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🎁 中英文稿：
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.
這是超自然神靈的發出的聲音？不。但這聽起來像馬可波羅在13世界橫跨中國羅布泊沙漠時聽到的聲音。馬可波羅當時描述說，這種聲音是由多種樂器演奏而來。但是事實上，人們認為這種聲音是一種不尋常的樂器沙丘的作品。
"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."
“這種聲音非常像大提琴的音調。”梅蘭妮•亨特說到。亨特是加州理工學院的一名機械工程師。沙丘在坍塌時，就會唱歌，“發出的聲音就跟用琴弓拉大提琴的聲音很像。”
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.
亨特和同事們記錄了離家比較近的加利福尼亞凱爾索和尤里卡沙丘“唱歌”時的聲音。他們使用很多地震檢波器——一種可以插入沙中的麥克風——並利用探地雷達繪製了沙丘的結構。
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."
結果表明，會唱歌沙丘的結構不同於沉默的沙丘——最上層是一層均勻乾燥的沙子——約5英尺厚，而下層則是濕潤的沙子。而那層乾燥的沙子就像樂器的主體——會對特定的頻率聲波進行陷波和放大。“樂器的大小就會設定你聽到的頻率的範圍。沙丘也是這個原理。”
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.
沙丘坍塌時，會發出不同頻率的聲波。這些聲波會穿越幹砂層，碰擊濕砂層，反彈後，碰擊沙丘上部的空氣，如此往復迴圈。與此同時，許多聲頻就會抵消消散。但是一些聲波擁有80赫茲的頻率。這些聲波就會往返運動，放大並發出轟鳴聲。該研究結果發表在《流體物理學》雜誌上。
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.
