Woofer Enclosure Construction
Nothing effects how well your woofer performs more than the quality of the enclosure it's mounted in. To build a quality enclosure, you must start with a quality design. The airspace inside it needs to be the right amount. The recommeded amount is generally supplied by the speaker manufacturer. It needs to be an enclosure that the woofer is compatible with. The enclosure must not flex or leak air. It must not vibrate or rattle.
The walls of your enclosure should be constructed of high strength that will not leak air. Two of the most commonly used materials are high density Particle Board and MDF (Medium Density Fiberboard). These are easily found at most area building supply stores or lumberyards. Generally, a thickness of at least 5/8 or 3/4 inch is recommended. In extremely high power applications 1 inch or thicker is used. The more rigid the walls are, the better.
As a woofer moves out, the air air inside the enclosure is thinned (rarefied), like a vacuum. If the box walls are not rigid, they will bend inward, causing cancellation of the front and bass response will suffer. The same applies as the woofewr moves inward, compressing the airspace and flexing the walls outward. If the spans more than 18 inches anywhere internally, it should be braced. Just remember to take in account of the area the bracing takes up when you figure your airspace.
You should use both glue and screws (or nails, staples) for the joint of the walls. If you choose to use screws, pre-drill the hole to make prevent the wood from splitting. Use a good, strong wood glue because a good deal of the enclosure's strength comes from the glue. For added strength, you can optionally glue strips of wood (cleats) along the inside edges of the joints. After the glue has dried and set up, add a bead of silicone caulk over all the joints to insure against air leaks.
The ability to correctly calculate the size (volume) of your enclosure is crucial to building the right box for your woofer. Most enclosures built will be similar to one of the four standard shapes shown below. Remember to take into account the thickness of your wood if you take outside measurements. Use the following formulas to figure the volume in cubic inches of your enclosure. Take the total cubic inches and divide that by 1728 to figure cubic feet.
Damping - "Stuffing the Enclosure"
Let's say you're planning on building a sealed enclosure, but after doing your calculations you find that it will be too large for the given area of the vehicle. There is a way to decrease the volume by about 20% without affecting sound quality. You can accomplish this by damping the box, stuffing it with a low density material like fiberfill (what they fill pillows with), long fiber wool or fiberglass (1.5 pounds per cubic foot of box volume). Make sure that you keep it away from the speaker - use bracing if necessary to insure this.
Damping can also be used for other reasons. Conventional Damping is not recommended for a vented enclosure, although lining the 3 box walls opposite of the vent with a thin layer may help reduce port noise. It's important that this does not interfere vent in any way. Stuffing an enclosure additionally can be done to adjust or vary the F3 of the the system or to smooth out a frequency bump.
