the Jon Risch site is very good for these. However, you do need to make a bunch of them. When room modes are excited a few placed in the corners will not make a very big difference. I often hear people tell me--these traps don't work. Well, they do, but room modes are compounded by the surface area of wall(s) and thus need a large portion of the wall covered to relieve the pressure that is causing the problem.
