Choose the loudspeaker to be housed in the cabinet. This is the biggest determinant of specifications for the box. Then, download winISD, an excellent speaker design software that is a free download.
Use the Thiele/Small parameters to finalize the enclosure size. They're provided with quality loudspeakers, but the information is also available online, especially at the Thiele/Small website.
Plug the Thiele/Small numbers and the speaker parameters into the speaker building software. The software will output all the specifications for building the cabinet. Ported designs and bass reflex designs are the same thing and produce more low-end with greater efficiency than sealed box designs.
Use 3/4-inch birch plywood for box construction. Buy the wood and cut the panels to size, or have the wood cut for a small fee from the seller.
Dry-fit the sides, bottom and bracing first. Bracing is an interior piece, the same size as the bottom piece, that increases box stiffness and helps minimize vibrations.
Glue the sides, bottom and brace if the dry fit matches up well. After gluing, countersink wood screws every 4 inches along seams. Clamp the pieces after screwing using furniture clamps, and let the assembly dry for eight hours.
Make cutouts for the back panel. If the construction is a powered sub, the cutout will accommodate the plate amplifier. If the build is for a passive sub, the back cutout is for the terminal cup that contains the inputs. Do the dry-fit, glue, screw and clamp sequence again, and let dry for at least six hours.
Attach the top. This panel may be inset in a rectangular design or auto box but can be larger than the larger if the sub is being used as a table top. Glue, screw, clamp and let dry as with the previous pieces.
Make the cut outs for the front panel to hold the loudspeaker and the port(s). Caulk all inside seams with silicone caulk before attaching the panel. Allow up to 15 hours for drying, since the caulk emits gases while drying that can erode speaker surrounds.
Build the port. Use 3-inch PVC pipe--the length determined by the speaker specifications. Flare the outer ends of the port for sonic purposes. Glue the port to the back of the front panel. Caulk around the port to complete the seal once the glue is dry, and again let the caulk dry for 10 hours.
Attach the front panel using the glue, screws and clamp sequence. Through the openings in both the front and rear panels, caulk with silicone caulk the seams of the front panel. Let this dry for at least 12 hours.
Sand the cabinet while the caulk dries, and fill countersunk screw holes with wood putty. Sand lightly a second time.
Install the back piece, a terminal cup or plate amplifier. Ensure all parts clear internal bracing; notch the bracing to create enough room. Glue the back piece in place. Caulk around the perimeter of the terminal cup or plate amplifier when the glue dries, and let the caulk dry thoroughly.
Attach the connecting wires between the back panel piece and the loudspeaker. Attach the loudspeaker with wood screws, then apply non-drying caulk around the cone's perimeter to form an airtight seal.
Finish the cabinet, if desired, with wood veneer, paint or stain. There are hundreds of finishes to choose from.When you build a subwoofer box to specifications for a car or home environment, the first specification that must be considered is the size of the loudspeaker/driver. The size of the driver determines the physical size and the interior volume of the box, regardless of whether the build is of a sealed design or ported enclosure. There is lots of help on building subwoofer boxes online, from free speaker design plans to software that handles all the engineering expertise that would otherwise be necessary.