I've always been afraid to ask just what the heck "comb filtering" means. It's mentioned all the time whenever the center channel subject comes up. I've seen/read a couple papers on it, but still don't get it.
I think I've experienced it. Send a test signal sent through the main speakers by using a CD with these tracks on it. Walk slowly across the path of the mains holding a sound meter pointed at the front of the room where mains are at. The reading will vary up and down as you walk across, but will remain steady when you stop at any point.
