Sound dampening is important for any speaker enclosure. It does a few things for you. First it keeps sound from coming through the enclosure's walls. You want to hear sound from the drivers & bass ports only. Second it reduces standing waves from bouncing around inside the enclosure & distorting the sound. The internal bracing in this design also helps with that. Lastly the dampening material helps soften the bass so its not boomy. This deepens the audio producing a more pleasant sound.

I used heavy duty carpet underlay. This stuff is made of all sorts of different fibers & is very dense. I applied it to just about every internal surface except directly in front of the bass ports. In addition to that, I balled up a good amount of soft batting material & placed a wad directly behind each woofer. This helps further deepen the bass & is important especially if the speakers are going to be placed near a wall, which they were to be.