Have you ever completed an enclosure that was a touch too small? Your low end sounds cramped but you don't want to or can't build a brand new enclosure. You throw in some polyfill and are blown away by the improvements. Then you go online to talk about it and everything gets confusing. You don't know why it works or what it really did, but you know that it sounds better. Maybe it's time you learned more about it! Firstly, there are a few primary types of fill that are used. This includes p