Why 90 deg phase angle between pressure and particle velocity for spherical waves

The_Lobster

My text says that as we approach a point source (of e.g. sound) from a distance, the phase angle between pressure and particle velocity will gradually shift from zero (in the far field/plane waves) to 90˚ (in the near field/spherical waves).

I can't find any intuitive explanation of why this is the case? Why do spherical waves have the phase shift, and plane waves don't, and vice versa?

Secondly, the text says that the particle velocity will increase with this increasing phase shift as we get closer to the source; but I have a feeling this might be explained by the above?

Thanks in advance! :)