REAL subwoofers!

By Stig Erik Tangen
20 Mar 1996

Box

Box front view Fig. 3. Box front view.
Box side view Fig. 4. Box side view.
Box top view Fig. 5. Box top view.

Parts list, all 19 mm HDF
Side walls: 957 x 638 mm, 2 pcs
Top and bottom: 957 x 510 mm, 2 pcs
Inner port wall: 866 x 600 mm, 1 pcs
Inner horizontal: 491 x 800 mm, 1 pcs
Inner verticals: 290,5 x 800 mm, 2 pcs
Driver support piece: 290,5 x 80 mm, 2 pcs
Spacers inside port: 53 x 938 mm, 2 pcs

I recommend building the boxes in 19 mm High Density Fiberboard (HDF). There is no point in building them more rigid than shown in my original plans, at least if you have no intentions of crossing higher than 80 Hz. Low frequencies have too long wavelengths to put the box walls or the air inside the box into resonance. The lowest modal resonance inside the box will be at about 190 Hz. A high crossover frequency will need a more resonance-free box, since these higher frequencies really can make the box walls sing! With exclusive sub-woofer use though (a low crossover frequency), the vibration level of this box is remarkably low. High mass is very important to keep the box stand still at high levels (spikes don't help much). Built in 19 mm HDF the original box weighs about 75 kg. This is less than optimum, so you should consider placing some heavy object on the top of the box to increase its weight. I've tripled the weight of my own subwoofers by adding 150 kg of amplifiers on top. This really a had good effect.

This box design reduces nonsymmetric acoustic load on the drivers' cones. This is done by mounting them at the end of a long symmetric tunnel, which to some extend protect them from the very powerful load from the port inside the box. The pressure which the port resonance creates inside the box is strongest at the port end. This pressure is distributed more uniform across the drivers' cones with this arrangement, which in turn makes the drivers operate in a more linear fashion.

A comment about the support piece for the drivers' magnets is also necessary. This piece is very important in order to get tight and uncolored bass. It 'drains' resonances from the driver's chassis, and makes it impossible for the driver's magnets (and chassis) to actually move the opposite direction of the voice coil, which it will without this support piece. It must be perfectly aligned so that it makes tight contact with the driver's magnet.

The box should be about 50% filled up with loosely packed fiberglass (building insulation). Leave free space near the end of the port. Do not obstruct the inner end of the port, nor the port itself.

From a modal resonance point of view, box damping should not be necessary, since modal resonances inside the box cannot exist below 190 Hz. However, fiberglass filling will increase the acoustical volume of the box slightly, and damp mechanical noises from the drivers. Sound coming from outside the box will also create audible reverberation inside the box if it's left undamped.

If you like to design your own box, please remember that a large port area is very important to minimize compression. The port area used in my box plans (about 300 sq. cm) is what I consider an absolute minimum. A large port area results in a very long port, so an alternative box design is restricted by the need for great port length. The port should not be folded or bent.

High quality speaker wire is as important in a subwoofer as everywhere else. Use good quality solid-core wires. Stay away from multi-stranded wire. I can happily recommend the Flatline cable. This is the most 'honest' cable I know, especially the bass is superb.

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