NO fiberglass in a speaker with open vents.
Acoustic foam can work very well, especially since you can easily adhere it within the cabinet and vary the density through-out it. I would suggest covering the entire inside of the cabinet with a layer of low density foam and then stuffing the open chamber in the top end of the cabinet using the same material. That is, if the port is located in the bottom section.
From there, you can vary the amount and density of stuffing within the open area between the woofer and the port. Don't go crazy here as all you want is just enough to very loosely fill the open space.
Not only will this provide what is probably a far more natural sound, you may actually experience a slight increase in bass extension. If you find the presentation to be lacking mid-bass ( warmth ), reduce the amount of foam between the woofer and ports. Most people will tell you not to block the area between the ports and the woofer at all, but that is because they don't know what good sound is and are used to high levels of sonic colouration. By placing a small volume of low density foam between the woofer and the port, you minimize the contribution of the port above its' tuned frequency. In effect, the port works as a port and not as a "leaky hole" radiating sound above the frequency it was designed to work at.
As a side note, carpet pad can work, but there are quite a few different types and densities available. This can be really hit or miss, depending on the materials and densities used. Dacron, polyester fiberfill, "acousta-stuff", etc... can all be used, but low density foam is by far the easiest to work with. Sean
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