Let's face it, installing car stereo equipment is no picnic. Putting speakers into places they simply weren't meant to be presents a challenge to even the most savvy of installers. And the more exotic the install, the tougher the task. While each aspect of system design brings a unique set of challenges to the installation, building custom speaker enclosures is perhaps the most creative aspect of the installer's job. Remember, there is no replacement for a pro-installer. They have years of training and experiance.
Straightforward in design, the sealed box is nothing more than an airtight enclosure whose purpose is to enhance speaker performance. When a woofer is installed in one, the sound waves that emanate from the front of the speaker cone are separated from the rear-firing waves. This improves bass response, since opposing waves can cancel each other out when they aren't isolated. Superb damping, good power-handling capability, and simplicity of construction make the sealed-box enclosure an ideal candidate for a variety of installations.
Vented enclosures – also known as ported or bass-reflex systems – are more complex than sealed boxes, but the extra construction work required to install a vent has its rewards. Vented boxes are more efficient than their sealed counterparts, since they channel sound waves from the front and rear of the cone into the listening environment. Other virtues of the vented design include better reproduction of low bass, a reasonably flat response curve, and low distortion – provided the box is properly constructed.
Tools Required
You won't need any special tools to build a speaker enclosure, just a tape measure or yardstick for measuring, a couple of pencils for drawing the cut lines, and a compass for drawing the speaker and vent cutouts. To make a straight, accurate cut, your best bet is a table saw; a circular saw is the next best choice, followed by an ordinary hand saw. To cut circular openings for the drivers and possibly a vent, you'll need a jigsaw. An electric drill will come in handy for drilling screw holes and can double as a power screwdriver. Other required tools are a hammer, a standard screwdriver, a razor knife, a caulking gun, and safety goggles.
Construction
Draw out each piece of the enclosure before cutting so you don't waste material. Once individual sections have been cut, temporarily assemble the box to make sure the pieces fit snugly together. One option to consider at this juncture is the use of internal braces, which, depending on the overall size of the box, may result in a sturdier enclosure. Usually, 1.5 inch x 1.5 inch strips of material will suffice. Generally, you'll need to add braces only for boxes that have internal volumes of more than 5 cubic feet.
If all of the pieces fit and the edges match up, the box is ready for permanent assembly. First make sure all joints are free of debris. Then slowly run a bead of carpenter's glue along the line where you'll be making the first joint. Carefully assemble the two pieces and secure them using screws or nails. Use one screw every 6 inches or one nail every 3 inches. If you're using screws, drill pilot holes to prevent the wood from splitting. And for a more polished look, countersink the screws. (Note that the type of joint that you select is not critical as long as the box is strong and airtight.)
Continue assembling the box but don't attach the baffle board – the front panel that holds the speakers – until you've made sure the enclosure is sealed. To do this, fill a caulking gun with silicone sealant and run a bead along the inside of each joint. When you're finished caulking the box, secure the baffle board in place.
With the box assembled, you are now ready to cut the speaker openings and the vent, if you've chosen a vented design. I've always found that it's easier to work on the wood once the box is assembled. The vent tube can be made of plastic, cardboard, or – for a square or rectangular vent – wood. First, calculate the center of each hole and draw a circle of the appropriate diameter using your compass. Make 3/8-inch starter holes with a drill and then cut out the holes using a jigsaw.
Once the speaker and port openings are cut, put some silicone sealant on the tip of your index finger and reach into the enclosure to seal the inside joints of the baffle board. It's very important for the enclosure to be completely airtight; air leakage reduces the system's power-handling capacity and causes distortion. Once the joints are sealed, I recommend covering three adjacent inside walls – back, top, and bottom, for example – with 1 to 2 inches of polyfill (available at fabric stores for about $1.50 a bag). The use of filler will help minimize midbass resonances.