The sound of pouring water
 

Has anyone noticed that pouring hot water into a mug sounds different then pouring cool water?

Try it out, it sounds very different.

Why do you think this is? My guess is that the steam escaping the hot water dampens the sound coming out of the mug.

Anyone else?

Also, how many of you noticed this? It's actually been bothing me since I was a kid.

The molecules in hot water are moving at a much faster rate than the molecules in cold water. Also, the molecules are much further apart. That little change could easily change the sound.

Not really.

The molecules are moving faster but the temp. difference is only about 70-90 degrees K. The cold water is still about 280 K so it's not a huge difference. K is proportional to the mean velocity squared so I think we're only talking about a 10-15% difference.

As for the molecules being further apart, that is negligible -- compare the volume of cold and hot water, it is nearly the same.

I think you need to start by thinking about where the sound is coming from. I don't think the water molecules themselves are emitting sound, but rather the pouring of liquid into a cylinder moving air around that creates the sound.

I am not certain but I think happymaan's theory is more likely.

A good way to test this would be to compare the sound of water at 10, 35, 60 and 85 degrees C. If Giltwist is correct, all four should sound different, whereas if happymaan is right, the 10 and 35 should sound very similar.

It could be the air bubbles in the water . I notice that when running hot water that appears white due to the air in it, it sounds more foamy than cold water in which all of the air is dissolved.

It's a long time since I had fluid mechanics, but I think this is because hot water is less viscous than cold water. The viscosity, along with flow rate, determines what type of flow you get: laminar or turbulent.
You can observe this if you pour both hot and cold water from the same kettle. Hot water looks lumpy and chaotic (turbulent) while cold water flows in a nice stream. Google for Reynold's number if you want the mathematical theory.

Also, I bet you pour hot water differently than you pour cold water, unless you're thinking hard about it. You pour hot water more slowly and carefully. Or at least I do.

Some very interesting suppositions here!

What you absolutely must consider is the change in density!!

As water is heated, its volume changes .. and density relies on volume ( density = mass / volume ).

One must consider how sound travels at different speeds through different media - as well as the frequency of sound produced by media at differing densities.

:D

Isn't there some ask dr. science webpage somewhere we can go to?

water is basically incompressible. unless it's turning into ice or vapor, it stays at VERY close to 1 g/cm3.

on the other hand, water viscosity changes quite a bit when heated. gal's discussion of laminar vs. turbulent flow is relevant--even if you don't have turbulent flow conditions when pouring hot water, you'll get smaller droplets forming when pouring hot water, making more of a drippy drip sound, than the plopping sound of large cold water drops.

here's a boring flash link:
http://www.grow.arizona.edu/water/wa...perature.shtml

viscosity is the answer. trust the scientists here.

Sapper isn't totally wrong.. compressibility isn't the same as thermal volume expansion. According to a table here, water at 100 deg is 0.95 g/cm3.

5% expansion over 100 degrees is likely trivial in this case.

I think you could produce more substantial changes in density by adding salts to the solution that wouldn't really alter the sound of cold water being poured.

I find the viscosity theory the most convincing.

Re: The sound of pouring water
 

I've noticed a similar effect while running the shower, waiting for the cold water to turn to hot water. I can hear when the hot water arrives.

well.. I havent taken thermodynamics yet.. but what about the transfer of heat to the mug itself? ceramic is a GREAT insulator and will only absorb a small amount of heat from the water... but that change in temperature will GREATLY affect the resonance of the sound generated on the ceramic... maybe thats why you get a lower tone? the ceramic is more flexiable and makes a duller noise than when the cold water splashes into the cup and makes a very obvious splashing noise...

maybe we should check the sounds different temperatures of water make in different material cups ;)

I love how quick people are to dismiss something that appears to be mathematically insignificant.

I suppose we can just dismiss 5% of the world's angular velocity without appreciation huh? WRONG! It would change the number of hours in a day to an appreciable degree.

Go find the equation relating velocity of sound to density of medium - then consider the velocity of sound vs. frequency!

http://www.gmi.edu/~drussell/Demos/refract/refract.html

The difference in medium would cause differences in wavelength when the sound goes from water to air, but the frequency (which is what we hear) is constant. Hence the speed of sound in the water is irrelevant.

Out of curiosity, I looked into the impact on wavelength.

v = speed of sound
B = bulk modulus, constant for water
rho = density
v = sqrt(B/rho)

with a 5% change over 100 degrees, the speed of sound changes by about 2.6%. v = f*lambda, f = constant, so the wavelength changes also by about 2.6%. Not that this means anything for our perception of sound, as noted above. Even if frequency somehow changed by this amount, the difference between, say, 500 and 510 Hz isn't the kind of thing that's going to make us say "whoa, it sounds totally different."

Changing the subject, I also like LStanley's answer.

sapper: if all the clocks in the world disappeared, do you think you would ever realize if the earth started turning 5% slower?

a large part of art of science is discerning between relevant and irrelevant.

Sounds like a B.A. talking ...

Your example is less than worthy of adressing.

Science has nothing to do with what you say - however, perhaps whatever it is you do allows for such an arrogant method. Science is all about testing a null hypothesis.

I think what he was trying to say is that a 5% change would not be noticeable to the human senses unaided. This is true when you're talking about time, like it or not. It's probably also true about the sound produced by the water.

Lordjeebus said:

with a 5% change over 100 degrees, the speed of sound changes by about 2.6%. v = f*lambda, f = constant, so the wavelength changes also by about 2.6%. Not that this means anything for our perception of sound, as noted above. Even if frequency somehow changed by this amount, the difference between, say, 500 and 510 Hz isn't the kind of thing that's going to make us say "whoa, it sounds totally different."

If I remember right, a note can go from flat to sharp in about 40 hz, and that is a noticable change. so a 5% change from 2,000 hz to 2,040 hz would be detectable.

As far as measuring time without clocks, ancient cultures like the central Americans and druids had accurate calenders, and they would have noticed if the earth were moving 5% slower.

Always noticed the difference, but never before put it to conscious thought... what an intriguing thing. Now I'll always be bothered by it too, even knowing why. Well, I guess the only thing to do now, is to further perpetuate this knowledge and let other people ponder this...

It would be noticable if you had perfect pitch or if you heard one right after the other. That 5% change is not going to produce the very obvious difference in frequency that you get between cold and hot water.

Complete agreement with lordjeebus. by the way, it's correct that a 5% change in frequency is about a half-step on the chromatic scale. (those who are not musically inclined, a half-step is the smallest difference in the conventional 12-note octave. the two notes in the "Jaws" theme are a half-step apart. sing it to youself. the two notes are almost the same, aren't they?)

Regarding the fact that a 5% change in the earth's rotation would be noticable, of course it would, even without druid calendars and whatnot--animals would probably feel out of whack, i'm sure the delicate ecosystem would undergo strange changes, certain species would disappear, weather patterns would change, etc etc., but ecology and weather are chaotic systems, where small changes can cause dramatic consequences. but if you are on some deserted island with no watches, you're telling me that you would be able to tell the difference if for a few days the earth slowed down by 5%? (you would probably feel like you got a little more sleep than usual, but you could just as easily attribute that to the partying you did the night before). or another example, if you were given a stopwatch that was 5% slower than it should be, would you be able to tell immediately that the seconds were ticking 5% slower than they should?

we are talking about (or WERE talking about) the very perceptible change in sound from hot water to cold water! unless you're going to argue that the change in sound as temperature increases is due to some sort of chaotic mechanism associated with the density of water, 5% is not enough to be significant in this case!

sapper: the null hypothesis is the foundation of the scientific method, but observing phenomena and discerning between significant and insignificant mechanisms at play is a large part in coming up with theories. what do you think we're doing now?? don't you think a few scientists have pondered this exact same question before us, before running whatever experiment they did to prove the point?

finally: i went through the trouble of finding a site that has viscosity info. the difference in viscosity between ice cold water and water at room temperature is about 200%, while the change in density for the same temperature difference is 0.3%!

Besides I would argue that, if two liquids have the same viscosity but widely differing densities, they would make pretty much the same sound when being poured. if anyone is interested in hearing why, just ask.