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Almighty Subwoofers
By Stig Erik Tangen
01 Jan 1997
Enclosure
My enclosure requirements were easy to accomplish, by recycling previously used ideas. 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 of colouration. I decided to build all enclosure walls of sandwich if possible, and use heavy internal bracing that should support the driver as well. Besides this, the enclosure shares design ideas with my previous subwoofer project. I ended up with a sandwich consisting of three layers of 12 mm HDF. This construction is a good comprimise between acceptable enclosure weight and low colouration. The internal bracing was done with 19 mm HDF. The resulting enclosures are 80x70x60 cm (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 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, I never use screws!
It is 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!
I took some photos during the building process. Links to the images follow below. The images may give a better idea of what the enclosures looks like. The photos are taken with a Kodak Fun Gold camera with flash (camera and film in one piece, REAL Crap!), and scanned on a far from high-end scanner, and then saved with 30% JPEG compression to reduce the file size. Needless to say, the quality could have been better.....
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