[Eternal Student (OP)]

Hi.

   So I went to start making a cup of tea but I couldn't remember if I had switched the kettle on.   I went back out to the kitchen and soon knew because I could hear the kettle making a gentle "whooo / roar" sound.   It's mid-range rather than being high pitched but some people do say the kettle is "fizzing"  (There are little bubbles of gas rising and I think people mistakenly describe what they can see instead of what they can hear).
    Why does it make that sound?   (I mean long before the water has reached the boil).

Best Wishes.

[Halc]

I have an electric stove and the sound is quite loud. It momentarily increases in intensity if you move the kettle say a cm or so to the side, exposing new 'colder' kettle-bottom to the hot portions of the burner.  But the sound comes from a gas setup as well.  It calms down to being quite quiet just before the boil sets in. My kettle has a fairly defective whistle and one needs to keep an ear out for the quite before the storm, so I've become more attuned to it.

OK, so I do think it is either noise from boiling right at the bottom. The gas makes it a very short distance before condensing back into liquid. But this can't be all since it would continue to make the noise once the boil is reached, and it kind of doesn't. Water at a full boil doesn't make a whole lot of noise at all. So maybe its the condensation going on (the bubbles ending) that makes the noise. That stops happening at the end and it just goes into a rolling boil.

[Petrochemicals]

The frequency seems to lower as the kettle gets hotter.

[vhfpmr]

The bubbles 'pop' as they re-condense, so it stops when the water reaches boiling point, and the frequency gets lower as the kettle warms because the bubbles travel further before they reach water cold enough to condense them.

Kettles with flat bottom elements are much noisier than the old ones with wound elements immersed in the water because the flat bottoms act like the skin of a drum.

[Eternal Student (OP)]

Hi.

    Thanks for the comments.   I don't know what's causing it which is why I asked.

    Sound is evidently a vibration in the air surrounding the kettle.  The main question is what bit or what thing is the main source or cause of the vibration.  The main candidates (I would think) are the water,  the body of the kettle,   OR  just the heating element.

  It seems that most people think it's the water that is the main source of the vibrations, probably due to water vapour bubbles being created at the bottom and condensing back into liquid higher up.   Could be.   There's a big change in volume moving from gas to liquid so there may be some rapid local pressure changes when a bit of steam condenses.   Those might travel like pressure waves through the water (either to the surface of the water or to the body of the kettle which will then also locally vibrate).   Ultimately those vibrations make the air outside the kettle exhibit the same sort of pressure changes (compression and rarefaction).
    Many variations are possible:  Perhaps the density changes for hot and cold liquid are sufficient,  phase changes from water vapour to/from liquid water are surplus.   If there are suitable convection currents set up then liquid water is rapidly cooled and those changes may be enough to set up pressure waves travelling through the water.  i.d.k.
    I'm inclined to agree that it's the water that is the main originator of the vibrations but I'm not keen to try experiments involving ruining my kettle by putting other fluids into it.   Maybe some fluid which has bigger / smaller density changes with temperature would make a different sound  etc.   Maybe if you add a magnetic stirrer in the bottom of the kettle and keep the fluid much more homogenous while heating the sound disappears.    i.d.k.

Best Wishes.

[Halc]

  It seems that most people think it's the water that is the main source of the vibrations, probably due to water vapour bubbles being created at the bottom and condensing back into liquid higher up.   Could be.   There's a big change in volume moving from gas to liquid so there may be some rapid local pressure changes when a bit of steam condenses.   Those might travel like pressure waves through the water (either to the surface of the water or to the body of the kettle which will then also locally vibrate).

Thought about it some more.  The bottom of the kettle acts as sort of a drum, amplifying action happening there and transferring  the vibration to the air.
So I place my bets on the creation of the bubbles from liquid state as it touches hot metal. The condensation just isn't going to do anything dramatic and will result in an inaudible fizz at best.

So the liquid sort of explodes into gas. That's the noise. Towards the end there is no liquid at the bottom and it all boils before reaching the metal. Hence less noise at the end.

And yes, if you boil water in a pan, you get the same noise, and it sort of goes away if you stir, only to return moments later when you stop. I think one can stir the water fast enough (at least when it's still pretty cool) that the water touching the very hot (well over boiling temp) metal still hasn't a chance to conduct enough heat away to boil instantly.