The odds are good that you have a tea kettle sitting in your kitchen. Maybe you don't use it to make tea--maybe you heat water in it to make oatmeal or coffee. But just about everyone has a kettle, or is at least familiar with the whistle kettles make when their water heats up. Strangely, even after hundreds of years, no one knew exactly why kettles began to whistle when they reached a certain temperature. Or, more accurately, we didn't know how.
The why is a simple answer: as steam builds up and heats up within the kettle, it causes vibrations. As the steam escapes the kettle more quickly, the vibrations grow more intense, and the whistling gets louder. But how does that produce the all-too-familiar whistling noise? "Although the sound of a kettle is understood to be caused by vibrations made by the build-up of steam trying to escape, scientists have been trying for decades to understand what it is about this process that makes sound," writes Phys.org.
As far back as the 19th century, John William Strutt, 3rd Baron Rayleigh and author of the foundational text, The Theory Of Sound, was trying to explain it. In the end...Lord Rayleigh was forced to concede that 'much remains obscure as regards the manner in which the vibrations are excited.'
Modern science, however, has cracked the case. A pair of researchers from Cambridge have tackled the problem and come up with an explanation. They modeled (in software) how air flows inside a kettle to explain why it creates a whistling noise. Once the air inside the kettle reaches a certain flow speed, small swirling vortices are created. And it's those vortices that can produce sound.
"As steam comes up the kettle's spout, it meets a hole at the start of the whistle, which is much narrower than the spout itself," writes Phys.org. "This contracts the flow of steam as it enters the whistle and creates a jet of steam passing through it. The steam jet is naturally unstable, like the jet of water from a garden hose that starts to break into droplets after it has travelled a certain distance. As a result, by the time it reaches the end of the whistle, the jet of steam is no longer a pure column, but slightly disturbed. These instabilities cannot escape perfectly from the whistle and as they hit the second whistle wall, they form a small pressure pulse. This pulse causes the steam to form vortices as it exits the whistle. These vortices produce sound waves, creating the comforting noise that heralds a forthcoming cup of tea."
The size and shape of the opening spout makes the kettle whistle rather than produce another sound. A shorter kettle spout produces a higher pitched whistle, and a longer spout produces a lower one.
But here's the coolest part: That classic kettle whistle isn't the only sound kettles make. Just as water begins to boil, the kettle produces another sound, which is always at the same pitch as the whistle. That sound is produced by the same effect that causes sound when you blow across the top of an empty bottle. Air in the neck of the bottle is bouncing up and down, while the rest of the air in the bottle is compressed and then bounced upwards. The whistle of the kettle does the same thing.
Now that these researchers have cracked the mystery of the kettle, they could likely design one that would produce no whistling noise when its water begins to boil. But who would want a silent kettle?