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Homework Help: Wave superposition and Doppler effect.

  1. Apr 25, 2013 #1
    1. The problem statement, all variables and given/known data
    A student is at some point between two sound reflecting walls. He's holding a tuning fork and runs towards one of the walls at 5m/s. The pulsating frequency he hears is 8Hz. ¿What's the sound frequency emitted by the tuning fork?
    Answer: 137.17 Hz.

    I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.
  2. jcsd
  3. Apr 25, 2013 #2

    Doc Al

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    Staff: Mentor

    In addition to the Doppler effect, you need to understand beat frequency. (Look that up!)
  4. Apr 27, 2013 #3

    Ok so, it is a beat frequency so

    f2-f1 = 8

    f2= f1+8

    using doppler's formula

    Assuming Vsound= 343 m/2


    The answer is supposed to be 137.17.... What am I doing wrong?
  5. Apr 27, 2013 #4

    Doc Al

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    Staff: Mentor

    You're on the right track, but there's a bit more to it. The two sounds that are creating the beat frequency are the reflections off the two walls. So you have to find those frequencies. Hint: For each of those reflections you'll need to apply the Doppler formula twice.
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