🚀 go-pugleaf

Hi.  I know this is possible because I've heard the military does it for
helicopter headsets; and I heard that the car companies will be doing it
too.  Does someone know of how the basics work behind active noise can-
cellation?

What I'm interested in is a system where you have a speaker or two
somehow produce a signal that is directly out of phase with the ambient
noise.  This seems impossible, since a speaker radiates in all directions,
thereby having a different phase at different points in the room.  Is 
there some way of getting an average phase or something, in order to
decrease the noise in the room/car?

I'd like references to articles on this subject, along with a simple
explanation if possible.

Thanks