Asked by: Tom Metcalfe, Andover
Most kettle whistles consist of two parallel metal plates with a hole running through them through which steam passes. The whistling sound is the result of this flow of steam making the air vibrate rapidly, but only now have researchers worked out how.
According to Ross Henrywood and Dr Anurag Agarwal at the University of Cambridge, there are two mechanisms at work. The first, known as Helmholtz resonance, occurs when the steam tries to push out of the whistle, only to run into the natural ‘springiness’ of air still in it. The resulting vibrations produce the first sounds from the whistle. But as the steam gets hotter, it pushes through the holes in the whistle faster, creating ripples of turbulence, and these generate the final note.
Robert is a science writer and visiting professor of science at Aston University.