Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Sound and power along the xaxis

  1. Sep 11, 2008 #1
    Two sources of sound are located on the x axis, and each unit emits power uniformly in all directions. There are no reflections. One source is positioned at the origin and the other at x = +164 m. The source at the origin emits four times as much power as the other source. (a) At which location between the two sources on the x axis are the two sounds equal in intensity? (b) At which location to the right of the source at 164 m on the x axis are the two sounds equal in intensity? Describe the locations by giving the distance from the origin.

    I attempted to solve this multiple times and it is incorrect each and every time.

    I made the origin I1, followed by the +164 I2, and I3 the location to the right

    I set I1 and I2 equal to each other by

    P1/4pi(164 + d)squared = P2/4pi(164 + d)squared

    P2/P1= dsquared/ (164+d)squared

    4P2/P1= dsquared/ (164+d)squared

    1/4= (164-d)squared/ dsquared


    164 + d/d= +-1/2

    i solved and got d equal to -290 and -96.667 and these were wrongg so i dont know what i did
  2. jcsd
  3. Sep 11, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    I'm not sure I understand what you mean by this. If there are only two sources, why do you need an [tex]I_3[/tex]? What is the intensity at the point x, due to (a)the source at the origin (b) the source at x=164 (c)both sources combined?

    The method of setting [tex]I_1[/tex] and [tex]I_2[/tex] is correct, but your expressions for [tex]I_1[/tex] and [tex]I_2[/tex] are not. For [tex]I_1[/tex], is the distance from the origin to the point [tex]x=d[/tex] really [tex]164 +d[/tex]? Why wouldn't it just be [tex]{\Delta}x=d-0=d[/tex]? And for [tex]I_2[/tex] what is the distance from the point [tex]x=164[/tex] to the point [tex]x=d[/tex]?

    This doesn't look right, the proper way to simplify [tex]\frac{P_2}{P_1}[/tex] would be to substitute in the equation [tex]P_1=4P_2[/tex] as follows: [tex]\frac{P_2}{P_1}=\frac{P_2}{4P_2}=\frac{1}{4}[/tex]

    Once you correct these mistakes, you should get the right answer. Try again and if you still have problems post your new attempt and I will go over it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook