A team of engineering researchers at the University of Cambridge in England have solved the long-standing mystery behind the kettle whistle.  The whistling mechanism of the classic stovetop kettle occurs when the speed swirls of steam are channeled through the spout.

The researchers explained that when the kettle boils, the steam moves faster and wobbles when it moves through the narrow part of the spout. When the steam escapes it gives rise to tiny 'vortices' (whirlwind), which at certain frequencies generates sound.

The team found that a kettle whistle behaves similar to Helmholtz resonator (air inside an empty bottle vibrates when blown across the top).  However, as the frequency increases, the sound is instead produced by small vortices in the kettle.          

"Pipes and vehicle exhausts are classic examples. Once we know where the whistle is coming from, we can potentially get rid of it," Researcher Ross Henrywood told Daily Mail UK.

For the study, the researchers pumped air into a kettle at different intensities. The team then recorded the sound the air made as it funneled through the spout. They also observed the frequency and amplitude data of the sound.

The researchers also used a two-microphone technique to note down frequencies 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."

The study has been published in American Institute of Physics.

Although traditional stovetop kettles have been used for several decades now, the reason why boiling water produces sound has never been scientifically answered. Prior to this finding, it was claimed that the sound was made when the vibrations caused by the built-up steam escaped through the wo metal spout plates, Telegraph reports.