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Homework Help: Mechanical waves and sound

  1. Nov 18, 2007 #1
    1. The problem statement, all variables and given/known data
    A sound source is located somewhere along the x-axis. Experiments show that the same wave front simultaneously reaches listeners at x = -7.0 m and x = 3 m

    a) What is the x-coordinate of the source?
    b) A third listener is positioned along the positive y-axis. What is her y-coordinate if the same wave front reaches her at the same instant it does the first two listeners?

    2. Relevant equations

    Change in phase = 2pi ( change in x) / lamda

    3. The attempt at a solution
    For a, since its the same wave front, i just took the mid point. I'm not sure if this is correct.

    For b. I know that if i can find the wavelength. I can find the velocity of the wave and since the time is the same i can use it to find the distance.

    But the problem is, i don't know how to find the wavelength.

    The above equation change in phase = 2pi ( change in x ) / lamda

    This equation gives me troubles. I don't have the change in phase, and i don't know lamda. SO how do i solve this? Also i don't know if the change in x is -7 - 3 = -10 or 3 - 7 = -4

    Please help
  2. jcsd
  3. Nov 18, 2007 #2
  4. Nov 18, 2007 #3
    Nope, my text book does a much better job than wikipedia, thanks tho
  5. Jul 13, 2008 #4
    i dun get it
  6. Jul 13, 2008 #5


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    Gold Member

    I'm assuming the source sends out a spherical wave, but you only have to the circular portion as it expands from the source form a bird's eye view. This means you must fit the two points to a circle.

    NOTE: reviewing your problem, it's 1d, not 2d, so you're solution is likely correct. It's similar to if you measured opposing points on that circular radius, ignoring the 2nd dimension of the circle.

    the velocity of sound waves is a constant through an atmosphere of uniform temperature (which you can also assume since you're in the bird's-eye view, ignoring vertical components). Maybe you're allowed to use that constant.
  7. Jul 13, 2008 #6


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    Homework Helper

    Hi Grapz,

    You don't need the wavelength; you can already find the distance. The problem says the two people on the x axis receive the same wavefront at the same time. When you used the midpoint to answer part a, what were you assuming about their distances from the speaker?

    If the person on the y axis has to receive the same wavefront, how far does he have to be from the speaker?
  8. Jul 14, 2008 #7
    you are assuming that the people are equal distance away from the source right? so that's 5 metres from the source.. so why isn't the distance on the y axis also 5 metres?
  9. Jan 11, 2009 #8
    because the third person is on the y-axis....so his/her x coordinate is 0....since this person is not directly above the sound source he/she is not 5 meters away
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