http://shure.custhelp.com/app/answers/list
Good info on a whole bunch of stuff
http://www.webervst.com/sptalk.html
Extensive speaker discussion, (search for the word "nominal")
http://whirlwindusa.com/support
Great articles, a little techhy but solid info (& another impedance/water analogy)
http://www.acousticdimensions.com
If you want to go headfirst into acoustics, this site will help
http://music1online.com/learningcenter.html
LOTS of in-depth articles on mics & sound
http://www.amptone.com/g068.htm
GREAT list of links, lots of info - You Must Check it!