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No clue where to start

  1. Nov 27, 2004 #1
    Hey,

    My question:

    Two loudspeakers face each other, vibrate in phase, and produce identical 440Hz tones. A listener walks from one speaker toward the other at a constant speed and hears the loudness change (loud-soft-lound) at a frequency of 3.0 Hz. The speed of scound is 343m/s What is the walking speed?

    Ok, where do I start?

    I can find the wavelength with 440Hz, and speed of sound.
    I'm guessing V=d/t? to find the walking speed. Help! :yuck:
     
  2. jcsd
  3. Nov 27, 2004 #2

    Astronuc

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    Staff: Mentor

    If the two vibrations are in phase, they are adding or reinforcing each other.

    What happens when an observer moves toward or away from an acoustic source? Doppler shift.
     
  4. Nov 27, 2004 #3
    See, the observer is moving from one speaker to another, so it's moving away from a source to a source. Is the frequency observed 3Hz, or is that the beat frequency?
     
  5. Nov 27, 2004 #4
    :uhh: Astronuc..? Still confused here lol.
     
  6. Nov 28, 2004 #5
    The doppler effect can't work here?
     
  7. Nov 28, 2004 #6

    Doc Al

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    Consider that the sound waves from each speaker will interfere with each other, creating a standing wave pattern. What's the spacing between the anti-nodes? (Hint: anti-nodes occur where the waves constructively interfere. For example, right in the middle between the speakers will be a spot where the waves are in phase.)
     
  8. Nov 28, 2004 #7
    hmm, let me see if I can get where you're going. The distance between the two speakers, let's say L. I want to find L do I can see what distance the man will walk, then find velocity, but I need time. Ok, I can find L from L=v/2f?
     
  9. Nov 28, 2004 #8

    Doc Al

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    The distance between speakers is irrelevant. Find the distance between the points of constructive interference. (Hint: That distance is related to the wavelength of the sound.)
     
  10. Nov 28, 2004 #9
    The spacing between the antindoes? isn't that dependent on the harmonics? if it was the 1st it would lambda/2, if it was the 2nd, then lambda? Wait, that's for nodes, for the 2nd the distance is lambda/2.
     
  11. Nov 28, 2004 #10
    Another thing(s): it says (loud-soft-lound), doesn't that mean when loud, we get constructive, soft -destructive. So wouldn't there be a node in the middle? I don't see how finding this will lead to the man's velocity, not yet anyway.
     
  12. Nov 28, 2004 #11

    Doc Al

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    Yes. The anti-nodes are separated by nodes.
    If you know the distance between anti-nodes (or nodes) and the man's walking speed you can figure out the frequency of loudness changes that he hears. For example: if the anti-nodes were 2 feet apart, and the man walked 2 feet per second, then he'd hear the sound alternate from loud-to soft-to loud every second: 1 Hz.
     
  13. Nov 28, 2004 #12
    But we dont' know the man's walking speed. Wait, so let's say that the distance between the antinodes were lambda/2. So to find lambda, lambda=speed of sound/frequency. So 343m/s / 440hz. ok so 0.77m/2 as distance between antinodes is lambda/2. I get 0.40m. He hears loudsoftloud at 3hz which is 0.33s. V=d/t, that would be .40m/0.33s, I get 1.20 m/s?
     
  14. Nov 28, 2004 #13

    Doc Al

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    Sounds good to me.
     
  15. Nov 28, 2004 #14
    Yes, Thanks!
     
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