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Homework Help: Reflection and transmission of waves

  1. Apr 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A river canyon is 147 m across as shown in Figure P.32. You are located on the river surface and clap your hands once to generate a brief acoustic pulse. You detect a time interval of 0.407 s between the twin echoes recorded on tape, analyzed when back home (?) in a physics lab. Where were you relative to the canyon walls? (Neglect multiple echoes.)

    Answer is m (from the closer wall)

    2. Relevant equations

    Not sure
    3. The attempt at a solution
    not sure
  2. jcsd
  3. Apr 7, 2009 #2


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    Hi begbeg42! :wink:

    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
  4. Apr 7, 2009 #3
    is this problem anything like where you drop a rock into a well and hear the sound ___ sec. later and have to find the depth? I know how to do this....but this problem confuses me b/c it involves horizontal distances...

    I know the speed of sound in 20 degree Celsius air is 343 m/s
    I'm not sure physically/intuitively what .407 sec. actually means? is it the time it takes you to hear the echo after you clap?

    im utterly baffled. and yes I have read about reflections of sound/echoes etc.
    sorry...and any guidance is appreciated
  5. Apr 7, 2009 #4


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    Horizontal and vertical are the same …

    but in this case you have two sounds, the .407 s is the gap between the two echoes :smile:
  6. Apr 7, 2009 #5
    t1(sound to travel to wall) + t1(sound to return to you)=a=2t(sound to travel to wall)
    t2(sound to travel to wall) + t2(sound to return to you)=b=2t(sound to travel to wall)
    4t(sound to travel to wall)=0.407 sec?
    t(sound to travel to wall)=.407/4?

    d(to wall)=343(.407/4)=34.9...which seems plausible but

    i mean this seems too simple...how does the actual width of the canyon=147 m come into play?
    Last edited: Apr 7, 2009
  7. Apr 7, 2009 #6


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    Draw a diagram

    what is a + b?

    what is a - b? :smile:
  8. Apr 7, 2009 #7
    t(echo1) - t(echo2)=0.407
    t(echo1)=t(sound to travel to wall) + t(reflected sound to return to you)

    d(from wall)=343t(sound to travel to wall)

    does the sound hit the wall, return to you, and then bounce back or....how exactly is the second echo produced?
  9. Apr 7, 2009 #8


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    ah … you're not getting this canyon thing, are you?

    there are two walls … one echo comes from each wall.

    Try again! :smile:
  10. Apr 7, 2009 #9
    wow....sorry about that

    i got it anyways...took me awhile, though I dont know why

    thanks anyways!
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