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Doppler together with beat

  1. Jun 25, 2012 #1
    1. The problem statement, all variables and given/known data
    In the figure, a motionless observer stands between sound sources A and B, which oscillate at 338 Hz and 342 Hz, respectively. The observer hears a beat. Next, when the observer begins moving at a constant speed on the straight line connecting A and B, the beat is no longer heard. Here, the speed of sound in air is 340 m/s


    2. Relevant equations
    fb=|fB-fA|....(1)

    Doppler equation
    f'= f([itex]\frac{v\pm v0}{v\mp vs}[/itex])

    v0 = observer's speed


    3. The attempt at a solution
    1st, I assume that observer walking towards A in order not to hear beat frequency.

    Thus, f[itex]^{'}_{A}[/itex]=fA([itex]\frac{v+v0}{v}[/itex]) (2)
    f[itex]^{'}_{B}[/itex]=fB([itex]\frac{v-v0}{v}[/itex]) (3)

    from (1); fb = |338-342|=4 Hz
    so |(2) - (3)| = 4 Hz and I got only 1 unknown to solve
    ∴v0 = 4 m/s
    but the answer is 2 m/s toward A, Oh!! Did I do wrong way or my assumption was wrong??
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 25, 2012 #2
    Hello IIK*JII
    When the observer doesn't hear beats its frequency should be zero or the two frequencies should be equal ,right?I am getting 2m/s as well if my calculations are not wrong. Maybe you have messed up your calculations somewhere by the looks of your method I suspect you have inverted the situation and still getting 4 beats .
    Rest is all correct .Even if you had assumed that observer walked towards B you'd getting the same answer in negative.
    Correct me if I am wrong.
    regards
    Yukoel
     
  4. Jun 25, 2012 #3
    Yukoel !!!!

    Okay, I understand it because I inverted the situation..

    Thanks a lot :)
     
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