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British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue.
英國物理學家瑞利勛爵因為1871年發現氬元素及解釋而變得舉世聞名。
But he also puzzled over this: sound of a kettle whistling.
但是他也一直飽受一個問題的困擾:那就是水壺沸騰時的鳴叫聲。
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.
瑞利勛爵知道水壺發出聲響是由于水蒸氣穿過厚厚蓋子中間有空隙的孔所致。
He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap.
他懷疑蒸汽經過蓋子孔隙時變得不穩定，是在間隙內引起了聲學回路。
But he couldn't prove it.
但他一直無法證明這一猜測。
Now two engineers at Cambridge University claim to have solved the puzzle—and proved Rayleigh wrong.
而現在，劍橋大學的兩名工程師宣布他們已經解決了這個難題—而且還證明瑞利勛爵的猜測是錯的。
The work is in the journal Physics of Fluids.
這一研究已在《流體物理學》雜志上發表。
The engineers found that a kettle actually whistles in two distinct ways.
工程師們發現水壺發出聲音有兩種方式。
It starts off with air vibrating in the gap between the layers of the lid, like when you do this and this.
開始是由于空氣在蒸汽和壺蓋間的空隙中引起振動，就像你在這里聽到的2種一樣。
But as the pressure builds, vortices of steam peel off from the jet exiting the lid.
但隨著壓力越來越強，蒸汽的漩渦從壺蓋的出口中脫離。
Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it.
每個漩渦創造的特定聲音波段是根據噴口的距離長度和內部當時所含的壓力所定，
Rising temperature means rising pressure, which produces a rising whistle which means it's time for tea.
溫度上升意味著壓力上升，而與此形成的哨聲是在告訴我們是享用一杯茶的時間了。
