I think it's more than one topic. There are lots of ways to deaden a speaker cabinet, but that's not really what the fiberglass is used for. You get more apparent enclosure volume with a fiberglass lined enclosure, so some of the volume you lose to drivers and braces and such can be reclaimed. It's not really "stuffing" like a sealed cab, and it's not really "deadening" like Dynamat. There used to be threads or links or something that was more easily located around here that spelled all this out, but I'm not finding it.