This question has haunted me for several years. I’ve got a couple of ideas, but no valid scientific reason to justify them. Each morning when I turn on the water for the shower, I turn the hot water on high to migrate the now-cooled water through the pipes. I can tell when to adjust the water temperature and enter the shower by an audible change in the water flow. What causes the sound to change between hot and cold water flowing through a pipe?
Hot water softens the seals in the faucet/spigot, allowing them to expand. They partially close and can restrict the flow of water and/or alter it slightly. The restricted and/or altered flow creates a different audio signature. It might be slower moving, be squezing through a partially closed faucet (now that the the rubber seals in the faucet has expanded) and/or it will flow along the pipe walls differently. It will exit with more or less force and could create a slightly different sound as it brushes against the pipe/faucet on the way out, too.
Sorry to contradict you OM, BUT …
Hot water flows FASTER than cold due to the lower viscosity of the hot water compared to the cold.
Also:
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Hot water is probably creating steam, which alters the medium through which you hear the sound. The air you’re listening through will make the water appear to get quieter or louder.
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Your hot and cold water systems are coming through different arrangements of pipes, meaning their pressure and flow rates, in addition to the viscosity, will be different.
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Hot water will have bubbles of dissolved oxygen present, which will definitely mess with its acoustic signature.
The pitch of propagating sound in a medium is related to density and elasticity.
Density is related to temperature. As the temperature changes density of the water also changes, affecting resonances of sound vibrations within the pipes.
I have always assumed that the difference in sound is due to the expansion of the pipes and valves in the system as they heated up, particularly the valve in your spigot. This will give the hot water a (slightly) different aperture size, and thus produce a different pitch than the cold. There may also be an additional effect produced by the increased velocity and decreased viscosity. My WAG.
What makes you think viscosity is the only significant variable? You’re completely ignoring Philster’s point about the effect of heat on the plumbing and fixtures. I can attest that in my house, the hot water tap flows a lot slower once it’s actually warmed up and spitting out hot water. I’m not making any claims about why that’s the case, but your proclamation that hot water necessarily flows faster is wrong.
In our house the cold water flows faster because of the higher mains pressure as compared to the pitiful tank pressure for the hot water.
The other factor is that I suspect much of the sound is due to vibrations in the metal pipes. The pitch of this vibration will definitely be temperature dependent just as an orchestra has to retune as the temperature changes.
Hmm… is the water hot enough to make steam? I wouldn’t think the water in the tank is close to boiling… Also, in my example I turn only the hot water on to get the cooled-down water passed through the pipes, so all the water (both cool and hot) are coming through the same pipes.
So, I figured that the density because of temperature would have something to do with it, but just didn’t have the physics reference on how it would affect the sound. I wonder if there’s a way I can test whether the sound is coming from the pipes, the faucet fixture, or the water itself? Of course, since they are all connected there will be some sound conductance (is that a real word?) between the three…
Any water is hot enough to make steam, even ice. Steam is just an easy term for water vapour, it forms over water at any temperature. The hotter the water the more steam it produces but there’s no lower temperature limit on steam production. You only need to look at the mist forming over the arctic and antarctic oceans to prove that.
No offence intended but …
It’s the viscosity, viscosity, VISCOSITY!
** Viscosity and Flow Rate vs. Temperature ** Stare the graphical visulation and vary the temperature to see the change in rate of flow. An 80 deg.F change in temperature results in a 120% increase in flow.
From your cite:
(emphasis mine)
We’re not talking about an experiment where temperature is the only variable and everything else is held constant. We’re talking about household plumbing.
Expansion/contraction of plumbing/faucet parts is Negligible. The coefficient of expansion of metals is inconsequential compared to the viscosity change in water.
Density is similarly materially unaffected as water is NOT a compressible fluid as is air or other gasses.
Too many opinions, lack of factual data in most replies. 
It’s the viscosity, viscosity, VISCOSITY!
Viscosity - very good… so there is a definite increased flow rate of the water, as well as less “friction”, correct? So would the actual source of the sound be the reduced friction on the pipes, other water molecules, water hitting the tub, etc, or directly related to the flow rate?
Thanks for the clarification on temperature of steam, Blake. I was specifically thinking of water in gaseous state, which should be above water’s boiling point, but hadn’t taken into account suspended water molecules in vapor form.
Y’all are good. Thanks for all the help!
Viscosity is a factor. However, driving it home like that shouldn’t discount the fact that cold pipes sound different than hot pipes. It should not discount the fact that rubber seals can almost shut off water that is hot (when it originally came out cooler).
So, while viscosity plays a key role, how can you look at a faucet that has almost entirely shut off the flow of water once it has warmed up and go blathering on about viscosity? Seriously, tossing around viscosity does bring in some science into this conversation, but if water is flowing and then the flow is halted to a damn trickle by a rubber seat/seal that has expanded, are we supposed to bow down to the viscosity gods?
- We’re not only talking about expanding metals. We’re also talking about expanding rubber seals.
- The coefficient of expansion of the object is irrelevant; it’s the absolute expansion that matters. A 0.1% expansion of a part can cut the size of an opening in half, having a much greater impact than a change in viscosity.
Let’s try this another way: how do you explain a decrease in flow as the water from the hot water tap warms up? Actual experience directly contradicts your “viscosity is everything” theory.
Another way of looking at it. Is the sound produced by a given volume of cold water flowing through a faucet the same as that of an equal volume of hot water flowing through the same faucet? I maintain that the noise is different due to the different settings of the faucet required to have equal volume flow rates. Yes, the change in viscosity of the water is the reason you have to adjust the tap to get equal flows, but it is the aperture size itself that has the greatest impact on the sound produced. And changing the temperature of the water WILL affect the aperture size due to the differential expansion of the dissimilar materials in the valve.
I guess the real test would be to manufacture a valve with VERY low expansion coefficient (perhaps a ceramic) and test it for chages in sound with temperature.
I agree that viscosity plays a role. But I have yet to be convinced that it is a greater role than that of geometry, because geometry is the major creator of turbulence and therefore of noise.
I take it that you’ve never looked under the hood of a car.
Check the radiator coolant overflow reservoir. Take note that there are two specified levels for the coolant based on the temperature, hot or cold.
And mercury isn’t a compressible fluid either. Its a liquid same as water. Why is it then that the mercury rises up the tube when the temperature gets warmer?
Wrong.
For example, see this column by our fearless leader himself:
http://www.straightdope.com/classics/a2_394.html
For the benefit of those who don’t click the link:
Having spent many years doing every kind of residential plumbing you can imagine (both as DIY projects and professionally for a couple of years) I can absolutely guarantee that the expansion of faucet parts is the biggest single reason for the change in flow from a faucet as the water heats up.
The change in sound is probably the result of many things, including difference in viscosity, dissolved gasses, expansion of the pipes, and change in flow.
And you’d might be surprised how much a 20 or 30 foot run of copper or galvanized pipe can expand. They don’t use pipe hangers that allow the pipes to slide back and forth for nothing.