[Ethan Winer]

Steve,

I suspect you'll be better off with four inch thick rigid fiberglass, two feet wide, and without a front panel or sealed enclosure.

When you add a membrane and seal the system it becomes a resonant trap that works over a narrower range of frequencies. Yes, you can target a lower center frequency this way, but at the expense of not absorbing all the other low frequencies that must be tamed too.

--Ethan