Pat Brown & Don Davis 
Sound System Engineering 4e [PDF ebook] 

Long considered the only book an audio engineer needs on their shelf, Sound System Engineering provides an accurate, complete and concise tool for all those involved in sound system engineering.
Fully updated on the design, implementation and testing of sound reinforcement systems this great reference is a necessary addition to any audio engineering library.
Packed with revised material, numerous illustrations and useful appendices, this is a concentrated capsule of knowledge and industry standard that runs the complete range of sound system design from the simplest all-analog paging systems to the largest multipurpose digital systems.