Box Building
Each type of sub requires it's own type of box. If a sub is installed in a box larger or smaller than what is supposed to, it will sound distorted/bad and could be destroyed. Boxes can be built in many shapes, but it is important that the box volume is calculated accurately to achieve maximum performance.
Materials
A box MUST be very sturdy. Most common building materials are 5/8" or thicker particle board or medium density fiberboard. If building a box with Plexiglas, do not use anything less that 1/2 inch thick. A common material used to mold complex shaped boxes is fiberglass, but it is very hard to work with, and require several layers for a smooth finish.
Gluing, Sealing
Glue should be used at all joints to fill any spaces. Any spaces will degrade the performance of your subs, not to mention the annoying noise air makes when being pushed out of a small hole. On applying the glue, let it cure for at least 24 hours before mounting the subs. This is a precautionary measure to protect the rubbers used to make the subs from the high fumes that some manufacturers glue products have.
Holding Joints Together
In connecting box joints, it is best to screw the joints every four inches or so using 2" - 2-1/2" screws. Pre-drill about 3/4" deep, so that screws do not split the wood at the edges, especially when working with particle/dashboard.
Should I have a Box for Each Sub?
It is advised to have a separate chamber for each sub. Even though not necessary, here are two reasons why such a directive should be taken: First, if one of the subs blows, then the volume of the box will be "twice" as big for the one remaining working sub. This could cause problems and even damage the other sub. The second reason is bracing. Building a box with a divider in the middle will make the box more rigid.
Ports
Ports must be built into your box design to channel out the air made by the sub's vibrations. If a pre-made port is not available, the most common material is PVC. PVC is very rigid, comes in different diameters. Cut the tubing at the desired length. Consider the volume the port takes up when calculating the box volume. Cut a hole in the box. Make sure the hole is as perfect as possible to minimize gaps between the box and the tube. A couple wood braces can be added for screwing the port top the box. Seal the gaps using a proper sealant (Evo Stick or even silicone can be used).
Bracing
Boxes that are more than a foot in width or length or height, should be braced so that the box becomes more sturdy. This can be accomplished with a piece of wood maybe 3 or 4 inches wide across the box). It is a good idea to put wood blocks on the corners for reinforcement. Always consider that blocks, braces, neon lights, etc. inside a box take up space and should be accounted for when calculating internal volume.
Damping/Filling
Damping increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. It is advisable to put damping material inside a box. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. Polyfill also "tricks" a sub into thinking it is in a bigger box. Play around with different amounts of polyfill until you get the desired results.
Making it look professional
Make the box surface free from holes and spaces by adding wood fill. If you decide to paint the box, then you should apply primer first. Carpet or Vinyl padding is the best covering to use since they easily cover any outer blemishes on the box and give the box a 'smooth' outlook. Be careful when cutting the vinyl or carpet since such cutters tend to be very sharp. Cut a piece of carpet (or vinyl) big enough to cover the whole box. Apply adhesive to both box and carpet (EvoStick works great). Wait about a minute and place the fabric over the wood. For the best fit, stretch the fabric when applying it. The fabric should wrap around and end in a place of the box that will not be seen. Do one side at a time, cutting excess carpet. If possible, add staples preferably heavy duty staples that can penetrate the box, to hold the fabric at the ends. NOTE: Do not cover each panel of the box before mounting it together since it would be impossible to find any 'leaks' that may exist in the box design much less fill these leaks.
