Info on compound/isobaric woofers for DIYers?

Get a copy of Vance Dickason's book from Madisound or wherever, it has enough info to get you started. Re Twl's comments, actually the best way to do compound is to mount the woofers back to back (magnets towards each other), in which case the inner woofer has to be wired in reversed phase. Also strongly recommend sticking to a sealed box type, forget about vented.

The primary purpose of Isobarik bass loading is to achieve deeper bass response in a smaller than ideal cabinet by isolating the radiating woofer from the air "spring" in the box by a second driven woofer mounted in a sealed chamber behind it. This can be effective, but there are a few concerns. First, the woofers must be well matched. Next, the parallel connection requires an amp that can drive a low impedance load, or you must use a pair of higher impedance(16ohm) drivers. Third, you still must use a box size that will not significantly impair the cone movement of the rear driver. The drivers must be wired in-phase. This method has been used to good effect in the Linn Isobarik line of loudspeakers. More info is available on the Speakerasylum web forum.