Sound Intensity and Sound Level

[leinad0213]

Homework Statement

The intensity of the sound from a certain source is measured at two points along a line from the source. The points are separated by 11.8 m, the sound level is 68.10 dB at the first point and 60.31 dB at the second point. How far is the source from the first point?

d = 11.8m
SL1 = 68.1dB
SL2 = 60.31dB

Homework Equations

SL = 10log(I/Io)
I = P/(4pi*r^2)

The Attempt at a Solution

Find I(1) and I(2)

I(1) = Io*10^(68.1/10) = 6.46*10^-6 = P/(4pi*r1^2)
I(2) = Io*10^(60.31/10) = 1.07*10^-6 = P/(4pi*(r1+d)^2)

So I set the two equations to I(1)/I(2) = (r1+d)^2/r1^2 = 6.04

When I solve for r1, the answer turns out wrong. I am fairly certain this is the approach I am supposed to take.

 

[Spinnor]

What value do you get for r?