whats the purpose of putting foam inside the box with your subwoofers?

i sell car audio at circuit city and ive noticed that the kicker boxes have foam inside them and the rest of them like sony and mtx dont have foam inside

Pollyfil is a sound absorption, dampening fiber that will deepen bass, provide cleaner midrange, tone down any unwanted reverberation and improve the dynamic extension of any speaker system.

It also helps eliminate standing waves, which is something like an echo inside the box. This will give you tighter and more accurate bass. It also helps to give a flatter frequency response to the overall sound, which is good. Besides this, it also helps reduce the vibration of the box, which results in less distortion. There is a big difference to the bass when you have an insulated box.

Damping also increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. It is advisable to put damping material inside a box. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. Polyfill also “tricks” a sub into thinking it is in a bigger box. Most sealed and vented enclosures require 1/2 lb. of dampening material per cubic foot of enclosure. For best results it is recommended to loosely fill the material throughout the enclosure.

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