunt）说“这种声音与你所听到得大提琴的声音非常相似。”当一边的沙子奔腾而下，沙丘就发出声音，“这种时候就像用弓划过大提琴上的琴弦一样”

Hunt and her colleagues recorded the sounds produced closer to home, by California's Kelso and Eureka Dunes. They used four dozen geophones—microphones you stick in sand. And they mapped the dunes' structure with ground penetrating radar. Turns out, dunes that sing are built differently from silent dunes—they’re topped with an even layer of dry sand—about five feet thick—on top of all the damp sand below. And that layer of dry sand is like the body of an instrument—it traps and amplifies certain frequencies more than others. "And so really the size of the instrument is really going to set the range of frequencies that you get. And that's the same thing with this dune." 

亨特和她的同事就近录下来自加利福尼亚的科尔索和尤里卡沙丘发出的声音。她们使用了48个地音探测器——一种插入沙中的麦克风设备。同时，利用探地雷达绘制了被测沙丘的结构图。结果表明，发出声音的沙丘与不发声音的沙丘具有不同的结构——发出声音的沙丘上面覆盖着一层均匀的干沙——大概5尺厚——在所有潮湿沙子的上面。那一层干沙层就像是乐器的主体——它能更好地抓取并放大特定频率的声波。“就像乐器的大小决定你听到的频率的范围一样，沙丘也是这样的。”

When sand avalanches, it shoots off sound waves of various frequencies. Those waves travel through the dry sand, hit the wet layer, bounce back, hit the air on top of the dune, bounce back, and so on. As that happens, a lot of the frequencies cancel out and dissipate. But certain waves have the goldilocks frequency, around 80 Hertz. As they travel back and forth, they amplify, creating that booming sound. The study is in the journal Physics of Fluids. [N. M. Vriend et al, Linear and nonlinear wave propagation in booming sand dunes]

当沙丘崩塌时，它发出各种频率的声波。那些声波穿过干沙，碰撞到潮湿沙层后反射回来，打到沙丘上面的空气，再反射回去，这样不断重复。这一系列的过程中，很多的声波频率都消失了。但是有一些声波就像金准则一般，大概80Hz左右。随着它们来回传播，它们的频率被放大，发出较大的声音。这一研究发表在《流体物理学》杂志上。

The bad news? Since the 'instrument' requires a uniform, thick layer of dry sand, Hunt says the dunes tend to fall quiet in the cooler, wetter months. Meaning you'll have to brave summer heat if you want to hear the dunes when they're really in tune.

坏消息是，由于沙丘这样的乐器需要一种均匀的，厚的干沙层，亨特认为较冷和潮湿的月份，沙丘会表现的很安静。也就是说，如果你想听沙丘真正的发出着调的声音，你必须勇敢挑战夏天的燥热。

（题图来源：wahyuinqatar.com）