# Homework Help: Difference in radius, same power and intensity?

1. Oct 9, 2011

### SPcle

1. The problem statement, all variables and given/known data
How much acoustic power propagates through a spherical surface (sound source in centre) with radius equal to 1.0m? with radius equal to 5.0m?

I = 10^-2 W/m^2
r = 1.0m or 5.0m
f = 50kHz
t = 2.0*10^-3s
IL = 100dB

2. Relevant equations
I = P/4∏r^2

3. The attempt at a solution
i figured out that when r = 1.0, power must be 1.3*10^-1W. However, when I work out the intensity when r = 5.0, i get:

10^-2 = P/4*∏*25
(10^-2)(4*∏*25) = P
3.1W = P

However, the answer states that the power for both spheres is 0.12W... What concept am I not understanding?

2. Oct 9, 2011

### ehild

It is the same sound source which produces I = 10^-2 W/m^2 at unit distance, so its power is 0.13 W. The same power propagates through any concentric spherical surface, with less and less intensity.

ehild

3. Oct 9, 2011

### SPcle

ooh, i see. so basically the power stays the same because it's the same sound source (a bat, in this case), but the intensity becomes a lot less in distance. i understand now, thanks!

4. Oct 9, 2011

### ehild

You understood it quick You are welcome.

ehild