Tea Kettles Stop Whistling in the Dark
瞭解茶壺吹口哨的成因 (in the dark 一無所知，不再對茶壺吹口哨的原因一無所知)
More than a century after relativity, physics can now explain how a tea kettle whistles. Wayt Gibbs reports
自愛因斯坦發表相對論以來，已經超過一世紀，物理學終於可以解釋爲什麼茶壺會吹口哨了。
文章出處:
http://www.scientificamerican.com/podcast/episode.cfm?id=tea-kettles-stop-whistling-in-the-d-13-12-09
British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue. But he also puzzled over this: [sound of a kettle whistling].
英國有一位物理學家 Lord Rayleigh，他最出名的是發現了氬，還有在1871年解釋了天空為何是藍色的。但他仍然苦苦思索著，茶壺的口哨聲。(原網站有文章聲音檔，可聽見燒開水的聲音)
Rayleigh knew that a kettle makes that sound when steam jets through the hole in a thick lid that has a gap in the middle. He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap. But he couldn’t prove it.
Rayleigh知道當蒸氣噴射通過厚鍋蓋上的洞，其在中間有個空隙，茶壺會因而發出口哨聲。他推測這蒸氣流在空隙中會變得不穩定，並且在空隙中形成了一種聲音的反饋迴路。但是他無法證明這項假說。
Now two engineers at Cambridge University claim to have solved the puzzle—and proved Rayleigh wrong. The work is in the journal Physics of Fluids. [R. H. Henrywood and A. Agarwal, The aeroacoustics of a steam kettle]
如今有兩個來自劍橋大學的工程師，宣稱已解開這個謎團，且證明了Rayleigh的想法是錯的。這項研究發表於"液動的物理學"[R. H. Henrywood and A. Agarwal, 蒸氣茶壺的氣動聲學]
The engineers found that a kettle actually whistles in two distinct ways. It starts off with air vibrating in the gap between the layers of the lid, like when you do this [sound of blowing over an empty bottle] and this [sound of whistling].
工程師發現事實上茶壺是以兩種截然不同的方式發出口哨聲。它首先是從空氣在鍋蓋層中的空隙振動開始，像是當你在做這個時[燒開一個空的茶壺聲]和這個[口哨聲]
But as the pressure builds, vortices of steam peel off from the jet exiting the lid. Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it. Rising temperature means rising pressure, which produces a rising whistle. Which means it’s time for tea.
但是當壓力形成時，蒸氣的渦流在氣流從鍋蓋離開時也消失了。每一個氣旋都會以固定的頻率發出聲波，而其頻率的不同，取決於壺嘴的長度還有壺內的壓力。溫度上升就表示壓力也在升高，這樣也會產生較高音的口哨聲。這也表示喝茶的時候到了。
