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Sound Wave Problem

  1. May 6, 2016 #1
    1. The problem statement, all variables and given/known data
    A handclap on stage in an amphitheater sends out sound waves that scatter from terraces of width w = 0.967 m (see the figure). The sound returns to the stage as a periodic series of pulses, one from each terrace; the parade of pulses sounds like a played note. (a) Assuming that all the rays in the figure below are horizontal, find the frequency at which the pulses return (that is, the frequency of the perceived note). (b) If the width w of the terraces were smaller, would the frequency be higher or lower? (Note: Assume the speed of sound in air = 343 m/s.)

    2. Relevant equations
    S = Sm*cos(kx-vt) maybe?
    3. The attempt at a solution
    I am clueless for this question, can someone suggest an approach? I don't know what the theory id behind this
    Thanks
     

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    Last edited: May 6, 2016
  2. jcsd
  3. May 6, 2016 #2

    gneill

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    Staff: Mentor

    Sound from a single source is bouncing off of a series of surfaces whose distances from the source are separated by a regular fixed increment.

    What's moving? How fast does it move? What's the path length difference for sound returning from adjacent terraces? What then is the time difference between returning pulses from adjacent terraces? Knowing the time interval between return pulses, what's the frequency?

    Surely you can quote relevant equations that pertain to velocity, distance, and time? How about period and frequency?

    You should be able to make an attempt.
     
  4. May 6, 2016 #3
    The sound waves are moving at 343m/s, the time difference is T=w/343 = 0.967/343=0.002819...
    Hence the frequency is f=1/T=354.705274 Hz
    Does that look right?
     
  5. May 7, 2016 #4

    gneill

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    Staff: Mentor

    Almost. Take a close look at the extra distance traveled by the reflected waves from successive terraces. Remember, the waves are reflected so they travel the same path forward and back:
    upload_2016-5-7_5-42-30.png
     
  6. May 7, 2016 #5
    Oh right! Shame on me for not seeing that.
    It should be T=2w/343 = 2*0.967/343=0.0056384..
    and f=1/T=177.35 Hz
     
  7. May 7, 2016 #6

    gneill

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    Staff: Mentor

    That looks good.
     
  8. May 7, 2016 #7
    Thank you very much for your help!
     
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