If you plan on designing or building speakers, especially open-baffle dipole types or subwoofers, you need to check out Seigfreid Linkwitz’s vast Linkwitz Lab site. He is a retired microwave engineer from Hewlett-Packard, and applies an engineer’s insight and detail to his designs without getting into the fatal “meter-reader only” mindset that many professional audio engineers fall into. Although he sells both products and design services, he doesn’t hold back in putting up his design details onto his website. It is now so large that he sells a CDROM of the entire site, which is especially helpful for people like me with slow dial-up internet access. I could go on and on about all the interesting parts of the site, but the best thing is to peruse it yourself.