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Homework Help: Interferene of Waves

  1. Feb 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Two point sources S1 and S2 emit idential sound waves of frequency 171.5 Hz as shown to the right. In terms of wavelength, what is the difference in the path length between the waves arriving at point P if
    a) L1 = 38m and L2 = 34 m
    b) L1 = 39m and L2 = 36m
    c) Assuming that the source separation is much smaller than L1 and L2, what type of interference occurs at P in situations a and b?

    *There is a picture: The paths of waves emitted from points S1 and S2 are straight lines that are converging twrds each other and intersect at P. Its sort of a triangle, w/ P at the vertex

    2. Relevant equations
    wavelength = f *v
    f = nv/2L --> n is harmonic number, but its not given?

    3. The attempt at a solution
    From that 2nd equation, I worked out that
    L = [tex]\lambda[/tex]n/2

    But I really don't understand what the question is asking. If the waves are identical, than how is their path length changed? And how do you express it "in terms of lambda"?
    Last edited: Feb 10, 2009
  2. jcsd
  3. Feb 10, 2009 #2


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    The idea behind this question is to figure out if the waves constructively interfere at point P, or destructively interfere. If the path lengths differ by a whole number of wavelengths then the waves will constructively interfere. If the path lengths differ by half wavelength, or an odd number of half wavelengths then they will destructively interfere. (Review these terms in your text if you ae not familiar with them). So you need to take your distances and divide by the wavelength to see what you've got.

    The concept of harmonics goes with standing waves, but this problem isn't about standing waves.
  4. Feb 10, 2009 #3
    Oh, ok thanks! It makes much more sense now. I see what the idea behind this is now, and thats what was confusing in the first place.
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