Calculate Intensity of Sound Wave at Cliff from Radio Source

[kokenwa]

Homework Statement

Michelle is enjoying a picnic across the valley from a cliff. She is playing music on her radio (assume it to be an isotropic source) and notices an echo from the cliff. She claps her hands and the echo takes 1.2 s to return.
(a) Given that the speed of sound in air is 343 m/s on that day, how far away is the cliff?
-i got the correct answer for this one, and it was 206m


(b) If the intensity of the music 1.0 m from the radio is 1.0 10-5 W/m2, what is the intensity of the music arriving at the cliff?


i can't find the answer for b. can someone help me out?

 

[tyco05]

Intensity is proportional to 1/r^2.

That is, if the source is twice as far away, the intensity is 1/4; three times as far away, 1/9 etc...
So if the source is 206 times as far away...?

 

[kokenwa]

i put in 2.35 x 10^-5 and it was still wrong

 

[tyco05]

how did you get 2.35 x 10^-5 ? - show your working.

this is not what I got.