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Homework Help: Wave Optics of sound

  1. Feb 3, 2009 #1
    Wave Optics!!!!!

    1. The problem statement, all variables and given/known data
    Sound with frequency 1240 Hz leaves a room through a doorway with a width of 1.19 m. At what minimum angle relative to the centerline perpendicular to the doorway will someone outside the room hear no sound? Use 344 m/s for the speed of sound in air and assume that the source and listener are both far enough from the doorway for Fraunhofer diffraction to apply. You can ignore effects of reflections.

    2. Relevant equations

    λf = v
    θ = (m+.5)λ / d (because no sound is heard and therefore it's destructive?)

    3. The attempt at a solution

    First I use the first equation to find λ. v/f = λ and therefore 344 m/s / 1240 Hz = .277419 m
    Then I set m = 0 and used the second equation to find the angle:
    θ = (m+.5)λ / d
    θ = (.5 * .277419) / 1.19 m
    I found that the angle was .1165 radians, but I know this is wrong. Help!!
  2. jcsd
  3. Feb 4, 2009 #2


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    Re: Wave Optics!!!!!

    What is the condition for a minimum in Fraunhofer diffraction?
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