Rochey, NC-25 is PDQ (Pretty Damn Quiet) - the whole point of THX certification is so schlockers can't pollute the experience. If you follow links to equipment, note the brands/models that are certified. See any Radio Shack PA systems listed? Me too...
I don't have access to about half my acoustics library at the moment, so can't give an exact spec for NC-25, but picture this - go to a mausoleum around midnight, preferably one in a remote part of tibet - put on earplugs, stop breathing, slow your heartrate to 3 BPM... OK, just kidding about the heartrate, and actually NC-15 is doable with the techniques John uses ASSUMING you are a really ANAL guy (NO SHORTCUTS) with a lot of time on his hands and a good budget... Steve