answer:Egg crate foam. It’s good enough for soundproof rooms, so why not? Hell, I know someone who, on a tight budget yet in need of a way to muffle their music space, just put up a whole bunch of empty egg cartons. While not as good as the foam, the shape of the stuff broke up the sound and served to muffle it a little bit. Regular foam won’t “soak up” the sound well though. You need something to scatter/diffuse it, and that irregular “egg crate” shape is great for that.