Designing the enclosure was easy to do, by recycling previously used ideas of mine. My current main system have enclosures made of sandwiched walls with internal bracing in addition. The sandwich consists of three layers of HDF boards, filled with sand inside. The sand filling is done by making a large cut-out in the middle HDF board, and filling the cavity with sand through a small hole, after the three boards are glued together. An enclosure build this way is very heavy and extremely free from coloration. I decided to build all enclosure walls of sandwich if possible, and use heavy internal bracing that should support the driver frame as well. Besides this, the enclosure shares design ideas with my previous subwoofer projects. I ended up with a sandwich made from three layers of 12 mm HDF. This construction is a good compromise between acceptable enclosure weight and low coloration. The internal bracing was done with 19 mm HDF.

 The resulting enclosures are 800x700x602 mm (depth/height/width) and weighs about 160 kg each (excluding the driver).

Their performance is very satisfying; there are virtually no signs of enclosure vibration!

The image shown here is is a 3-D drawing of the enclosure, with one side removed. The driver's magnet is supported by the internal bracing. The shaded parts are sandwiched boards, and the rest 19 mm HDF. All parts are glued together, I never use screws.

All parts maked grey on the 3D image of the cabinet above are 'sandwiches' made of 3 layers of 12 mm HDF, the middle filled with sand. The easiest way to do this is to use three equally sized boards, and make a large cut-out in the middle board. I made a square cut-out leaving a 70 mm wide rim. The open space inside should be supported by spacers to avoid warping. All other boards are 19 mm HDF.

The vertical braces inside the enclosure are mode of one piece of HDF, the horizontals are three separate pieces. The spacers inside the port runs all the way back to the rear wall (this is hidden on the 3D image), and so do the braces. The front baffle consists of two layers of 19 mm HDF.

The bracing just behind the driver must be lined with a heavy asphalt damping material, like the type used for resonance damping in automobiles. This provides a semi-elastic connection between the driver and the bracing . This reduces possible vibrations in the driver’s frame, and certainly contributes to less sonic coloration.

It is, as always, necessary to use acoustic damping inside the enclosure. I've covered the side walls, top and bottom (the port, really) and the top 3/4 of the back with 75 mm fiberglass (building insulation). It is important to avoid obstructing the port. Damping of the port must not be done, even though the very long port has strong resonant modes at 250 Hz and above. Since this is safely above the crossover frequency, it does not cause any audible problems. The port area is also large enough to avoid turbulence noises at normal listening levels.

Mounting screws are included with the JBL drivers, but I do strongly recommend using hexagon or torx screws instead. The JBL-supplied screws are simply too weak. Not one of my screws survived mounting the driver!

Parts list for one subwoofer: