Originally Posted by A_D
the shape of the box isn't of huge importance as long as its NOT a complete cube. the depth, height and widtch can't be the same length. you need to tend to stay away from this. i think it has to do with the sound waves bouncing off the back wall and since all the sides are the same, the wave bounces back at the same length it was produced, and that will create cancelation. i'm not quite sure
but you can pretty much make any dimensions you want (as long as the sub has enough mounting depth and width). TWO sides can be the same length if desired but all 3 can't.
If you're really really lucky (or unlucky) this might happen, but, even with a perfect cube box, the amplitude of the lower frequencies is so large that this really doesn't happen often at all. But yes, to be on the safe side, don't make a perfect cube.
And also, when calculating your box volume, don't forget to factor in wood thickness and sub displacement.