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Wave question.

  1. Feb 12, 2009 #1
    Ok, so basically the question on a test was if a a sonic boom occurs because an airplane travels as fast as sound what is it an example of. a) interference b) Doppler effect c) something else d) something else.

    The correct answer which my teacher marked was interference, although I think it's doppler effect and my teacher was like if you can come up with an argument supporting that it's doppler effect he would give me the point back. Considering that my grade is an 87 and i still have 6 weeks to bring it up an A i still need the extra point cus it brings my grade up to an 88.

    like this is how i explained it to him:

    because when you're standing infront of the airplane the frequency is higher because the waves "pile" up, but when you're standing under the sonic boom the frequency is lower because the waves are just going by.

    but my teacher needs a more "technical" argument. so can someone please help me with this.
  2. jcsd
  3. Feb 12, 2009 #2


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    Personally I'd go with constructive interference.


    Remember no matter how hard Cinderella's stepsisters tried they couldn't get all their toes into the slipper.

    Try looking at the other 12 things you got wrong if you want to mine a point.
  4. Feb 12, 2009 #3

    That link doesn't provide any help.
  5. Feb 12, 2009 #4


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    Hi iFat! :smile:
    Well, you try to word it technically :wink:

    start with the three regions (I) in front of the boom (II) in the boom (III) behind the boom …

    what do you say the Doppler effect is, and why, in each of those three regions? :smile:
  6. Feb 12, 2009 #5


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    Not to your thesis any way.

    The idea behind constructive interference in adding the amplitudes of the sound waves to produce the shock wave, is more closely related than any notion about increasing frequency as part of any Doppler effect.

    I will admit that the question in not a sound one to begin with. But as one phenomenon - boom - is more related to a shock wave as opposed to the other being more a phenomenon of an increase in frequency ... don't call me as an expert witness in your case.

    Perhaps your teacher will be impressed if you demonstrate a more in depth command of the matter?
  7. Feb 12, 2009 #6
    Does this work?

    "I can see how it is constructive interference. Constructive interference is where the sound waves add up in order to produce a shock wave. But however since we didn't learn what a shock wave is, it is closely related to Doppler effect because as you are in front of the boom the waves add up making a higher pitch, and when you're in the middle of the boom the pitch of the boom stays constant, and when you're behind it, the frequency is lower which makes the pitch lower"
  8. Feb 12, 2009 #7


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    how?? why?

    (in fact … i can't remember the answer to this … can the plane be heard at all in front of the boom?)
  9. Feb 12, 2009 #8

    You see the picture to the farthest left. Like if you're behind the airplane, where the waves start adding up, there is more frequency where as when you're behind the airplane like farthest to the right it takes time for each wave to pass you which makes the frequency lower which makes the pitch lower. Right?
  10. Feb 12, 2009 #9


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    No. Your perception is of the trailing cone of the shockwave coming at some angle from the source. At higher Machs the wave is sharper to the direction of motion and may not even be heard on the ground as I recall.
  11. Feb 12, 2009 #10


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    No. Give it up.

    The middle picture exactly shows the more constructive interference nature of the phenomenon. Making a frequency argument is not compelling.
  12. Feb 12, 2009 #11
    I got my argument.

    Thanks guys!

    It doesn't hurt.
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