B What causes that water kettle sound?

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Summary
What makes that hush sound as water heats in my kettle?
When I turn on my water kettle, I hear that familiar "hush" sound as the water heats. What exactly is that? The surface of the water vibrating? Warmer, expanding air leaving the kettle? I know it can't be a function of the water temperature itself because when I turn the kettle off the sound stops a moment later though of course the water temperature doesn't immediately drop back to room temperature. It's also not the kettle alone since it doesn't make the same sound without water in it.
 
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How hot is the water when the sound begins?
If it happens when the water is close to boiling then the cause could be vapor bubbles collapsing. Heat goes into the water faster than it can dissipate and a bubble forms. The bubble leaves the heat source surface and the surrounding water is below the boiling point. Without the heat input the bubble collapses and the impact of the condensed water filling the void caused a vibration.
Does the pitch change when the heat is increased or decreased? Does it stop when the water reaches boiling?
 
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The sounds starts pretty quickly, and the water is initially at room temperature. I'm unable to adjust the temperature of the applied heat, and I haven't noticed a pitch change. And yes, as it approaches boiling there are a few seconds of silence, then the noisy boiling starts. (I assume that part has to do with the latent heat of state change?) I was just reading about this bubble collapsing phenomenon elsewhere, sounds credible to me!

Now I'm wondering, why are the air molecules in the water more likely to coagulate into bubbles as the water temperature increases?
 
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Is there an open container that allows visual observation? Expanding atmosphere bubbles will not collapse from vapor to liquid, their appearance may be different in shape and action from water vapor bubbles.
Do all forming bubbles collapse upon rising from the heat source? What visual changes occur as the noise begins?
If frequency is unchanged as the water heats does the loudness (intensity) change? Why?
 

Bystander

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Summary: What makes that hush sound as water heats in my kettle?

What exactly is that?
"That" is the sound of differential expansions of a non-uniform/non-equilibrium heat transfer process, or processes.
 

BvU

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Kettle isn't accessible for observation. Repeat with a cooking pan and describe carefully what you hear and see. Preferably reading off a thermometer as well, otherwise record the timing.
 

sophiecentaur

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Summary: What makes that hush sound as water heats in my kettle?

I hear that familiar "hush" sound as the water heats.
Summary: What makes that hush sound as water heats in my kettle?

when I turn the kettle off the sound stops a moment later
Boiling in a kettle is quite a complicated, one shot event and it's much more involved than at first sight, I think.

I hear this every day and I think it's due to what goes on right on the surface of the heating element. Heat transfers from the element surface by Conduction to the nearby layers and that first effect will be due to small bubbles of air coming out of solution. The bubbles are small because of the small amount of dissolved air near the surface. Once the supply is turned off, the local temperature will return pretty quickly - hence the fast response time. This. I think, is what used to be referred to as "singing".When bulk convection becomes established the water will be cooling the element surface better, this sound will change and it will stop when the dissolved air concentration drops and the bulk temperature is high enough.

If you turn on a kettle that's already been thoroughly boiled, things change; that sound tends to be different (at least in the kettles I know and love), with a rash of tiny explosive pops and cracks (no dissolved air but lots of water available to form bigger bubbles which insulate the surface and the local temperature gets to 100C). I think this happens after a small delay with silence and it is a small scale version of 'bumping' that you can get when you heat up a pan with a smooth bottom. Again, once convection gets established, the local bumping tends to stop and the general 'boiling sound' takes over, eventually.
 

anorlunda

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The bubble leaves the heat source surface and the surrounding water is below the boiling point. Without the heat input the bubble collapses and the impact of the condensed water filling the void caused a vibration.
That was always my theory of the sound.

I used to have a transparent Pyrex coffee pot. I watched it approach boiling every chance I got. I could see bubbles rising from the bottom, then disappear as they rose into colder water above, and my theory is that the sound asked about in this thread is caused by implosion of some bubbles.

As the kettle approaches boiling, rising bubbles do not shrink and implode. They start growing in volume because hydrostatic pressure reduces as they rise. That accounts for the noise stopping just before full boil.

During a full boil, one can see cone-shaped columns of bubbles rising from nucleation sites on the bottom. The acute angle of those cones is on the order of 20 degrees.

Boiling in a kettle is quite a complicated, one shot event and it's much more involved than at first sight, I think.
Indeed. I used to have a fantasy about a benefactor to finance me while I spent the rest of my life modeling in detail what happens in a non-equilibrium open or closed kettle. It would be a work analogous to Michael Faraday's famous The Chemical History of a Candle .

It is even possible that one or two of those imploding bubbles might produce sonoluminescence, although too little to detect. That is a fascinating phenomenon in physics.

Bringing water to boil in zero gravity would be even more challenging to model. If the bubbles do not move away from the heat source, that upsets all the dynamics and heat transfer.
 
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Thanks for all the replies! Had a busy morning, much to process here..
 

sophiecentaur

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As the kettle approaches boiling, rising bubbles do not shrink and implode.
An explanation is still needed why a kettle that's already been boiled sounds different (from cold) when it starts off. Perhaps the tiny air bubbles which come out of solution form nuclei for the steam bubbles and allow them to be formed smaller.
 

anorlunda

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An explanation is still needed why a kettle that's already been boiled sounds different (from cold) when it starts off. Perhaps the tiny air bubbles which come out of solution form nuclei for the steam bubbles and allow them to be formed smaller.
Are you talking about those marvelous electric kettles, that only UK citizens seem to have.

Not an answer, but my guess is that because they are metal and because they have feet, they may amplify noises that the rest of us don't hear, just because of sound damping. Any chance of a good quality audio recording? One with fresh water, and again with previously boiled cold water.
 

sophiecentaur

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Are you talking about those marvelous electric kettles, that only UK citizens seem to have.

Not an answer, but my guess is that because they are metal and because they have feet, they may amplify noises that the rest of us don't hear, just because of sound damping. Any chance of a good quality audio recording? One with fresh water, and again with previously boiled cold water.
I have heard the noise from a thin bottomed camping kettle too. You may well be right about more solid based kettles.
The noise I hear is more of a squeal initially with fresh water. This is followed by crackling noises
, perhaps due to the smaller area where the heat source operates.
I am busy tomorrow but I will try to get a recording for PF. I can’t think that i’m The only one to have heard a ‘singing kettle’ though.
Central heating boilers sometimes squeal and there would not be much air in that water. More confusion but local hotspots can occur when there is line scale.
 
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We have kettles too (we call them jugs), some of them are plastic but have similar boiling habits. The noise ultimately comes from bubbles, just as in the noise of a river. You will notice that as scale builds up on the inside of the kettle the noise of boiling increases until you clean the scale off. As for the hot versus cold I believe that the air has been boiled out of the cold water, so less bubbles.

Cheers
 
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I'm curious to hear this "singing kettle". I've heard the term, and I always assumed that was the whistle on the old stove-top tea kettles (funny name, even in the US people don't put tea in that kettle, it's used to boil water for tea).

And in Charles' Dickens "A Christmas Carol", they talk about the pudding "singing in the kettle".

... Oh, such a goose and a pudding singing in the kettle.
Though I'm familiar with sizzling sounds coming from a pot of water on the stove as it reaches a boil. I just never thought much about it. Sometimes condensation forms on the outside of the pot, and that boils. I guess that water is a by-product of methane combustion? CH4 plus O2 forms some H2O?
 

sophiecentaur

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I'm curious to hear this "singing kettle".
The sound is not as common as I thought / assumed from past experience. I am still convinced that it's due to dissolved air, because the water in an electric kettle has hardly warmed up at all then that sound happens. Moreover, I am trying to reproduce the sound with my Dualit kettle but it has a flat element on the bottom - unlike the traditional type with a loop element. To be fair, the effect with a loop element in an older kettle is what I remember.
We now tend to use softened water occasionally and that cuts down limescale. I believe the limescale could also be to blame (nuclei to form the bubbles). The expression "singing" comes from way back before electric kettles to perhaps it could be the limescale and caused when a kettle is put on a hot range stove with local high temperature.

I have managed to record the sound of fresh tap water brought to the boil and the noise starts very soon after switch on. Life routines and another user have meant that a kettle of already boiled water never has a chance to cool down to room temperature. I'll have to decant it slowly into another vessel and let it cool right down and try again, to see if the early bubbles are not there.
 
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Just boil and cool some water in a stove top pot. If the theory is that dissolved oxygen is related to the sound, a 10 minute boil and then cool down will greatly reduce the O2 content.
 

sophiecentaur

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Just boil and cool some water in a stove top pot. If the theory is that dissolved oxygen is related to the sound, a 10 minute boil and then cool down will greatly reduce the O2 content.
You shamed me into recording the cooled/ boiled water and I made two mp3 files.

I think the start of the fresh water sequence is a bit more toppy and the once boiled water is more gritty. The sound of bubbles comes earlier from the fresh water and I think that's when the Oxygen starts to out. Perhaps there is still some Oxygen coming out during the boiling with may be affecting the size of the steam bubbles too. Its a bit like Astrology though; you can hear what you want to, if you are not careful.

The iPhone was in the same place on the worktop for both recordings and there was more or less the same amount of water each time. Judge for yourselves and (better still) try something similar. Home experiments are always good - are they not @OmCheeto ???:smile:
OH dear. You can't attach audio files. I will have to find somewhere to put them.
I'm on it, I'm on it !!!!!
See how it goes.
 
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I'm curious to hear this "singing kettle". I've heard the term, and I always assumed that was the whistle on the old stove-top tea kettles (funny name, even in the US people don't put tea in that kettle, it's used to boil water for tea).

And in Charles' Dickens "A Christmas Carol", they talk about the pudding "singing in the kettle".



Though I'm familiar with sizzling sounds coming from a pot of water on the stove as it reaches a boil. I just never thought much about it. Sometimes condensation forms on the outside of the pot, and that boils. I guess that water is a by-product of methane combustion? CH4 plus O2 forms some H2O?
I think the "singing kettle" in the Dickens' example may be poetic language but there really were singing kettles. Before the advent of kettles that automatically switched off when boiled there were kettles with a whistle in the spout that were activated by water vapour/steam. Perhaps they had them in Dickens' time?

Cheers
 

sophiecentaur

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I think the "singing kettle" in the Dickens' example may be poetic language but there really were singing kettles. Before the advent of kettles that automatically switched off when boiled there were kettles with a whistle in the spout that were activated by water vapour/steam. Perhaps they had them in Dickens' time?
The steam operated whistling kettle is still alive and well in boats and camping equipment. Nothing nice about that noise; it's just insistent and will only stop when the kettle is removed from the hob or when it boils dry.
I would say that there is a very common 'singing' noise to be heard from domestic central heating boilers than from modern kettles. There is never (?) any boiling inside domestic boiler pipes because that would burst the very low pressure system; the temperature of the water is kept low by a high flow rate of water through the tubes. But when the 'heating' kicks in, it's very common to hear the high pitched squeak, groaning or singing as the boiler starts to heat the water. I would love to hear an explanation of that because there will be very little dissolved gas left in the static mass of water in that system.
 

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