I have a few walls that I would like to have as much acoustic isolation as possible. These walls will be done with staggered studs. I figure that dead air is good, but perhaps some open cell spray foam (or some other type of sprayed material would be better.)

I heard that I could put multiple applications of wallboard; this sounds expensive, but I guess I could for one particularly important location. (How exactly would that be done?)

This particularly important location is a toilet and urinal on the wall directly behind the den, and I don't want the "Archie Bunker syndrome". I will pay any price to make sure that a toilet flush is not heard by someone sitting next next to the wall in complete silence.

Anny ideas or suggestions?