After doing some reading.... it seems standard polyfill would work fine at 1lb/ft^3. So that would mean I would use 4 lbs. However, it seems like my box is on the larger side of recommended specs. Isn't polyfill used to make the sub think it's in a larger box and eliminate reflection inside the box? Perhaps a more specific amount would be suggested.
If you are going to use just Polyfill, then fill the whole cavity just to the point where there is no compression. You have to fill a sealed speaker, or it sounds bad. Do not compress the fill, but completely fill the space.
Now the Polyfill will damp the enclosure, but won't do much to stop reflections, which is why I suggested lining the walls with mineral wool (Rockwool).
You can buy the Polyfill at Walmart, it is very cheap.
When I last bought it, the brand they had was Morning Glory, Premium Polyester Fiberfil. It is in the crafts department, and sold to crafters who make there own bedding etc.
When I built
these speakers, it took a huge quantity, and I bought it by the cart full over several days. On my last trip, when the bins of the stuff were getting low, I was approached by three members of store security in uniform. They said, "we have been watching you, and have noted you have bought an awful lot of this. Could you please tell us what you are doing with it? To which I replied: - "I'm using to to damp aperiodically damped transmission lines." That of course sent them into a right tizzy. I'm convinced they thought it was some kind of terrorist plot!
