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Homework Help: Sound with Wind Present

  1. Apr 24, 2007 #1
    1. The problem statement, all variables and given/known data
    A factory whistle emits sound of frequency f. On a day when the wind velocity is w from the north, what frequency will observers hear who are located, at rest, (a) due north, (b) due south, (c) due east, and (d) due west, of the whistle? What frequency is heard by a cyclist heading (e) north or (f) west, toward the whistle at a speed u?

    2. Relevant equations
    Frequency heard by stationary observer of a sound source

    (1) moving toward the observer: f/(1 - vs/v);
    (2) moving away from the observer: f/(1 + vs/v).

    Frequency heard by a moving observer of a stationary sound source when

    (3) the observer is moving toward the source: f(1 + vo/v);
    (4) the observer is moving away from the source: f(1 + vo/v).

    f is the frequency of the sound source, v is the velocity of sound in air, vo is the velocity of the observer and vs is the velocity of the sound source.

    3. The attempt at a solution
    This problem is a bit tricky for me because it discusses a new concept: waves on a moving medium, viz. sound waves in the wind. I thought of a simpler situation: a transverse wave on a moving string. In this situation, if the wave moves in the same direction as the string, an observer will see a faster moving wave than one in which the string is not moving right?

    The question is: Do I consider the wind as a moving sound source then?
  2. jcsd
  3. Apr 24, 2007 #2
    Not sure. From intuition I would, but I would clearly state as an assumption that v plus c add vectorially.
  4. Apr 25, 2007 #3
    I checked the answer in the book. It seems that they ignore the velocity of the wind because the answers to (a) - (d) are all f. I wonder if this was a trick question.
  5. Apr 25, 2007 #4
    Yea, the advection for wind + c is fairly minimal, dissipation is not.
  6. Apr 25, 2007 #5
    What do you mean by "advection for wind + c"? I don't get it. If I'm ignoring the wind, would the answers to (e) and (f) be the same?
  7. Apr 25, 2007 #6
    No, the first four parts assume (or not)a vector addition of wind and c, whereas e,f are based on the motion of the observor.
    Last edited: Apr 25, 2007
  8. Apr 25, 2007 #7
    You keep referring to c. What is c? Also, the cyclist is moving toward the whistle so it doesn't matter if he is moving towards it from the south or from the west.
  9. Apr 28, 2007 #8
    c is the speed of sound. a-d involve at rest bodies, while e,f are at motion relative to the whistle.
  10. Dec 9, 2009 #9
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