How should you design bracing for a DIY subwoofer? It's very common for DIY enthusiasts to get it wrong. There are many different ways to brace a subwoofer enclosure, but the one shown here is one that will yield the best performance possible without going crazy.
What are we trying to achieve?
With a subwoofer box, the critical aspect is stiffness. In a fullrange enclosure, the challenge is different as we are also interested in damping qualities. Accelerometer measurements of fullrange enclosures show benefit from mass damping with lead, but for a sub box this merely adds weight. With a sub we simply want to make the box stiff. We can achieve this with curved walls, exotic materials, very thick walls but the cost effective way to do it is with bracing applied to reasonably thick walls.
So if we start with 18mm MDF, we might first double the thickness for the baffle, then brace internally. Each brace should touch 4 walls, and 3 intersecting sheets should be used. We then make cut outs for the driver and amp (if used in the box), and openings so that we don't have enclosed air cells.
Here is an example:
This bracing system could be improved slightly but making the cut-outs circular, although the differencs isn't critical.
Another example can be seen here of a 10" subwoofer with a downfiring driver.
A similar bracing design could be scaled up for 12" or 15" drivers. As the enclosure gets larger you may choose more bracing sheets, but as a general guide aim to break up the box with a grid no greater than 200mm.