How to build a subwoofer box

Step 1 – Cutting the boards

We started by measuring and cutting the main pieces of MDF for the front, sides, back and top of the box, using a table saw.

Don't have a table saw? Don't worry. If you purchase your MDF at any large home improvement store, they should be able to cut it for you for a small fee. (Shout out to Cameron at my local Lowe's. He turned the 6'x8' sheet of MDF into planks that would fit into my car.)

You could also use a circular saw, just be careful to keep your cuts square (smooth, flat cuts help to ensure that the box seals well).

We cut seven pieces total – top and bottom, two sides, the back, and two identical pieces for the front (since it was to be a double thickness). Once this is done, the rest of the process is like assembling a model kit or prefab furniture.

Step 2 – Draw the circle

After the pieces are cut to size, use a compass, or the template that may be included in your subwoofer's packaging, to mark the woofer cutout on one of the identical front pieces. We used the cardboard packing insert as our template.

Step 3 – Prepping the front panels

If you choose to use a double-thickness of MDF for the front panel (this method is recommended — it provides an extremely strong, non-resonant mounting surface for the sub), fasten the two identical front pieces together using plenty of carpenter's glue and several sheet metal screws. Also, the double thickness will serve to strengthen the box as a whole.

If you don't use a double thickness of MDF for the mounting surface, you should definitely plan to use bracing elsewhere in the box for added strength. In fact, it's never a bad idea to use bracing no matter what, especially if your box is larger than a cubic foot. The box will be subjected to extreme internal pressure, so the stronger it is, the better.

The easiest way to add bracing is with 2"x2" strips of lumber. Glue and screw these along at least two of your box's internal seams before attaching the top and bottom.

Step 4 – Assemble the box

While the front panels were clamped and the glue was drying, we assembled the rest of the box.

We drew a bead of glue on the panel edge and pressed it against the attaching panel. We used wood clamps to hold the panels together until we got screws in place. The glue, not the screws, is what will ultimately seal the box, so don't be afraid to pour it on.

MDF can be prone to splitting, so we pre-drilled the screw holes, then inserted the screws. It's times like this that it really pays to have both a drill and a bit driver!

We fastened the pieces together using 2" drywall screws. Some of the glue will squeeze out during this step — you can wipe it off the outside of the box using a wet rag, but it's OK to leave it on the inside edges (it'll actually help with the seal).

Step 5 – Attaching the final sides

When assembling the box, you might find that it’s a little out of square. This is where wood clamps can really help to pull things into shape. When you screw on the last two sides (front and back, in our case), it should pull things back into alignment.

Note: the largest sides of the box should overlap each of the smaller sides to provide the greatest strength.

Step 6 – Cutting the opening for the woofer

Using our drill and a large bit, we made a hole near the inside edge of the circle we had traced, large enough for our jigsaw blade to fit in.

We cut out the circle with a jigsaw, gave it a quick hit with sandpaper, and the woofer opening was complete.

Step 7 – Adding the terminal cup

Flipping the box over, we used a 3" hole saw to create the opening for the terminal cup. If you don’t have such a large hole saw or if you have a square terminal cup, you can use the drilling/jigsawing procedure we used on the front of the box.

After smoothing the opening with rough sandpaper, we ran a bead of silicon caulk around the edge of the terminal cup and screwed it into place using 1/2" sheet metal screws.

After you've glued-and-screwed all the panels and cut the holes, you're just about done.

Step 8 – Test fitting the woofer

The next step is to drop the subwoofer in and make sure it fits. If the sub is a tight fit, use coarse sandpaper or a rasp to adjust the opening.

This would be a good time to mark the locations of the screwholes with a pencil so you can pre-drill the holes after you remove the woofer. We happened to do it at the end of the process.

Step 9 – Seal the interior

To make double sure that everything was sealed and airtight, we waited for the glue to dry and ran a bead of silicon caulk over all of the box's internal seals. We used a hand-held tube of caulk, since it would be tough to get a caulk gun down inside the box at this point.

Let the caulk cure for 12-24 hours before putting the subwoofer back in. Some silicon caulk releases acetic acid fumes while curing, which have the potential to destroy subwoofer surrounds.

Step 10 – Installing the subwoofer

After the caulk had cured, we added some non-hardening rope caulk (found in the weatherstrip section of the hardware store) to act as a seal beneath the woofer frame.

Next, we dropped the woofer back into the hole and used a pencil to mark the screw hole locations, then removed the subwoofer and pre-drilled holes for the mounting screws.

Finally, we connected a length of speaker wire from the terminal cup to the subwoofer, placed the subwoofer back in the box, and screwed it down. We were very careful to not let the drill slip off of the screws and ruin my shiny new sub.