box calculator

Let's say you need your box to be 100 litres, so that's 100,000 cubic centimetres. 50 x 50 x 40cm internal will do it, then add the wood thickness on each side. If there's a port, recessed baffle or other bulky internal feature, work out the dims of that including wood thickness first then add that to your internal volume requirements. Ditto any bracing.

Start with that bigger number and divide it by two numbers that you like and see if the remaining number sounds good with those two, then tweak. A calculator might do these easy steps for you but the devil's in the details, and that's what will consume your time if you want to do it properly. Practicing in the basics manually will make it easier to handle the tricky details when they arise.

You've got to include driver/horn displacement, for example. The manufacturer's specs, if they list it, will be for the entire unit, not mounted to a baffle unless stated, that can actually make a couple of litres of difference on a big driver or horn. If there are no manufacturer's specs for the driver just estimate it based on cylinder and cone dimensions, you can eyeball it from a picture if necessary and it'll be near enough.

If you imagine a cube, you will get L, W and H fundamental standing wave all occurring at the same frequency, so possibly very audible. And the same at the 1st harmonic and so on. Very spikey output.

With the dimensions adjusted, the trick is to spread these fundamental and harmonics, so that only one ever occur at a time, that way you get the flattest "Box Output" possible, before wadding and internally bracing to further disturb standing waves and resonances.

1/ internal box ratios are best if they are multiples of irrational numbers so that a standing wave across one set of opposite box walls does not reinforce any of the others. Be careful with the square root of 2. 2/ the golden ratio (phi) is an appealing irrational number. Due to one of its properties it is easy to calculate the internal box dimensions. Take the total required gross volume (ie includes volume of back of driver & all bracing) and find the cube root. That is D1. Then multiply D1 by phi for D2. Take D1 and divide by phi to get D3. Choose any side for the driver. Each has it's advantages and disadvantages. A Classic GR, sets D1 to width and D2 to height. Rotating it such that D1 is now depth fits today's aesthetic better.  3/ if one dimension starts to become significantly larger than the others (wintermute mentioned 3x) then the box becomes a quarter-wave resonator (ie a TL) if it has a hole/port/terminus in it, and a half wave resonator if sealed.