As a wave propagates, 

For a plane wave, intensity (energy crossing per unit area per time) is constant at all points. But for a spherical wave, intensity at a distance r form a point source of power (P), energy transmitted per unit area per unit time is given by 
 $I=\frac{P}{4\pi r^2}$
 $\Rightarrow I\alpha \frac{1}{r^2}$

But the total intensity of  the spherical wave over the spherical surface centered at the source remains constant at all times.
For line source $I\alpha \frac{1}{r}$