Hey fellow competitors, are you ready to get your hands dirty? Our "sub box saga" continues this month, but we're not discussing theory or talking about sound waves. It's time to put down the calculator, pick up the power tools, and get down to the business of building your subwoofer enclosure.
This month's column is based on a question I received several months back from Curt, a development and materials engineer at Johnson Controls. He asked me about the differences in box construction materials, and how they relate to sound quality in particular. He was basically debating whether to build his enclosure out of wood or fiberglass for his Jeep Cherokee. There are many things to consider when building your box, so let's look at the pros and cons of several different construction methods so you can decide what's best for your ride.
No matter which box construction material you choose, two golden rules apply to all of them -- it has to be sealed and solid if you want it to sound good. Remember a couple months back when we discussed the purpose of an enclosure? It's basically there to block (or at least tune) the back sound wave from your subwoofer to prevent cancellation. A leaky box will not only let some of these backwaves out, but it will also screw up the effective volume of your enclosure by releasing some of the internal pressure created when the woofer compresses. The "solid" part is also very important, because if the box can flex and vibrate, these vibrations will negatively affect your sound quality and sound pressure level. Keep these two rules in mind when building your enclosure, and you'll be much happier with it in the long run.
The first and most traditional enclosure material out there is wood. There's a reason why it's been a mainstay. Wood is heavy, solid, and easy to attach together, making it fairly simple to create a good-sounding enclosure. There are many different types of material, including plywood, particle board and MDF that you can buy in 4-foot by 8-foot sheet form. Of all of these, I think MDF is the best, mainly because it is solid, consistent (no air pockets or soft spots), and very easy to cut and work with. Wood glue and some screws or a nail gun is all it takes to hold this stuff together, so it doesn't take very long to build a basic enclosure. Wood also has natural qualities that make it great for absorbing reflective sound waves inside your box, so it's a good choice from a sound quality perspective.
There's no perfect material, and wood is no exception. Since it comes in sheet form, it's pretty easy to build a square or rectangular box for your car, but not so easy to make it curvy to fit the shape of your interior. MDF is also really heavy, with a simple sealed box for two 12" subs weighing around 40-50 pounds before you even put the subs in there! This will negatively affect the overall performance of your car, so it's something to keep in mind when designing your sub system. Also remember that this stuff cannot get wet, so if you're designing a system for your boat, you'd better find some marine-grade plywood instead.
Another good enclosure material to consider is fiberglass. It has also been around for a long time, and it's used to build everything from boats to Corvettes to the seats on the rides at Magic Mountain. Fiberglass is glass fiber mat or cloth, reinforced with a resin that hardens to create a solid piece. Since it is built up in many layers, any desired shape or thickness can be achieved with fiberglass, making it extremely versatile. As our friend Curt correctly points out, 1/4" of fiberglass is roughly equivalent to 3/4" of MDF from a structural standpoint, so our box for two 12's might only weigh 15-20 pounds if it's built from fiberglass instead of MDF. Fiberglass can be shaped to fit any contour or curve, so it works great for integrating your sub box into your vehicle. As we discussed last month, the rear corners and spare tire well of most cars work great for a sub location, and fiberglass can be molded to fit the contours of these areas. Molding the enclosure in this way maximizes available space, making the box less intrusive into the useable space of the car. In addition, it is completely waterproof, so it can be used anywhere.
If you decide to use fiberglass, get ready for a steep learning curve. There are many pitfalls to mixing, laying, and shaping this stuff, so be prepared. It also takes a lot longer to build a fiberglass enclosure than an equivalent-sized wooden box, since there are many more steps involved. Even though fiberglass and MDF can be made to be structurally similar, the fiberglass box will be more reflective internally due to the natural properties of the material. It's usually necessary to line the enclosure with a dense flexible foam or fiberglass insulation to achieve the level of damping necessary for a good-sounding box (this isn't a bad idea for an MDF box either). Finally, fiberglass is a much more expensive material, so it costs quite a bit more than wood.
There are other options as well. MDF and fiberglass are compatible materials, so it's often easier to build a combination of both to achieve the desired enclosure. The latter can be used to form a curved floor shape, while the former can be attached to the sides and the top to finish the box. If you have a square-shaped area in the vehicle to start with (picture the back of an SUV), then it makes sense to build the floor and sides out of MDF, and then use the fiberglass on the top to give it some curvature and style.
If you want to get really creative, you can go out and find an unusual material with which to build your enclosure. I've seen acrylic tubes, cardboard, steel, and even honeycomb aluminum used for this purpose -- as long as it's solid and sealed, you'll be OK. One competitor from years back even built his enclosure out of concrete. It weighed several hundred pounds but it did get him some creativity points. The sky's the limit, so go out there and have fun with it and I'll see you in the lanes.