# How intensity and amplitude depends from a distance r

1. Nov 8, 2009

### AndreAo

1. The problem statement, all variables and given/known data
A line source of sound (for instance, a noisy freight train on a straight track) emits a cylindrical expanding sound wave. Assuming that the air absorbs no energy, find how (a) the intensity I and (b) the amplitude sm of the wave depends on the perpendicular distance r from the source.

2. Relevant equations
Intensity = Power/Area

3. The attempt at a solution
I thought that the area would be something like the base of a circular cone, and the area of it base would A $$\propto$$ r². Then I $$\propto$$ r⁻². Is the reasoning right?