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Sound waves

  1. Feb 4, 2006 #1
    I have been studying sound waves, however, i have not seen in my books nor online how sound waves "move." I know that sound waves are longitudinal waves, meaning, that they move parallel to the direction of displacement, but it seems like they just travel in one direction although they reflect off things. For example all we cover in school is just a picture of them as: ||| | | | ||| | | | |||. (for you, the reader, probably know the close together lines represent compression and the far spaced ones represent rarefraction. Im just explaining what the "drawing" represents.) now, that makes me think that they are moving in one direction (it cant be like that) because if it were a person behind me on an open field would not be able to hear me if i speak. I would say that sound waves would move in a ripple way. Not exactly like a ripple since ripples occur in water. Could someone explain to me how sound waves move so that a person behind me could hear me in an open field? Thank you.
  2. jcsd
  3. Feb 5, 2006 #2
    When sound waves meet objects, they diffract around them. And that is why we can hear them around obstacles. For the person behind you in an open field, the sound waves when they come out of your mouth, diffract or 'spread' in all directions, because the mouth also is an aperture, and due to this spreading, reach that person.

  4. Feb 5, 2006 #3

    Meir Achuz

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    Sound waves are usually depicted incorrrectly in elementary texts
    (probably so they can show nice pictures like the one in your post).
    The authors and publishers think that makes them easier to understand.
    Actually, sound is a presssure wave and pressure is a scalar with no direction. Diffraction phenomena and effects like what you mentioned in your question are actually easier to understand in terms of a pressure
    wave. The air molecules do oscillate back and forth as in your picture, but that is because their motion depends on vector derivatives
    (the gradient) of the pressure.

    The person behind you hears you not so much because of diffraction, but just that the pressure wave spreads out in all directions like the waves when you drop a rock into water. If you use a long megaphone, the sound is more directional.
  5. Feb 5, 2006 #4
    So, it would be somewhat like a ripple, thank you very much
  6. Feb 7, 2006 #5
    Yes, indeed like ripples. Just as ripples of water spread out in waves from a stone dropped on the surface, sound waves spread out in a material in circles.

  7. Feb 14, 2006 #6


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    Sound waves are cyclic or random variations in the air pressure. A sound wave is really a (spherical) shell of a higher pressure moving away from the source till it hits a reflective object. The rebound is the "echo" or similar spherical shell of pressure variation. One or more such shells may start out from the source. One shell would appear as a "bang" or a "sonic boom", many would appear as a vibration or a note or a continuous sound.
    It is impossible to show a 3D description on a 2D paper.
  8. Feb 15, 2006 #7


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    Ugh, no!!!
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