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Your driver’s spec sheet tells you how much air it wants behind it. This designer turns that number into a real box: outside dimensions, a cut list for every panel, a cutting layout for a sheet of MDF, and, for ported boxes, the port length for the tuning you want. It accounts for the wood, the driver, the port and any bracing, so the finished box has the volume you actually asked for.
- Choose sealed or ported, your driver size and how many drivers.
- Enter the net volume and displacement from the spec sheet. Typical values are filled in to get you started.
- Pick a shape, or choose “fit a space” and enter the height and depth you have, for example in a trunk.
- For a ported box, set the tuning frequency and port width. The port length is worked out for you.
How the designer works
Box volumes on spec sheets are net internal volumes: the air left once everything inside the box is accounted for. The designer starts from that, adds back the space taken by the driver, the port and any bracing to get the gross internal volume, then finds internal dimensions with that volume. Adding the wood thickness gives the outside size, and each panel is sized from there.
The driver sets some minimums: the baffle has to be big enough for the driver to fit, and the box has to be deeper than the driver’s mounting depth. If those minimums make the box bigger than your target, the designer tells you so you can add filler blocks.
For ported boxes, the port length comes from the same Helmholtz formula our port calculator uses. The port’s own volume counts against the box, which is why slot-ported boxes are often bigger than people expect.
Does box shape matter?
Less than you might think. At subwoofer frequencies, the sound waves are far longer than the box, so internal standing waves are rarely a problem. Shape matters mainly for three practical reasons: the box has to fit your space, it has to fit the driver, and big flat panels flex more than small ones. The golden-ratio option gives well-balanced proportions; “fit a space” is usually the right choice for cars.
Building tips
- Use ¾-inch (18 to 19 mm) MDF. It’s dense, cheap and easy to cut accurately. Plywood works too and is lighter and more water-resistant.
- Glue and screw every joint with a good wood glue like Titebond II, then run a bead of glue or silicone along the inside seams. A sub box has to be airtight.
- Brace big panels. Any panel wider than about 14 inches benefits from a brace or dowel between opposite walls. That’s what the bracing allowance is for.
- Cut the driver hole with a router and circle jig such as the Jasper 200. Use the cutout diameter from the driver’s spec sheet, not its nominal size.
- Seal everything: a terminal cup for the wire, and foam gasket tape under the driver’s frame.
- Measure your wood. Sheet goods sold as ¾ inch are often a little thinner. Enter the real thickness for an accurate cut list.
Frequently asked questions
How accurate does the box volume need to be?
Within about 5 to 10% of the spec sheet is fine. Small differences shift the response only slightly. Sealed boxes are especially forgiving, and polyester fill can make a slightly small sealed box behave as if it were bigger.
Should the port volume be included?
Yes. The air inside the port and the wood around it take up space that the driver can’t use, so they have to be added on top of the net volume. The designer does this automatically.
MDF or plywood?
MDF is the standard for home and most car boxes: it’s dense, flat and inexpensive. Good-quality plywood is lighter, stronger at the edges and handles moisture better, which can matter in a car. Either works if the box is braced and sealed.
Can I build the box bigger than recommended?
A little, yes. A larger sealed box gives slightly deeper, less punchy bass and lower power handling. A larger ported box lowers the tuning unless you lengthen the port, so re-run the numbers rather than guessing.
What tuning frequency should I use?
For home theater, 28 to 32 Hz gives deep extension. In a car, 32 to 38 Hz is common because the cabin boosts the low end anyway. Always use a subsonic filter a few hertz below the tuning to protect the driver.