[thedoc (OP)]

Allan Grant asked the Naked Scientists:



   Why does a tea kettle make an increasingly louder sound as it approaches the boiling point and then start to quiet down as it reaches 212 degrees?



What do you think?

[Colin2B]

The sound is due to bubbles collapsing. At first these are dissolved air bubbles which expand and then collapse, this produces a low frequency. As the water gets hotter vapour bubbles form on the heating elements and they collapse at a higher frequency - around 1kHz.
I'm not so sure of the mechanism nr boiling, but as the water get hotter the bubbles get smaller and the frequency rises starting to move above hearing frequencies and so appears to get quieter.
I've met people who claim to be able to tell the temperature from the sound and using a thermocouple they appear to be reasonably accurate.