How Much Energy Hits Each Eardrum from a 2500W Sound Source 20m Away?

[Potatochip911]

Homework Statement

Standing 20m away from a tower generating 2500W of power how much energy/s hits each of your eardrums?
Surface area of eardrum=55mm^2
Assume that the speaker generates sound uniformly in a forward hemisphere.

Homework Equations

I=P/(4πr^2)
r=20m
P=2500W

The Attempt at a Solution

I=2500/(4π*20^2)=0.497W/m^2
Surface area of eardrum=55mm^2=55*10^-6m^2
0.497W/m^2*(55*10^-6)m^2=2.7*10^-5W
The actual answer is 5.5*10^-5W, not sure what went wrong.

 

[TSny]

Note that the sound spreads over a hemisphere (not a sphere).

 

[Potatochip911]

Note that the sound spreads over a hemisphere (not a sphere).

Oh that's sneaky, I'll have to pay more attention to that stuff in the future.