Many people use tea kettles every day; what they may not know is that researchers have long been mystified about what makes it whistle.
For the first time, researchers have fully described the mechanisms involved in the familiar whistle, an American Institute of Physics news release reported.
Previously, all scientists knew about the inner workings of the tea kettle was that vibrations were created by the buildup of steam as it escapes through haps in the kettle's metal plates.
Researchers at the University of Cambridge decided to solve the mystery.
The team found the sound originated at the spout, which has steam flowing through it. Through their studies they were able to identify "two-mechanism process of whistle production."
"Their 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 news release reported.
The researchers believe this could help solve problems with noisy pipes and car exhaust.
"Pipes inside a building are one classic example and similar effects are seen inside damaged vehicle exhaust systems," Ross Henrywood, the study's lead author, said. "Once we know where the whistle is coming from, and what's making it happen, we can potentially get rid of it."
The researchers looked at a number of simplified kettle whistles and forcing air through them at different intensities in order to make the discovery. The team recorded the sound the air made as it went through, they then plotted the frequency and amplitude data to analyze any trends that may exist. They also used a two-microphone technique to pick up frequencies inside the simplified 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 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," the news release reported.
The instabilities hit the second whistle wall and form a "pressure pulse." The pulse forces the steam to form vortices as it spews out of the whistle. The vortices are believed to be responsible for the nostalgic sound.
