Silly plumbing (or maybe physics) question

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Sounds like it's all about the diffence in bubble-density between hot and cold water. Google "hot chocolate effect". Here are a couple of links:
http://www.acoustics.org/press/143rd/Rossing.html
and
http://www.kilty.com/coffee.htm
"Physicist Frank Crawford noticed this same effect in hot chocolate (American Journal of Physics, May 1982), and explained it as being due to the dependence of the speed of sound in water on the bubble density. What we are hearing are longitudinal oscillations of the water column. Crawford heard pitch changes of nearly three octaves in a tall glass cylinder. Gas bubbles reduce the speed of sound in the liquid, which lowers the fundamental mode of the liquid column. As the foam clears, the speed of sound rises and so does the pitch. Crawford called this 'the hot chocolate effect.' "
--------------------------------------------------------------------- Malcolm Hoar wrote:

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wrote:

Bingo, we have a winner, I think. Thank you very much for the pointer.
The hot water is saturated/supersaturated with dissolved air. The aggitation caused within the faucet and the pressure reduction on exit cause many bubbles to form. Hence a "softening" effect not unlike that created by an aerator.
This seems very consistent with the nature of the sound change I've observed. Much more plausible than a simple density change effect.
Mystery solved, to my complete satisfaction, at least!
Back now to the regular schedule of electrical code issues and broken appliances and other problems for which the solution is caulk, duct tape and/or WD40...
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On Thu, 11 Jan 2007 21:44:46 GMT, snipped-for-privacy@malch.com (Malcolm Hoar) wrote:

There is a big difference between 200 degree coffee water and 130 degree tap water. I wouldn't be to sure that the coffee effect is what you are hearing.
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Take a bottle or can of soda. Heat it up to 130 degrees, shake the container (c.f. water forced through faucet) and remove the cap (water exiting spout)...
Yes, I realize that's an extreme case but there's little doubt in my mind that there will be significant bubble formation with the hot water faucet that could easily account for the change in sound.
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It's the sealing washer of the hot water valve that gets soft when hot and muffles the vortex area around it.
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On Thu, 11 Jan 2007 18:44:28 GMT, snipped-for-privacy@malch.com (Malcolm Hoar) wrote:

I think it's the expansion of the metal in whatever constriction is causing the harmonic in the first place, but a reaction of an anti-scald valve to the hot water would also do it.
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Malcolm Hoar wrote:

Hot water is happy water.
"Hi, ho, hi, ho, it's off to work we go..."
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wrote:

Sounds strange, but true in a way. Hot water is more excited.
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Mark Lloyd
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Malcolm Hoar wrote:

I have seen this with well water and with a water-main system.
Cold water can hold more air in solution than hot water- Water under pressure holds more air than water at 'room pressure'
The cold water that comes out 1st keeps its air in solution (at least for a longer time), the hot water has its air come out of solution as soon as the water gets past the valve.
The noise is the air bubbles forming or bouncing around etc...
Dave
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snipped-for-privacy@yahoo.com wrote:

Yup!
Well it's not a noise created by the bubbles per se. But I am persuaded that it's the bubbles that soften, dampen and change the pitch of the sound created by the running water.
I have endured this "itch" for several years. It really does feel good to finally scratch it ;-)
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Malcolm Hoar wrote:

I have had times when it was relatively quiet until the hot water arrived, so I think its supportable the bubbles themselves (during formation, or going through a restriction) make noise.
Dave
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snipped-for-privacy@yahoo.com wrote:

Hmmm, I don't think that's an effect I've observed but you make a good point. I can see how that might happen.
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Malcolm Hoar wrote:

How water is expanding parts in the valve, usually rubber washers.

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I had a faucet that would do thid. If you just turned the hot on a bit, when the sound changed, the flow dropped - almost to zero. I figured it was the expansion of parts in the valve since the effect was so obvious.
Bob
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It's definitely an observable phenomena. The effect of heating up the faucet valve may be more significant than you think. Certainly you have observed some types of faucets that greatly reduce the flow when hot water reaches them. Another thing I can think of that may account for a sound change, is that hot water has very little dissolved air in it compared to cold water.
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Malcolm Hoar wrote:

I didn't learn much there. But ... hot water is higher pressure, moving faster, higher frequency across the joints? Kind of like a steam whistle?
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On Thu, 11 Jan 2007 18:44:28 GMT, snipped-for-privacy@malch.com (Malcolm Hoar) wrote:

I have definitely heard it in my shower.
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Yeah, there's little doubt in my mind now -- it's the bubbles. The hot water is supersaturated with dissolved air. It's agitated by passing through the faucet and then subjected to a pressure reduction. All that air is released in the form of bubbles. So what we hear is the difference between a stream of water and a stream of something approaching a foam -- very different sounds.
I must say, solving this little mystery has been MOST satisfying ;-)
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