[jacquesm]

Three dimensional speaker arrays are definitely possible, a simple one would be a cube, a bit harder would be a cylinder.

Yes, it is extremely hard. The question is though, can it be done? Phased arrays are insanely hard as well, they do work though.

In the 80's there was a free space noise cancelling demonstration on a cross roads in a major city somewhere, I can't seem to dig up any references to it. IIRC that was a completely analog setup, I have no idea how it worked but they had a volume within which background noise was substantially reduced. I can't stand it when I am 100% sure that I read something but I can't find it.