# Simple Power/Intensity problem

1. Feb 12, 2008

### DylanB

1. The problem statement, all variables and given/known data

A sound source emits uniformly in all directions. It produces a sound level of 80dB at a distance 30m. What is the power of the emitting source. (W)

2. Relevant equations

dB = 10log(I/Io)

Io = 10^(-12) W/m^2

I = P/A

3. The attempt at a solution

80dB = 10^(-4) W/m^2

sound disperses in a sphere..

I = P/(4*pi*r^2)

P = 10^(-4)*4*pi*(30)^2

= 1.13W

This answer is not listed, although multiples of it, 2.26, 3.39, 4.52 are choices.

2. Feb 12, 2008

### rootX

oo well, I think you did it right. I also get same thing =P
See the answers carefully , you will notice that a) is 13 and things on left side are erased, so it must be 1. on the left!

Anyways, good luck with the exam tomorrow!

P.S. funny how I also got stuck on the same question.

3. Feb 12, 2008

### DylanB

ah i see it. I found it odd that I was getting a wrong answer for the easiest question on the exam. thought maybe Rohan was playing tricks on me.

yeah good luck yourself (seems to be a good share of waterloo students here), no more bell curve anymore to take away the wrong answers!