when should u use polyfill?

I use polyfill in sealed boxes - about one pound per cubic foot of enclosure space.  Polyfill works by absorbing some of the pressure waves from the sub and converting that energy to heat.  This makes the box "appear" bigger to the sub.  If you have room in your install to make the box as big as you want, you probably don't need polyfill.  If you use it, do what Rob suggested and pack it in very lightly, like tufts of cotton candy.  You want all the fibers loose and free to vibrate.  It won't work if the stuff is clumped back in one corner of the box.

Scott Gardner

usually polyfill is used only when your enclosure it too small for the sub. polyfill can create the illusion that the box is up to 20% larger. if your box is only a cuft and you need it to be 1.25 cuft then you would add polyfill. if your box meets the requirements of the sub then i would advise against using polyfill in the box.