Why does the kettle get louder as the water heats up but then quieter as it reaches boiling point?

We put Allan's question to physics expert Caroline Steel... Caroline - So, in a kettle, you have a heating element at the bottom and that heats up quickly to actually above one hundred degrees. So the first thing to heat up in a kettle is the water around the heating element. This hot water around the heating element forms tiny little bubbles which then rise up through the kettle, but as these bubbles rise, they rise into an area of colder water. So the bubbles cool down and condense and kind of implode, and the bubbles sort of slaps against itself making a really loud noise.

But as the kettle heats up throughout so all the water inside the kettle is one temperature. As these bubbles form and rise through the water they don't reach an area of colder water, so they actually make it all the way to the surface and then they pop on the surface. But this popping sound on the surface is a lot more gentle and quiet than the sound of the bubbles kind of imploding on themselves within the kettle.

So, if you watch your kettle boil, as it's making a loud sound and coming up to boil you'll see lots of tiny little bubbles in the main body of the water, but you won't see them reach the surface. And then once the kettle has reached boiling point and these bubbles make it to the surface, you'll see them expand and pop but that popping noise is actually quieter than the imploding bubbles in the main body of the water.