A brilliant lecture by industry legend Floyd Toole that every self-respecting audio professional should watch!
M-Noise™ is a new scientifically derived test signal whose crest factor as a function of frequency is modeled after real music. M-Noise should be used with a complementary procedure for determining a loudspeaker’s maximum linear SPL. The M-Noise Procedure contains criteria for the maximum allowable change in coherence as well as frequency response. When the loudspeaker and microphone are positioned as prescribed by the procedure, reductions in coherence are expected to be caused by distortion. Although higher precision methods for measuring distortion exist, coherence has the advantage that it can be calculated for wide-band signals such as M-Noise as well as music.
If loudspeakers could somehow magically occupy the same point in space, our work would be considerably less difficult. Unfortunately, actual, physical loudspeaker enclosures prohibit us from doing so, which is when "Shit hits the fans" (plural). Physical displacement between multiple loudspeakers is one of the prime challenges in sound system design.