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Phonons and their properties

  1. Jan 26, 2004 #1
    Is there any general information that I can get about these discrete acoustical particles? I know that they have the same wave-like properties as photons, but they also have quite a few differences (i.e. sound by itself diminishes over time and does not exist in places with lack of a medium such as air).

    What im curious about is the energy related to phonons. What happens to the phonons when sound fades? Suppose im playing an instrument, the air I supply to it causes the air to vibrate in the instrument, thus providing sound. Where does the energy transfer to?Does the wavefunction of the phonon gradually fade from, lets say sin[x] to 0?
  2. jcsd
  3. Jan 27, 2004 #2


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    Somehow, based on your question, I have the impression that you are associating phonons with sound vibrations in AIR. Phonons are not well-defined in air (or gasses for that matter). Rather, phonons are the normal modes of lattice vibrations in SOLIDS. It is one of the more common form of "elementary excitation" of solids. It exists due to the lattice ion vibrations and requires a nearest-neighbor, next-nearest neighbor, etc. interactions, which one doesn't have in a typical gas state. Not only that, there are acoustic and optical phonons that occur depending on how those ions are displaced relative to one another during the normal modes vibrations.

    An elementary explanation of what phonons are can be found at the Hyperphysics site (which is a good resource to start with when one has any basic physics question) at


  4. Jan 27, 2004 #3


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    What wave-like properties do they have that are the same?

    They are energy; discrete "packets" of it. A lattice can only gain or lose sonic energy in the amount of these packets.

    What is the "wavefunction of the phonon?" A phonon is an amount of energy. Imagine a crystal is vibrating at a certain amplitude at a pure frequency. If this vibration decays (emitting sound into the air), it will do so in a stepwise fashion. Each step down in amplitude is the loss of a phonon. Of course, these steps are extremely small, because the frequency is sonic.
  5. Jan 27, 2004 #4
    Ah, I see. So phonons are just energy vibrations that propagate through the bonds in the solids. And since phonons are not well-defined in air, they disspiate compared to photons (which not only can travel through a gaseous medium but also through a vacuum). I get it now. Thanks for the help ZapperZ and turin.
  6. Jan 28, 2004 #5


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    I'm afraid that you still might have a slight misconception. Think of a phonon as a unit of energy, like a Joule or an eV (only a phonon is much smaller than even an eV). It is not quite this simple, but I think it will give you a better picture than referring to a phonon as an "energy vibration that propagates." Saying this would be like saying an eV is a vibration that propagates. Though, if saying this about an eV makes sense to you, then I may just be arguing semantics, and for that I would appologize.
  7. Jan 28, 2004 #6
    I never thought about it as a unit, but it makes sense.

    No need to apologize, this is a pretty new concept to me. Its more complex than I thought it would originally be. But its a learning process, and I appreciate your help.
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