How does one go about calculating what size of port walls to use, if they're wanting a certain tuning frequencies? I.E. I've got plans for a box that's 2cubes at 36Hz, but I need a higher tuning frequencies. I'm wanting to raise the tuning of the box, but keep most of the properties of the plans the same. I know I'm going to have to change the port length of the port wall, but I don't know how much?