[jacktaylor]

Jack Taylor asked the Naked Scientists:
   
Why does water from the hot tap sound different to the cold tap?

What do you think?

[Ken Hughes]

Hi Jack,

Hot water is less dense than cold water since it has expanded, the amplitude of vibration of its molecules is greater and so it takes up more volume.
The speed of sound in any medium is dependant on the density of the medium.
For air, the speed of sound is around 600mph or around that number.
For water (much denser than air), the speed of sound is much less (I don't know the number).
To get an idea, think of bird song in air, compared with whale song under water. The frequencies are much different.

[chris]

Hi Ken

nice answer - but why should the speed of sound in the medium make a difference to the way it "sounds"?

Chris

[Pikaia]

Mine sounds exactly the same! The difference in density is negligible, so it would not affect the sound, and in any case, the frequency of a wave is not affected by the density of the medium, only the speed and wavelength are affected.

I suspect that the real answer is simply due to the hot and cold water being at different pressures and flow rates, because the sound is produced by the water hitting the sink. Try adjusting to the same rates and see if there is any difference.

[imatfaal]

Pikaia - is it possible that the different density (and I guess viscosity) could lead to different flow/droplet characteristics?

[Fozzie]

None of these explanations seem to explain why your get a much lower, hollow sounding note if you tap a spoon in the bottom of a mug filled with boiling water, compared to a cold one. If you do this with a flat-bottomed mug of fresh coffee, the frequency of the sound increases over the course of a few seconds, but this might be due to air bubbles in the liquid?

[imatfaal]

Fozzie

That's to do with micorscopic bubbles in the water introduced by the coffee making process - the new scientists last word section had a long debate on this.  I am sure it will be on line somewhere.

[Geezer]

When you tap the mug, the mug and it's contents act as a resonator. The propagation rate is affected by the temperature of the liquid, so the resonant frequency of the ensemble changes.

Howzat!

[imatfaal]

Not out!  Well I think so.  Speed of sound would vary with the square toot of density (and compressibility) but how much would that really vary with temperature?  The special case I have read about is why stirring a cup of coffee can lead to a change in pitch of the ringing of the teaspoon on the mug.   That is due to microscopic air bubbles that are introduced by the freeze dried instant coffee (that some heathens still insist upon drinking) and are removed by the stirring action.  The air bubbles change both the density and the bulkmodulus and thus change the note.

I remain unconvinced that two identical cups with same amount (mass or volume hmmmm) would sound noticiable different.  I will report back

Edit

Well just tried it - and I cannot hear the difference after a couple of seconds (ie when the gas bubbles from the boiling process have gone).  One lot of water is cold the other is off the boil and the note sounds the same pitch.  Once I work out how to up load video I will post it.

Well now I am unsure
http://www.flickr.com/photos/22711004@N07/5816494654/in/photostream

I won't say which is the hot water.  And i realise how dodgy the experimental set up is

[Geezer]

I suppose this proves it's always a good idea to prove the pheomenon actually exists before we try to explain it  [] []

[Fozzie]

Nice video, but I see two problems with this experiment. a) The water needs to be boiling and I mean boiling. As it has been standing in the cup for some time, the temperature has obviously dropped beyond the point where the effect is manifest. b) The cups are standing on a surface which could dissipate the sound. It's better to hold them in the air and tap the bottom of the inside of the cup with the spoon, not the sides. A mug with straight sides and a flat bottom the same diameter all the way down also works better.

This definitely works and I have just amazed some work colleagues with a demonstration in the kitchen!

The other sound effect I have noticed which you can do quite easily is to pour some boiling water out of a kettle onto a flat surface and then do the same with cold water. There is an obvious different in the sound.

[imatfaal]

You will note in my frankly very poor write-up that I agreed that for a few seconds after boiling the pitch noticeable changes and that this is due to gas bubbles in the water.  On the point of putting cups down or holding them - this will not effect the note's pitch and might even damp out some unuseful harmonics, but does allow uniformity.  I will get the microphone out and capture the change with boiling water if I have the time today - should be a slow day at the office :-)

[techmind]

I agree that with water emerging directly from a tap, the main factors why hot and cold may sound different include the different pressures (in the UK, in older houses the hot water is fed from a tank in the loft so only has a few metres of water pressure). The viscosity and speed of sound will be slightly different but (without calculating it) I'd be a bit surprised if that made a noticeable difference. The position of the tap within the sink/basin and relative to nearby walls may colour the sound as might the shape of the tap opening (for mixer taps the hot may come out of the centre and the cold from an array of small nozzles around the outside) - or even for separate taps one may tend to get more bunged up with limescale than the other. Another significant factor is likely to be dissolved gas - heating the water tends to remove much of the dissolved gas, so if your cold water was particularly gassy to start with this is likely to cause a difference in sound when the taps are run.