Building subwoofer boxes is something that, while primarily for business purposes (see e.g. www.MidwestDJs.com), has also become something of a hobby for me. As with any good hobby, there is no shortage of opinions on the optimal configuration. The internet (and even books) hold countless designs and methods which purport to be the loudest, most efficient, or best by virtue of any number of unique and often-times strange techniques. Some suggestions are clear conjecture while others offer scientific (or at least pseudo-scientific) foundations for their benefits. One technique which has often fascinated me is the concrete subwoofer box.

If you've haven't had an occasion to run across this idea before, I'll give you a little bit of background. When constructing a subwoofer, one supposedly (I say "supposedly" because I haven't scientifically tested this theory—though my own empirical observations support this) important design consideration is to make the enclosure as stiff as possible. Basically, the theory goes, energy that is spent on vibrating the walls of the box is energy that isn't being used to generate sound. Additionally, when the walls of the enclosure vibrate, they can contribute to "sloppy" bass because they keep reverberating after the actual speaker has stopped.

Obviously, when it comes to easily accessible construction materials, concrete is pretty high up there on the stiffness scale. However, if you look at some of the plans for actually constructing a concrete sub box, it becomes pretty clear that these boxes are not really all that practical. More importantly, for my particular use, they're not at all mobile. Nevertheless, every time I go to design and build a new sub box, the concrete box concept always comes to mind. This time around, I was determined to replicate that same effect with some more practical materials.


Over the years, I've done numerous tiling projects in my home. I know (as anyone who is halfway decent at tiling knows) that one of the keys to a long-lasting tile installation is to lay tile only over a very stiff surface. Flex in the underlayment leads to cracked grout and, in the worst case, cracked or loose tiles. In order to achieve a very stiff sub surface, concrete backer board is installed over usually wood subfloor. Unfortunately, until recently, concrete backer board was a pretty awful product to work with. In it's classic form, it's heavy, relatively thick, and the edges basically crumble off. For tile work, you're usually bonding the edges together with thinset and fiberglass tape, and covering it with thinset to adhere the tile, so the aforementioned downsides are only an inconvenience during the tiling process. However, any use of that particular material for a subwoofer box would almost certainly be a disaster.


In the past few years however, several new tile backing board materials have been introduced which are much more pleasant to work with while maintaining the same stiffening effect of the old concrete backer board. One such product is a relatively thin, very solid material called GreenE-Board (http://www.greeneboard.com/). In my opinion, this product has all of the attributes that make it a great choice to providing a ton of stiffness without excessive weight (hence making the box still portable) and without complicating the project too terribly. Also, it's very reasonably priced (less than $10 for a 3'x5' sheet at my local home improvement store).



For this particular project I chose to build my box out of 3/4" birch plywood (a popular choice for speaker boxes) and then line it with GreenE-Board. As the photos show, the procedure was pretty straight forward—simply build the box as normal, apply a layer of subfloor adhesive (e.g. Liquid Nails), and use plenty of screws to secure the GreenE-Board. The GreenE-Board adds some weight to the box (which is a good thing, for various reasons) but it's not ridiculous. While I don't have any quantitative testing to support my observations (mainly because I'm not going to build two full subwoofer boxes to scientifically test my theory) I believe that the resulting enclosure is stiffer and more solid than any previous enclosure that I've constructed. Also, I believe that especially when it comes to the stiffness-to-weight ratio, this method of construction would be hard to beat without using very exotic materials.



Perhaps at some point I'll build a comparable box without the GreenE-Board lining and test it against the lined box to get some hard numbers. Or, perhaps someone else will do that for me and share the results. In any case, my new sub cab hits hard and goes low. And, at the end of the day, that's all I was really going for...