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 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.