Silly plumbing (or maybe physics) question

[Malcolm Hoar]

My bathtub is located some distance from the water heater. I turn on the hot faucet only and wait. The tub starts to fill, with cold water initially. After a while, the water becomes hot. That's all fine and as expected.

However, I can tell when the water is hot by *listening* to the flow. The sound of the water changes quite significantly and I've been trying to figure out why.

Clearly, the hot water will be slightly less dense than the cold but I have a hard time imagining this would cause an audible change in the sound. Also the metal faucet will expand and that might change the sound of the water whistling through the valve. Again, it's hard imagine the thermal expansion of fractions of a millimeter causing such an audible effect -- think about the size of changes between two notes on a piano, guitar, flute or whatever.

This happens in my current home but I have a vague feeling I've seen (heard) the effect at other homes too.

Any other theories/inputs? Anyone else even observed this effect?

[avid_hiker]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Would you have well water? After filling for a while, would you be hearing more water pressure when the well pump kicks in? Do you get the same sound with just the hot water running?...cold water off.

[Pason]

Have you every noticed how the pitch changes when you stir sugar into your coffee?

[Malcolm Hoar]

No well (or pumps) here. Regular city water supply.

I've generally noticed it with the hot faucet full on and the cold faucet off.

[Malcolm Hoar]

No, I only add coffee to my coffee ;-)

But I suspect you're describing a different manifestation of the same effect. If so, the change is likely in the reverse direction because I presume the sugar would make the coffee more dense.

Your data suggests the effect is indeed density related I think.

[DerbyDad03]

Follow my logic here:

- Cold is the absence of heat.

- Cold water is just water.

- Hot water is water with heat added.

- Therefore, hot water is thicker than cold and squeezing it through the pipes makes a different sound.

QED

Malcolm Hoar wrote:

[CptDondo]

I've always thought it was the sound of the rubber gasket in the faucet itself. As it heats up, it gets more pliable and thus its vibration characteristics would change. It may even pop in or out, reducing water flow....

Pure conjecture on my part, though.

[Todd H.]

I have no more authoritative a take, but am leaning toward it being the warmer faucet bits than the actual water modulating the sound as well.

-- Todd H.

[Chris Friesen]

I've noticed this as well. Regular copper pipe.

Chris

[DerbyDad03]

-

- - However, I can tell when the water is hot by *listening*

- - to the flow. The sound of the water changes quite significantly

- - and I've been trying to figure out why.

-

- I've noticed this as well. Regular copper pipe.

-

- Chris

Chris,

What were you doing at Malcolm's house?

[Ermalina]

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:

and

"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:

[tnom]

It's the sealing washer of the hot water valve that gets soft when hot and muffles the vortex area around it.

[Goedjn]

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.

[Malcolm Hoar]

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...

[HeyBub]

And ice is the thickest water of all...

[HeyBub]

Hot water is happy water.

"Hi, ho, hi, ho, it's off to work we go..."

[spamTHISbrp]

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

[Malcolm Hoar]

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 ;-)

[Joseph Meehan]

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

[Bob F]

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