quoted from www.subbox.net ..."It is advisable to put damping material inside a subwoofer box. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. This increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. Polyfill also "fools" a sub into thinking it is in a bigger box. Play around with different amounts of polyfill until you get the desired results."

and here's a link to another discussion about it.

it's usually recommended to fill half the box with polyfill. i did it in my old car and it did sound better with the polyfill.

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