VERY IMPORTANT: Exposing Acoustical Myths by Russ Berger
Acoustical design is burdened by many time-honored misconceptions. LOOK AT THE TRIPLE LEAF SUBHEADING.
Art Ludwig's Sound Page
A really GOOD site from a VERY clever guy.
IPA Acoustics Handbook: How to use perforated steel sheet
This is a VERY GOOD publication of the Industrial Perforators Association (IPA), related to the use of perforted steel sheets, as wel for broadband absorption as well for tuned resonators.
Mikrofonaufnahmetechnik und Tonstudiotechnik
German language, but lots of references to English documents and sites. I know this person, he lectures acoustics and studio techniques in Berlin, and wants to share all his know-how with anybody. For Germans, there is no better, more extensive site. For English people there is an extreme amount of info (only navigating a bit more difficult).
Fast info about noise, vibration, room acoustics, Reverberation Time and much more.
SAFLEX acoustical guide
This is the (very good) acoustical guide of SAFLEX Laminated Glass. Use the menu to the left to navigate through the different chapters. You can also download this guide. It's free, you only have to register your name.