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New Energy (Audio)

  1. May 3, 2010 #1
    I have idea, audio is vibrating wave of air, we know how to make energy from movement (movement energy) very loud audio can vibrate membrane and from that vibrating we can make energy (axiom of movement energy) but that quantity of energy is minor
     
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  3. Jun 25, 2010 #2
    hi,


    yes that is true, and yes that energy is very minor, that is basically a microphone though, isnt it?, i wouldnt call sound a form of energy, where would the loud sound come from? there is no point generating sound by using energy and then converting that sound into energy again, is there?
     
  4. Jun 25, 2010 #3

    sophiecentaur

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    A fair idea, in principle, but the devil is in the actual numbers - as in many engineering problems.
    When you consider that a tiny battery in a hearing aid can give an (hearing impaired) user an audible signal in their ears for several days, you will have a qualitative idea of how much energy is actually available from sound (by thinking in reverse). An environment in which there would be enough sound energy to make it worthwhile converting it into another form ( say electricity) would be so loud as to be painful or worse.
    It is, I suppose, worth considering 'extracting' the sound energy that exists inside heavy machinery but improving the actual efficiency of a machine would probably be of better value in terms of energy use. We would be talking in terms of energy quantities of the order of those used in solar lights for gardens etc. rather than providing a serious amount of electrical energy.
    But don't give up on inventing novel energy conversion ideas - after all, those damned parking meters are driven by solar energy nowadays and we all LOVE them, don't we?
     
  5. Jun 25, 2010 #4
    Well, you could operate your computer with your voice instead of moving a mouse and keys.
     
  6. Jun 25, 2010 #5

    sophiecentaur

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    You could CONTROL it by voice but you wouldn't be supplying the power to drive it.
     
  7. Jun 25, 2010 #6

    Borek

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    This question shows up again and again. My guess is that's because of the powers installed in audio amplifiers. It is nothing unusual to see 2x100W, which suggests amount of energy that can be harvested from the sound should be of the same order of magnitude. But it is not.

    Honestly - I have no idea why. The only thing I can think off is that our method of producing sound is incredibly inefficient.
     
  8. Jun 25, 2010 #7

    sophiecentaur

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    You are right. Good quality sound reproduction is very inefficient compared with just rubbing your back legs together (if you are a cricket).
     
  9. Jun 25, 2010 #8
    The motion of air molecules is very erratic, a lot of kinetic energy is lost.
     
  10. Jun 25, 2010 #9
    I wouldn't want to try and power my computer with a hand crank, little yet my vocal cords. I am talking about a smaller amount of energy, the movement of a mouse.
     
  11. Jun 25, 2010 #10

    Borek

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    Lost? To what?
     
  12. Jun 25, 2010 #11
    Lost to us, not to the universe. :)
     
  13. Jun 25, 2010 #12

    Borek

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    Explain how it is lost, I have no idea what you mean.
     
  14. Jun 25, 2010 #13
    Lost by increasing entropy. For example if you had a sealed container with 100W speaker in it you could transfer the energy in its entirety to another speaker that would not require another 100W amplifier.
     
  15. Jun 25, 2010 #14

    Borek

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    Still no idea what you mean, nor how it is related to the fact that motion of air molecules is erratic.
     
  16. Jun 25, 2010 #15
    Using a fan to push a book off the table wastes a lot of energy.
     
  17. Jun 25, 2010 #16

    Borek

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    It has nothing to do with the fact that air molecules motion is erratic.
     
  18. Jun 25, 2010 #17
    Sure it does, you use 100W to accelerate a membrane at a very high acceleration, then that energy is distributed into the air all around you, now try to collect all that KE again as it escapes in every direction as far as miles away at the speed of a bullet.
     
  19. Jun 25, 2010 #18

    Borek

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    Fact that energy escapes in every direction has nothing to do with the fact that molecules motion is erratic. It is based on macroscopic properties of the medium, not on the microscopic mechanism. You don't have erratically moving molecules in solids, yet the sound travels in every possible direction, very similarly to what happens in gas.
     
  20. Jun 25, 2010 #19
    Ok, the point is the energy is in the KE of the air molecules and is not easily collected.
     
  21. Jun 25, 2010 #20

    russ_watters

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    Energy in moving air is lost to heat, due primarily to viscous friction in the air and absorption by objects.

    In addition, sound tends to be somewhat omnidirectional, so it is difficult to capture a significant fraction of the ouput of a speaker unless you surround it.

    Also, a home amplifier that delivers several hundred watts is truly earsplitting in a small room, when playing loud sounds. But when your TV speakers are delivering sound that you can talk in a normal conversation overtop of, the speakers are putting out single-digit watts of power. So it isn't just that sound is tough to capture: it is also that there just plain isn't much energy in most day-to-day sounds.
     
  22. Jun 25, 2010 #21

    sophiecentaur

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    The thermal motion of air molecules has a little or no effect on the sound energy passing through it. (Although the actual speed of sound depends upon the temperature) However, some sound energy is absorbed as it propagates through air and this contributes to a bit more random motion (i.e. it raises the temperature a little bit)
     
  23. Jun 26, 2010 #22

    Borek

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    Again, where is the erratic motion of the molecules involved? While tiny part of the sound energy is lost to the thermal energy, most of it passes as if the air was perfectly elastic. Also note that at this stage there two kinds of KE - one is KE due to sound wave propagation, other is KE due to thermal motion. They are basically not related and and should be not mixed.

    That's what I am aiming at. In the context of sound propagation erratic motion of molecules is far on the list of the things that should be taken into account, even if - no doubt about it - together with almost perfect elasticity of collisions they are what makes the air behave the way it does. But when we are talking about the sound propagation, it is enough to concentrate on the emergent properties, like compressibility and viscosity, and treat the air like continuous medium. Mentioning erratic motion only murks the water.
     
  24. Jun 27, 2010 #23

    sophiecentaur

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    If there weren't "erratic motion" of air molecules, the temperature would be zero. The speed of sound would be very small because the molecules would take a vast time to bump into their neighbours and transfer the sound pressure. (Nonsense scenario, really but I quote the example to make the point).
    These ideas are much better approached in the classical way, for describing the waves and, if you need to, the kinetic model to account for the (classical) modulus and density of the gas.
    An ideal gas can be treated in this way but a 'real' gas will introduce loss mechanism to absorb some of the sound energy as it passes through.
     
  25. Jun 27, 2010 #24

    Borek

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    I think you are missing my point. We don't need kinetic theory of gases (be it real or ideal) to describe mathematically sound wave propagation. For that we can treat air as a continuous medium. We do exactly the same when dealing with diffusion - (almost) nobody treats diffusion as a random process based on erratic motion, what we usually do is we solve Fick equation. My guess is that granularity of the medium becomes important when the wave length becomes comparable with distances between molecules - that means at least 10 GHz frequencies.
     
  26. Jun 28, 2010 #25
    Ok I see what your saying Borek, even though the molecules motion is erratic, it does not much effect the sound energy propagation.

    So the reason that the energy is hard to capture is because it speeds away in every direction at the speed of a bullet and travels through most materials.

    Russ, you are correct a 100W speaker at full volume is deafening.
     
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