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Homework Help: The velocity of sound

  1. Oct 22, 2016 #1
    1. The problem statement, all variables and given/known data
    Will the velocity of sound change if the source moves?

    2. Relevant equations
    The velocity of velocity of sound in air equals:
    $$V=\sqrt{\frac{\gamma RT}{M}}=const$$


    3. The attempt at a solution
    I think it doesnt change. But it seems to conflict with the equation :
    $$\vec{v_{air}}=\vec{v_s}+\vec{V}$$
     
  2. jcsd
  3. Oct 22, 2016 #2

    haruspex

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    Please explain what each velocity represents in that last equation.
     
  4. Oct 22, 2016 #3
    In room reference frame
    ##v_{air}## is the velocity of sound that observer receives. ##v_s## is velocity of source and ## V ## is the velocity of sound if source doesnt move.
     
  5. Oct 22, 2016 #4

    haruspex

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    I don't understand.
    In the room reference frame, the source, receiver and air may all have a velocity, and the sound has a velocity relative to the air.
    Let's call these vs, vr, va, and V respectively, all measured in the same direction.

    What does your equation look like in those terms? You seem to be saying that the sound arrives at the receiver with speed V+vs-vr, but that is not true. No matter how the source moves, the velocity of the sound waves relative to the room is V+va, and relative to the receiver is V+va-vr. The frequency is another matter.
     
  6. Oct 22, 2016 #5
    Sorry .I seem air and receiver dont move. And source comes near observer. So the velocity of sound that obsever receives v=vs+ V
     
  7. Oct 22, 2016 #6

    haruspex

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    No, as I posted, the velocity of the source does not affect the velocity of the sound waves. It does affect the frequency.
     
  8. Oct 22, 2016 #7
    Thanks a lot for helping. I understanded.
    Answering this question helps me prove the Dopper effect.
     
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