FWIW, Following is a description along with a few conclusions on speaker placement based on my own investigation over the past 14 years or so into multichannel recording of live-performance music and optimal means for the reproduction of those recordings on the scale of a medium sized room or small multichannel outdoor playback arrangement. To be clear, my investigation is not directly connected with home cinema nor standard layouts for Dolby, DTS, DXS, Auro, and what have you, but does use the same basic playback arrangement as starting point. My goal is the most convincing "you are there" experience I am capable of creating for listeners within the sweet zone, regardless seating/standing position or head orientation. Listeners are free to turn sideways, turn around, or what have you and the illusion should remain robust without audible gaps, as would be the case at the live event. All this infers and relies upon a direct link between recording and reproduction methodologies. How this applies to you all is open to general interpretation. Take from it what you will and feel free to ignore!
The material being reproduced are live music performances recorded using multichannel microphone arrays numbering from 4 to 8 or more microphone channels. I use the standard L/C/R speaker arrangement across the front. Beyond that, I've found I want surround channel speakers evenly arrayed around the listening area to seamlessly recreate an enveloping reproduction of diffuse hall ambiance, discrete reflections, and both diffuse and discrete audience reaction. Unlike cinema surround or studio-produced music for surround playback, there is far less direct sound object image placement required from the surround channels, it is more about seamless immersion, however there is still some direct imaging required - its not just about reproducing a seamless reverberant and audience applause ambience, listeners are able to point toward the locations of specific audience members coughing, reflections off the rear wall of the hall, things like a bar off to one side when applicable, and locate specific environmental sounds heard at outdoor concerts with relative accuracy, etc.
I generally use 7 to 9 reproduction channels, distributed horizontally as LCR + 4, 5 or 6 surround speakers. Beyond 7 channels (arranged typically with Ls/Rs located at ~+/-90 degrees and Lb, Rb at +/-150 degrees) I shift the forward most surround pair to around +/-80 degrees, place the next pair at ~+/-130, and an adjacently-placed pair angled so as to point to either side of the primary listening position at 180 degrees. Depending on the source recording, that center-back pair is feed either either a monophonic rear-facing microphone channel with the signal phase-rotated +/-90 degrees to each speaker (effectively creating a dipole with null facing the central listening position), or a rear-facing stereo microphone pair with a Mid/Side ratio adjustment increasing its stereo-width and reducing its center Mid content which otherwise can cause front/back confusion in the center listening position. This odd arrangement for the center-rear surround location works well because it links ambient surround distribution across the back "filling the hole" for non-centered and non-forward facing listeners, while simultaneously "getting out of the way" for centered, forward-facing listeners.
This arrangement achieves an even distribution of 50 degrees between adjacent speaker positions outside of the front L/C/R sector all the way around the playback area. Shifting the front-most surround pair toward the front serves somewhat as "front wides" filling the gap between the L/R speakers and the surround speakers. Given my stated goals, I'm quite pleased with this arrangement in several incarnations indoors. I'm currently working up a portable outdoor version of the system using small satellite speakers all around, bass managed to a pair of stereo subs placed at the +/-90 degree locations. That is an interesting problem in that it both avoids problematic room issues while at the same time receives no beneficial room interactions.
Hope you all find this interesting, and perhaps applicable generally, even though it's a unique solution tailored to a specific goal that differs from what typical home cinema setups are aiming for.
