This is a simplified plan to build a basic sealed enclosure.
The first step is determining how much air space the speaker is designed for. Most audio speakers, especially sub woofers, come with a data sheet that tells you how much air space needs to be inside the box for best performance. This number usually reflects what is needed for a sealed enclosure to get "flat" performance. "Flat" means more or less equal performance across the range that the speaker is capable of reproducing. Ported boxes, and band pass boxes produce a stronger reproduction of a certain frequency range or "peak" and get much more complicated to build. Go to the link in the resources section of this article if you want to tackle a more complex enclosure.
Next, draw your enclosure out. If it is going into a car you have to take into consideration where the box will be located, and how much room you have to work with there. In a home you will probably just make a rectangular enclosure. To determine what dimensions your enclosure should be take the manufacturer's recommended air space, and build your enclosure to have this much space inside it. Sounds simple huh? Well to really do it right you need to take into account the air space the speaker itself takes up. This is usually in the spec. sheet. If it isn't, a fairly accurate estimate is: an 8" woofer takes up .03 cu. ft., 10"-.05 cu. ft., 12"-.07 cu. ft., and a 12"-.10 cu. ft. Remember to add this to the size of your enclosure. If your box is going into a car, and has to be an odd shape you need to separate each section into a shape you can easily find the volume for, and add the results together. For a triangle form an imaginary outside square to find the volume, and roughly divide by half. The more irregular the triangle the less accurate this is. Remember volume is in cubic units of measure, HxWxD, a 1 cu. ft. box is 1728 cu. inches because 12x12x12=1728. So you can divide the manufacturer's recommended air space by 1728, to get the length of one side of your enclosure in inches if it were perfectly square. This can be useful when deciding the dimensions of your enclosure. If you only have 6" of vertical space, and need a 1 cu. ft. box it can be 6"x24"x12". You will have to use some geometry skills with complex shapes. The closer to the exact air space you build the box the better it will sound. This is why you want to draw it out first, and redraw it several times if necessary to have a shape that fits the place it will be mounted, and has proper air space. Remember if you are mounting two speakers in one box the air space has to be double, and a center divider will make it sound better.
Next build the box using your drawing as a pattern. You can even use cardboard, and assemble a mock up of the box if it needs to be very odd in shape, and use the cardboard as an exact pattern. Use coarse deck screws about 1 1/2" long. Pre-drill the holes with a 1/8" drill bit so the plywood doesn't split. If your speaker didn't come with a template for cutting the mounting hole, use a string, and pencil to make a perfectly round hole for the speaker(s). Screw a deck screw in about halfway in the exact middle of where you want to mount the speaker, and measure the exact radius of the speaker mounting lip. It isn't exactly the nominal size of the speaker. I.E. it isn't always 5" for a 10" speaker. Measure the lip exactly from side to side. Remember to either buy wire lugs to mount the wires through the box or at least drill a small hole, and run your wires through it, and seal it. I recommend buying lugs from an electronics dealer. They look nice, and work well. make sure your cuts on the plywood are precise, and square or your box won't look very attractive. Remember to take the blade curf into account. The curf is the amount of wood the blade removes when it is cutting. It is usually 1/8".
If the speaker didn't come with a gasket, use multi-purpose weather strip from a hardware store. Glue the weather strip very close to the outer edge of the speaker hole. Most weather strip is self adhesive, if not buy 3M trim adhesive or something similar. Seal the box from the inside with silicone sealant. Seal the wire lugs where they come through also. When you mount the speaker I recommend using "T" nuts from the hardware store. They hold much better, and make it easier to remove the speaker, and put it back again while you're building your enclosure. Just drill a hole of the size recommended in the instructions with the "T" nuts (usually 1/4" or 3/8"), and tap the nuts into place in the hole with a hammer. Now cover the box with some nice carpet, or rhino line the outside. Be creative, and make it look good. Be proud of your work.
