Understanding the Amplitude Difference in Spherical and Plane Waves

[kasse]

Why is it that the amplitude of an harmonic spherical wave decreases with increasing r, while this is not the case for an harmonic plane wave?

 

[-Vitaly-]

I don't know what those are, but maybe the spherical wave spreads its energy in all directions, while the plane wave conserves the energy by moving in 1 direction.

 

[PullMeOut]

That's right spherical waves are moving in all directions as we define it spherical coordinates.But plane waves like sound are move in 1 direction and go towards that direciton.