If you have decided
to build the enclosure yourself, then you must understand what
limitations you have for it's internal volume, and locations of
speakers or ports. These figures can be derived using enclosure
modeling software or can be obtained from your dealer.
To get started, carefully measure the area in which you wish to place the
enclosure. Be sure to watch for a secure place
to mount the enclosure, account for the fact that the enclosure is most likely
to be built outside the car, and loaded in. You must then determine the exact internal
dimensions of the enclosure. Some enclosure designs are forgiving in terms of
I'd rather know exactly what I was building.
You now have three choices in picking your subs. You can experiment with some of the shareware enclosure design programs found on the web. You can take this internal volume number to a reputable car audio shop and have them help you pick the right speakers for your kind of music. Or you can fire us to design the enclosure for you (If you are interested, see the Services page).
There is one very important question that must be asked of you before they start to design or choose an enclosure. What type of music do you like. If this isn't determined, or you haven't somehow made it already obvious (your hats is on backwards, your jeans are around your hips, you shoes are undone and the biggest of the 4 gold chains around you neck says 'M.C. D.J. MIXMASTER BASS), then make sure it is stated clearly. Anyways, you get the point. It's better not to assume that they know what you want. So tell them to be safe. If they reach for a generic speaker box 'off the shelf' and expect to sell you a USD, Focal or any other reputable speaker, you might want to head for another shop. Don't be afraid to question their means.
The design of your enclosure is based directly upon the selection of speakers you are intending on using and the space you have available. I always see what internal volume of enclosure will provide the most useable room in the vehicle, while providing the type of sound the customer requested. I will take the volume, lets say 3.25 cubic feet for this example, and determine what speakers will operate well in this size. Many people will be able to tell you from experience that certain speakers will work very well. But, they must be able to explain why one is better than the other if you ask.
The criteria for speaker selection for your enclosure will be
frequency response, power handling, control and price. If you
choose a Rockford PowerDVC15, and cram it into this box, it's not
going to play very deep. If you use a single Pioneer or Clarion
10 inch, or pretty much any 8-inch, it's going to exhibit very
limited physical power handling. Your shop should be able to give
you some information on physical power handling for the enclosure
you intend to build, or have built.
The physical location of the drivers and the ports (if there are any) in the enclosure will also play a large role in the ability to reproduce accurate bass. I try and design an enclosure that allows the ports to be on the same surface as the drivers and to be sure that they are located in a location which is as central as possible with respect to the walls of the enclosure. The ports should be as large as possible, while staying away from internal surfaces. I use twice the diameter of the port itself as a minimum distance from any internal surface as a general rule, and it seems to work quite well.
Building your box
Certain materials have become the standard for speaker enclosures. The most popular is MDF (medium density fiberboard). It has several other names, Rangerboard, Weinerboard, etc.. It is basically sawdust and glue. It is nice and dense, and fairly non-resonant. BUT, it can split if not assembled correctly, rendering it useless. All enclosures should be glued (generic wood glue works fine in quantity (helps seal the box as well)), and nailed together with an air-nailer or the like. If you must screw the box together, ALL HOLES MUST BE PRE-DRILLED!, then drywall screws work fine. Any large panels should be braced to prevent flexing. Any flexing of the enclosure is throwing away sound. In both quality and quantity.
The weirder the shape that the enclosure takes, the better chance you have of preventing resonance's and standing waves. A sphere with a small flat portion to mount the driver on would be marvelous, all other things being equal. A minor drawback to this is that measuring the internal volume can get quite difficult, and this is a very important number for anything other than a sealed enclosure, as it will drastically affect the port tuning frequency, which will alter the sound quality and even worse, the systems power handling.
Before we close this article, I want to touch on an issue has come to my attention in the past, and has recently been refreshed in my memory. It is an issue that is critical to the performance of 99% of the subwoofer systems built around the world.
Most people choose MDF for their enclosures. Believe it or not, MDF is not airtight. All the hours you have spent sealing your seams with Silicone, Latex Caulking, and the like, while important, are only half the problem. During a recent visit by Dave Gumienny of Kicker Car Audio, he reminded me about the breathability of MDF. He told a story of one of the training courses they did years ago when he worked for PPI, where 5 small pieces of MDF, 3/8 thick, were stacked, and all where lifted by a vacuum into the air.
If you choose to use MDF, you have two readily available solutions to seal the cabinet. Paint the interior or exterior with a gloss or semi-gloss paint, or even a varnish. Anything that sinks into the wood, and will seal it. Your second option is to coat the interior of the enclosure with fiberglass resin. This is very rigid as well.
I hope this information helps you to build a better speaker enclosure.