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Diffraction or Distraction?

  1. Nov 28, 2006 #1
    Diffraction or Distraction??

    A row of seats is parallel to a stage at a distance of 8.1 m from it. At the center and front of the stage is a diffraction horn loudspeaker. This speaker sends out its sound through an opening that is like a small doorway with a width D of 0.072 m. The speaker is playing a tone that has a frequency of 2.0 104 Hz. The speed of sound is 343 m/s. What is the separation between two seats, located near the center of the row, at which the tone cannot be heard?
     
  2. jcsd
  3. Nov 28, 2006 #2

    berkeman

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    Staff: Mentor

    You need to show your work in order for us to help you.
     
  4. Nov 28, 2006 #3
    Oops, I forgot to add that ... LOL ...

    D = 0.072m (given)
    f = 2.0 X 10^4 Hz or 20,000 Hz
    v = 343 m/s (given)

    ... I know for the second part ... sin (theta) = (lambda / D)

    ... solving for lambda ... lambda = velocity / frequency ...

    ... lambda = 343 / 20000

    ... therefore: lambda = .01715m


    ... this is where I'm stuck.
     
  5. Nov 28, 2006 #4
    Am I missing an equation?
     
  6. Nov 29, 2006 #5
    If the wavelength of a sound is bigger than the person/object/insect/whatever it is hitting, the sound won't be heard/clear. Also if there is not enough room for the wavelength to pass through, it will not work either (diffraction), apply this concept to your problem.
     
    Last edited: Nov 29, 2006
  7. Nov 29, 2006 #6
    I don't understand ... I was reading from my Cutnell and Johnson 7th edition physics book - but, I couldn't understand the diffraction concept ... can you elaborate?
     
  8. Nov 29, 2006 #7
    Last edited: Nov 29, 2006
  9. Nov 29, 2006 #8
  10. Nov 29, 2006 #9
    I went to the Hyperphysics website ... and got the jist of it ... but, it's not clear to me yet ... any other suggestions or walkthroughs?
     
  11. Nov 29, 2006 #10
    Read those pages over and over until you get it, alternatively you can search on google for "diffraction". I think that between your textbook and the two websites you should have enough information to grasp the concept. What exactly don't you understand about it?
     
  12. Nov 29, 2006 #11
    I'm more of a "hands-on" math concept guy ... rotational kinematics and torques killed me last chapter ... I don't know what equations to use to set up a problem. Make sense?
     
  13. Nov 29, 2006 #12
    I solved for lambda ...

    ... velocity = (lambda * frequency)

    ... solving for lambda ...

    ... lambda = (velocity / frequency) ...

    ... lambda = ( 343 / 2e4 )

    ... lambda = .01715m
     
  14. Nov 29, 2006 #13
    I know ...

    ... sin of theta = lambda / D

    ... therefore, I concluded with ...

    ... sin of theta = ( .01715 / .072 )

    ... sin of theta = .238 ...

    ... but, I'm not trying to find an angle ... am I?
     
  15. Nov 30, 2006 #14
    It doesn't seem that I'm looking for an angle ... does anyone else have an idea?
     
  16. Nov 30, 2006 #15
    Is there an equation to calculate diffraction?
     
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