Frank (University of Southampton, UK) Fahy 
Sound Intensity [PDF ebook] 

The advent of instruments capable of measuring sound intensity, which represents the flow of energy in sound fields, has revolutionised audio-frequency acoustical metrology. Since publication of the first edition, two International Standards for the use of sound intensity for sound source power determination, and one International Standard for sound intensity instrumentation, have also been published. A number of International Standards have also been developed.