# Interesting question - sound waves and temperature

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I have been pondering this for a while now. Since anything moving has energy and therefore has a temperature - and when there is more motion that means more energy and higher temperature. I have often wondered if you took two rooms, identical in size, and set them at the exact same temperature, but in one you played loud music and the other you kept silent. Would the temperature be raised in the room with music? Since the sound waves would vibrate things in the room, there would be more motion. They would also cause the air molecules to vibrate.

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I have often wondered if you took two rooms, identical in size, and set them at the exact same temperature, but in one you played loud music and the other you kept silent. Would the temperature be raised in the room with music?

Yes, but not very much. If you had a focussed sound... think "laser" for sound... you could heat up a small spot very significantly. I think some military research involves this idea for weapons. However, just your regular stereo speakers are not enough to keep you warm in the winter.

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Right you are Taq. If you make some sound waves in a room then these waves are transmitted through the air by the atoms moving around and knocking into each other. This means the atoms will have a higher kinetic (moving) energy and thus a higher temperature.

However remember that air molecules, in an average room, would normally move around quite a lot anyway. Whilst the sound waves will make them move in a specific direction (away from the sound source) their actual speed might not increase much, if at all.

An example would be if the all the air was blowing towards the sound source. Then by turning on the music you are pushing the air molecules in the opposite direction to where they are going - you are slowing them down. In a room the molecules will all be moving in random directions, but I use this "wind example" to demonstrate my point.

If there is any effect it is very small, and you wouldn't measure it normally. As for theoretically, well, yes it would increase the temperature, unless what I said in my 2nd & 3rd paragraph is significant.

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However, just your regular stereo speakers are not enough to keep you warm in the winter.

Oh really? I was going to save on heating by playing consant music! lol No, I knew that, just had to make a joke. 5614, that's very true about the air molecules.

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The spread of sound is actually nothing but the spread of vibrations from one molecule to another. A definition I found best for waves (as we're dealing with sound waves) is that waves represent the spread of shakes on an elastic medium. Air has the feature of being elastic as the molecules can move freely.

And as sound is spread with the "transport" of vibrations from one molecule to another, then while transporting the vibration, molecules hit each other and a result is the generation of heat. But that's just too little to measure (like 5614 said)

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The acoustic power of very loud music is in the tens of watts. 90 dB, which is pretty loud, is about 20 Watts. The power to most speakers runs along some pretty wimpy wire, which should give you an indication that they don't draw much current and therefore don't convert much electrical into acoustic energy.

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Has anyone heard that snippet of trivia that goes...

"It would take you 8 years of non-stop yelling in order for you to produce enough sound energy to heat up an average cup of coffee".

Now I am not too sure about the validity of such a statement but if it is true, then I'd say the temp. increase due to sound is so small that it would require very very sensitive equipment to pick up.

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Has anyone heard that snippet of trivia that goes...

"It would take you 8 years of non-stop yelling in order for you to produce enough sound energy to heat up an average cup of coffee".

Now I am not too sure about the validity of such a statement but if it is true, then I'd say the temp. increase due to sound is so small that it would require very very sensitive equipment to pick up.

Depends how loudly you yell. The thing is, that perceived sound needs to increase by a factor of 10 to sound twice as loud, because our perception is nonlinear. Shouting might only represent a small fraction of a watt. So let's do a back-of-the-envelope calculation:

8 years for a 250 mL cup raised 60 ºC requires about 60 microwatts continuously over that span (assuming 100% efficiency and ignoring the heat loss from the cup), which corresponds to 77-78 dB and that's louder than normal conversation volume.

http://en.wikipedia.org/wiki/Sound_power

Order-of-magnitude-wise, the statement seems OK. Certainly not egregiously wrong (assuming no calculator errors)

edit: Oops. forgot a conversion — I was using calories instead of Joules. You need ~240 microWatts. That's 83-84 dB

Edited by swansont
fixed numbers

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After a bit of thought I'll expand my answer to say this:

In any undisturbed room (ie. where the air molecules can be assumed to be moving totally randomly, Brownian motion) when you play music the room will heat up.

The reason is that whilst the particles moving towards the speaker will slow down, this will be cancelled out by the particles moving away from the speak which speed up. The inbalance that causes net heating is where particles are moving very slowing relative to the progessing wave. These particles (whether they're moving slowly towards or away from the sound wave) will all speed up. Thus there is a net energy gain.

I suppose maybe this is stating the obvious... but anyhow!

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All good points but is everyone forgeting the simple fact that the sterio will heat up? Yes the sound waves will heat the room up by a fraction of a degree but the stereo itself is producing 100s of times more heat. Plus any dancing will produce also produce heat.

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All good points but is everyone forgeting the simple fact that the sterio will heat up? Yes the sound waves will heat the room up by a fraction of a degree but the stereo itself is producing 100s of times more heat. Plus any dancing will produce also produce heat.

That's a separate effect, though. If you were going to run this as an experiment, you'd put the amp in a different room. And I'm sure you can find a scientist or two that don't dance.

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