Rob - a word for the record about insulation. There's quite a bit to it.Common wisdom is that wood is 'better' than glas, etc. , which I believe oversimplifies the situation considerably. The various materials act differently, so even at equal cost, specific application criteria apply. I ran extensive insulation comparisons at Thiel in the 80s. We landed at pure wool felt on the back wall of midrange enclosures - nothing else comes close to how well that acts in controlling reflections and so forth. In fact, we called the various engineered foams, batts, etc. "wishful engineering" due to their relatively poor performance. When it comes to filling cavities, polyester fiber-fill of various densities and fiberglas are the front runners. Wool has a bad habit of 'unloading' as frequency drops, which is contrary to what is often optimum. Polyester falls somewhere in the middle. Fiberglas has enough crooks and nodes to stand up to bass resonances without squirming. It outperforms the rest in my experience. That opinion is shared by some other designers of note.
Big problems with 'glas include irritating dust and a sharp hand. But wait! all 'glas' is not created equal. Thiel's 'glas is not builder's fiberglas insulation, it is a clean, graded product for industrial use. I got our fiberglas certified to our insurer's satisfaction.
If you want to compare different kinds of stuffing, I suggest measuring the outcome with REW, etc. to make sure you aren't compromising performance. Your correct dose will depend on outcome, not on comparative density, mass or volume. Most of the non-'glas products are likely to unload the bass to produce a lower Q with less flat response. Some folks might like that. Jim's goals were toward flat, uncolored bass extension with a 0.7 damping factor, which he considered the ideal. Some folks might prefer a fuller, looser bass from other materials. Let us know what you learn.
