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I apologize if this is a naive question, but I never really learned about this. I'm curious as to what happens to sound waves after they are "used"? For example, if I say something to you verbally, then a sound wave is transmitted and picked up by your ears, but what happens to the wave after that? Does it float away into space? And if sound waves are never destroyed (not saying that is the case, but if it is), does this theoretically mean we can reproduce or attain all sound waves from the history of man, including every word ever spoken?

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3 Answers

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No, It's not possible. Sound ultimately transforms into heat energy. You can not reproduce all the useful and useless sounds from history. In general all energy is ultimately converted to heat energy and heat energy flows from an object at higher temperature to an object at lower temperature in order to attain thermal equilibrium.

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Terminology correction: there is no such thing as heat energy. It's called internal energy (or thermal energy, depending on what you're speaking about) and heat is just a process whereby energy is transferred. –  Marek Mar 21 '11 at 8:37
    
@Marek, exceptions of Your rule are: heat of combustion, specific heat, heat of vaporisation, heat of melting, etc... –  Georg Mar 21 '11 at 10:03
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@Georg: all of those terms are historical and have little to do with the concept of heat. These days one prefers notions such as "enthalpy of fusion". Only specific heat is a possible exception but in this case the full term would be specific heat capacity and so it's only indirectly related. –  Marek Mar 21 '11 at 10:10
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Just one more aspect to consider is the comparison with the electromagnetic waves from radio and TV broadcasts. One could ask the analogous question here:"what happens to them?" The answer is that some were broadcast also into space (when transmitters were entirely undirectional) and so are still available for reception, albeit only by aliens light years away and very attenuated.

Sound waves stay on Earth in the atmosphere, and become part of that atmospheric motion, which is gaseous motion. This motion exists because there is a non-zero temperature for the atmosphere (say 300K). So the sound wave attenuation will reduce the given wave intensity, and corresponding particle motion, and corresponding air pressure difference (which is sound) below that of the ambient atmosphere. In a good (outdoor or indoor) auditorium the sound can last for longer and travel further, but eventually it will reach the general level of atmospheric noise.

So I suppose that with sound (unlike with EM waves in space) one could say that the medium ultimately destroys the message.

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To supplement sb1's answer I would like you to think the same question about waves of the sea, even if directional, like the wake of a boat.

Sound is waves of motion of all the particles that compose the air, similar to the waves of the sea. The shape and energy that the waves of the sea carry become finally thermal motion of the water, the air, and the sand if they break on a beach.

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protected by Qmechanic Jan 31 '13 at 8:27

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