The plate at the back of the muffler isnt really the baffle. USUALLY you will find the baffle to be a tube (and the various manufacturers of pipes each have their own design) with perforations in it. It goes into the megaphone, back to the collector with a disc there to force the exhaust gasses to pass thru it. As the gasses come down the perforated tube, they generally run into another plate that forces them out into the "resonation chamber" (which would be the can or megephone, whatever you want to call it) which is usually packed with some sort of heat/fire resistant material such as fibreglass or steel wool or asbestos, whatever. Then it passes where the plate stopped it, and goes back into the tube, and out the end of the pipe. Every pipe is set up differently, as i said. But this is the general idea behind it.
