Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do sound waves carry momentum?

  1. Jun 9, 2008 #1
    do sound waves carry momentum? any momentum at all (no matter how small)?

    the obvious answer is that they carry energy but not momentum but i am wondering whether they dont theoretically carry some very tiny (usually negligible) amount of momentum in the same way that light, even though it can normally be thought of as being massless, theoretically does have some very tiny amount of mass.
  2. jcsd
  3. Jun 9, 2008 #2
    sound waves are vibrating molecules, and everyone of these molecules move and thus have a momentum.

    Hey I got an idea, if we can get a formula for momentum for sound, ie vibrating particles, then we can use it the other way round to determine the properties of the "medium" that light eventually travels through, right??? ;)
  4. Jun 9, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Wrong, light does not propogate in the same manner as sound. The properties of materials which effect light transmission are pretty well understood.
  5. Jun 10, 2008 #4
    Okay so do sound waves have momentum or not? I believe they do....as do light waves.

    I know sound waves move a lot like a wave and light as a wave function consisting of particles (photons) thus having a wave-particle duality.
  6. Jun 10, 2008 #5
    Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

    However I seem to have some problem in seeing the momentum of an acoustic wave with a classical approach. :/ There are certainly cases where an acoustic wave can "push" an object, for example in the case of a "shock wave".
  7. Jun 10, 2008 #6
    or better yet, bass frequencies....

    you can "feel" bass
  8. Jun 10, 2008 #7
    It was more like an idea IF there is a medium that light travels through (not any "material"), but more like the luminiferous aether that people thought existed a while ago. Do we really know everything about the properties of light? I doubt that.
  9. Jun 10, 2008 #8


    User Avatar
    Science Advisor

    Phonons carry pseudomomentum. Summed up, the momentum of all phonons usually cancels to 0. Otherwise the center of mass of the crystal would have to move.
  10. Jun 10, 2008 #9

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    Sound waves have momentum and dispersion, a phase and a group velocity. That's one of the reasons why gongs, when struck with a soft mallet, have long attacks.
  11. Jun 10, 2008 #10

    same here.
  12. Jun 12, 2008 #11
    sounds pretty dumb to say sound waves move alot like a wave when sounds waves are in fact waves... why do you think they call them sound WAVES haha... of course this coming from a yankee fan... haha just bustin your balls
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook