Mystery of the kettle whistle solved at last: Speed steam is funnelled through spout creates the sound

Breakthrough: Engineers say the speed which steam is funnelled through the spout is why a kettle whistles

When you finally get home after a hard day’s work, the whistle of a stove-top kettle can be a truly comforting sound.
Until now, scientists have been unable to explain what makes the noise.
But a group of Cambridge University researchers say they have found the reason – and it’s to do with the speed steam is funnelled through the spout when you’re making that all-important brew.
They explained that as a kettle boils, the air moves faster and wobbles when it travels through the narrowest part of the spout.
Then as the steam escapes it creates tiny ‘vortices’ – whirlwind-like eddies that can produce sound at certain frequencies.
Although the finding might sound trivial, it could help solve other problems. Researcher Ross Henrywood said: ‘Pipes and vehicle exhausts are classic examples.
‘Once we know where the whistle is coming from, we can potentially get rid of it.’
Results show that as the kettle starts to boil, the whistle behaves like a Helmholtz resonator - the same mechanism that causes an empty bottle to hum when you blow over the neck.
However, above a particular flow speed, the sound is instead produced by small vortices - regions of swirling flow - which, at certain frequencies, can produce noise.
The study tested a series of simplified kettle whistles in an apparatus by forcing air through them at various speeds.

The resulting sound produced by rushing air was recorded, the frequency and amplitude data of the sound were plotted, then analysed to identify trends.
A two-microphone technique was also used to determine frequency inside the spout.
Vortex production starts as steam comes up the kettle’s spout and meets a hole at the start of the whistle, which is much narrower than the spout itself.
This contracts the flow of steam as it enters the whistle and creates a jet of steam passing through it.