In article , Mark Lloyd wrote:
On Thu, 11 Jan 2007 18:44:28 GMT, (Malcolm Hoar)
wrote:

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?

I have definitely heard it in my shower.

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 ;-)

--
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~|
| Malcolm Hoar "The more I practice, the luckier I get". |
| Gary Player. |
| http://www.malch.com/ Shpx gur PQN. |
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