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Drawing wave interference

  1. May 9, 2013 #1
    1. The problem statement, all variables and given/known data
    Two speakers are emitting identical sound waves with a wavelength of 4.0 m. The speakers are 8.0 m apart, directed toward each other. At each of the noted points (a-e) in the above diagram (at 2,3,4,5, and 6 meters, respectively), is the interference constructive, destructive, or somewhere in between?


    2. Relevant equations
    Δr=|r2-r1|


    3. The attempt at a solution

    Ok, so, mathematically, I understand this with no problem. I know that a, c, and e are constructive and that b and d are destructive, based off the formula.

    I get that. What I don't understand, is how exactly the waves appear on the picture. The image we're given has the line without the wave included (attached if interested). They are equally spaced and have the same exact wavelength, so wouldn't that mean that the waves perfectly overlap? I am having the hardest time imagining how these two waves will look. When I draw them both out, they are both literally identical (they go one right on top of the other). What am I failing to see? Sorry if this is a stupid question, but I am not so great at seeing these sort of things sometimes.

    Thank you for any help!
     

    Attached Files:

  2. jcsd
  3. May 9, 2013 #2

    Simon Bridge

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    The waves should not be identical - they should have the same phase at their respective sources and travel in opposite directions.

    It may help you see if you sketch cosine waves instead of sine waves.
    It may also help if you repeat the sketch for several different times so you see how the resulting superposition evolves over time. What kind of wave should you have?
     
  4. May 9, 2013 #3

    TSny

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    Look here and scroll down to Two sine waves travelling in opposite directions create a standing wave and see if that helps.
     
  5. May 9, 2013 #4
    Ohhh, ok. I think I get it now (points a, c, and e are constantly flipping between constructive interference up top or at the bottom, while points b and d are never constructive). Thank you so much!
     
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