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Sound Interference and Problem

  1. Dec 17, 2007 #1
    Hey! I got two questions. Hoping someone can please help me out.

    1) I'm doing a physics assignment and there's a question about constructive/destructive interference of sound.

    One wave is square and its on top of and the other wave is a triangle and its on the bottom. The square is bigger than the triangle. I'm thinking its a destructive interference since they are both on a different plane. Therefore, when the waves meet, I have the square with a missing portion of the shape of the triangle.

    Something like this: http://img503.imageshack.us/img503/6374/physicsinterferencefu3.png

    Can someone help me with this. This is what I have but my friends have something else.


    2) An ambulance is moving away from you and its siren is making a sound with a frequency of 457 Hz. It the original frequency was 620 Hz and the temperature is 25 C, how fast is the ambulance going?


    Someone please help. I'm desperate.
     
  2. jcsd
  3. Dec 17, 2007 #2

    Doc Al

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    Staff: Mentor

    1) Your answer looks good to me. When the two waves overlap they will look as you drew it.

    2) This is a Doppler effect problem. Look it up!
     
  4. Dec 17, 2007 #3
    Thanks for the reply.

    For the second question, I knew it was a Doppler Effect problem. I did the calculations too. However, my final answer for Vo turns out to be something unbelievable, as in something couple of times the speed of sound. I'm thinking I made a mistake. Could you please double check for me?

    Thank you very much. Truly appreciated.
     
  5. Dec 17, 2007 #4

    Doc Al

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    You're right, those numbers aren't particularly believable. :wink: (Could be a typo.)
     
  6. Dec 17, 2007 #5
    Just out of interest, is the answer you got 124 m/s? Because that's what I got.
     
  7. Dec 18, 2007 #6

    Doc Al

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    I thought you got an answer that was several times the speed of sound? Show what you did.
     
  8. Dec 18, 2007 #7
    Well I did it again and I got something along the lines of 124 m/s.

    f2 = f1(vs/(vs+vo))

    f2/f1 = vs/(vs+vo)

    f2(vs+vo) = vsf1

    vo = vsf1/f2 - vs

    So when I plug in the digits, I get 123... for vo.

    To prove it, you just just plug in vo and the other values into f2 = f1(vs/(vs+vo)) and f2 should come out to 457, just like in the question.
     
    Last edited: Dec 18, 2007
  9. Dec 18, 2007 #8

    Doc Al

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    You are correct. (I messed up my calculation earlier! D'oh!)
     
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