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Can't understand the question of a sound problem.

  1. Jan 29, 2017 #1
    1. The problem statement, all variables and given/known data
    Two kids are positioned as in the following figure and A emits a long sound. d=100m, x=40m, vsound=340m/s
    L53Qbq4.jpg
    How long (t) does the sound emitted by A has to be such that B perceives a signal that lasts 2t?


    2. Relevant equations


    3. The attempt at a solution

    Seems like an easy problem. I don't understand what do we mean by "long sound", and what does it mean that a sound lasts t seconds. Does this mean that A can hear the sound for t seconds and B has to hear the sound for 2t seconds?
     
  2. jcsd
  3. Jan 29, 2017 #2

    gneill

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    Hi kadesh, Welcome to Physics Forums!

    By a "long sound" it means a constant sound that is emitted for some time t. Imagine that A has, say, a musical instrument such as a trumpet and he blows a note for a period lasting t seconds, then stops.
     
  4. Jan 29, 2017 #3

    BvU

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    Bad translation or bad rendering of the problem statement ?: there is no t in there, so 2t is meaningless.
     
  5. Jan 29, 2017 #4
    t is the duration in seconds of the sound emitted by A. We need to find that. I just named it t such that we can easily talk about it. Sorry about the misunderstanding.
     
  6. Jan 29, 2017 #5

    Nidum

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    Is this a primary signal plus echo signal problem ?
     
  7. Jan 29, 2017 #6
    Thank you for your greetings! Well, then if B hears the sound for 2t seconds then it hears the original sound for t seconds and the echo for t seconds?
     
  8. Jan 29, 2017 #7
    Yes, indeed. My cousin asked me to help her with it. It's been some time since I solved this kind of problems and I forgot how to interpret them.
     
  9. Jan 29, 2017 #8

    BvU

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    That would be a good approach. Make a sketch: time horizontally and vertically the yes/no (or 1, 0) of B hearing the sound.
     
  10. Jan 29, 2017 #9

    gneill

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    That would depend upon the geometry. B should start hearing sound first directly from A, but later from both A and the reflection from the wall. When A stops making sound, B will still be hearing sound from the wall for some time. The question is, how long should A keep making sound (then stop) in order for B to hear sound for twice as long as A made sound?

    You would do well to make some sketches showing the sound's progress over time.
     
  11. Jan 29, 2017 #10
    Thanks again for your answer but you kind of lost me there... The graph shouldn't be a horizontal line (y=1) from 0 to 2t?
     
  12. Jan 29, 2017 #11

    gneill

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    Make a few sketches of the setup and superimpose a wave, or block, or arrow, or what ever your choice is to represent the progress of the sound signal. Here's my version for when the sound first reaches B from A:

    upload_2017-1-29_9-30-3.png
     
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