Electrical & Audio Guides
Basic Sub Enclosure Design Ideas
This page provides the basis for designing sub boxes, the measurements here may not be exact for your sub or you may wish to change the design of the box. Many subs will work well in a wide range of box volumes, however, people often try to build boxes that are too large thinking this will give them a better bass response. This is wrong and what actually happens is that you loose the tightness of the bass ie. the sub can flap about too much, and move in and out too far, this also can cause clipping where the voice coil of the sub hits the end of it's travel. Counter to that it is just as bad to put a sub in a box that is too small for it this results in loss of the lower frequencies due to a "choking" effect on the sub ie. it is unable to move far enough. Either way, using the wrong volume of box can damage the sub over time.
Below are some example box shapes with dimensions for the more common volume requirements. All the dimensions are external and allow for box wall thickness using 18mm MDF which is the ideal material to use. Each box volume has also been altered slightly to allow for an average 0.15 cu ft displacement for a 10 inch or 12 inch sub. A box interior volume tolerance of ± 5% is hardly ever noticeable so things like cross braces and connecting terminals can usually be ignored in all but the smallest boxes.
Square box
(all dimensions are external)
| Nominal Interior Volume (litres) |
Nominal Interior Volume (cubic feet) |
Height "h" (inches) |
Width "w" (inches) |
Depth "d" (inches) |
|---|---|---|---|---|
| 14.2 | 0.5 | 14 | 15 | 8 |
| 28.3 | 1.0 | 14 | 16 | 12 |
| 42.5 | 1.5 | 16 | 20 | 12 |
| 56.6 | 2.0 | 16 | 26 | 12 |
| 84.9 | 3.0 | 18 | 33 | 12 |
| 113.3 | 4.0 | 18 | 36 | 14 |
To convert from litres to cubic feet multiply the value in litres by 0.03532.
To convert from cubic feet to litres multiply the value in cubic feet by 28.134.
Wedge Box
(all dimensions are external)
| Nominal Interior Volume (litres) |
Nominal Interior Volume (cubic feet) |
Height "h1" (inches) |
Width "h2" (inches) |
Width "w" (inches) |
Depth "d1" (inches) |
Depth "d2" (inches) |
|---|---|---|---|---|---|---|
| 14.2 | 0.5 | 14 | 14.32 | 15 | 9.5 | 6.5 |
| 28.3 | 1 | 14 | 14.32 | 16 | 13.5 | 10.5 |
| 42.5 | 1.5 | 16 | 16.28 | 20 | 13.5 | 10.5 |
| 56.6 | 2 | 16 | 16.28 | 26 | 13.5 | 10.5 |
| 84.9 | 3 | 18 | 18.25 | 33 | 13.5 | 10.5 |
| 113.3 | 4 | 18 | 18.17 | 36 | 15.5 | 18 |
To convert from litres to cubic feet multiply the value in litres by 0.03532.
To convert from cubic feet to litres multiply the value in cubic feet by 28.134.
When building the box, 18mm MDF is the best material to use as it provides the rigidity needed for best acoustics. If the box is flexible then sound quality will be lost. Don't forget to cut a hole in the front panel for the sub and another in the back for the input terminals before starting to assemble the box!
The box also needs to be well sealed so use proper wood glue, liquid nails, or a similar gap filling adhesive, it's best to screw the parts together as well as gluing them, as the glue may not be strong enough to withstand the constant vibration caused by loud bass. The best screws to use are twin fast or chip board screws as these grip as much of the wood as possible, they need to be at least 1.5 inches long and countersunk. To finish the construction the inside needs to be sealed completely. This can be done using any ordinary sealant. Acrylic sealant is the best as it dries hard and so adds to the strength of the box. Some speaker manufacturers advise caulking the inside of the box, this is generally pointless unless you're really fussy. All caulking does is reduce standing waves inside the box and produce flatter bass, ie. more of a thud than a boom, this isn't so important in cars as the boot space usually alters the sound anyway (Fiesta boots are brilliant at emphasising bass!).
Once the box is together and all the glue and sealant has dried properly, check it's strength by carefully trying to bend it. The box shouldn't be able to flex in any way. It's not essential to cover the box but it is best to carpet it as this will provide a better seal around the sub and terminals when they're screwed on. An alternative to covering would be to use door frame draft excluder round the sub and terminals when fitting them as it's not going to be possible to seal round these on the inside!
Bass Ports:
If a sub needs a port on the box then the manufacturers data will say what size port is needed and they're quoted volume will be for a ported box. This just means that once the box is designed an extra hole will need to be made in the front for the port so go in. Most of the time it's better to use a sealed box with a sub as this produces tighter bass and unless it's a "free-air" sub then it's probably designed for a sealed box anyway, so will sound best when mounted in this way.
And that's it. One sub box designed and built. All it's needs now is fitting in the car. Unless it'll only fit in one place, it's best to move it around until it sounds the best and then fit it there.
To aid with the decision as to what type of box you need and what size it should be there are some freeware programs available. The AJ Sealed Designer uses the technical details of the sub to determine the optimum box volume for best sound reproduction, and aids with the design of a simple square box. The other program allows design of different shaped boxes and allows the addition of a bass port, giving the changes required when making a ported box.
AJ Audio Subwoofer Box Enclosure Design Software
