Hi Rich, every thing I am doing is based on the information contained in Adrian Celestinos' doctoral thesis.
http://vbn.aau.dk/files/12831869/AC-phd.pdf
The bass produced by the stereo system escapes the room through all six sides of the cube assuming an above ground structure. It may also be absorbed by the room's contents. As a result of this there is no need for the rear woofers SPL output to match that produced the front speakers. The only requirement is that the bass extension of the rear sub-woofers must equal that of the front system. Theoretically, for the front system to produce a planer bass wavefront, many woofers must be recess mounted in the front wall and like wise the cancellation of planer wave before it hits the rear wall theoretically requires another wall full of woofers.
Celestinos' work applies the idea to real rooms in a practical manner. Experimental data shows that two woofers located in the front at the floor/wall boundary and equidistant from the corners will generate a close approximation of a planer wavefront at frequencies below 125Hz. Two subs placed on the rear wall in the same locations as the subs in the front of the room and operated 180 degrees out of phase with the appropriate delay will be enough to create a matching opposing wavefront and efficiently cancel out the bass wavefront before it impacts the rear wall and reflects causing standing waves. In fact according to Celestinos measurements plus and minus 3dB throughout the listening room is possible. This approach can be applied to any room whether rectangular or a perfect cube. The exception is the L shaped room, which of course is what I have to deal with, and he discusses this situation at the end of his paper.
What is not discussed is the effect on the imaging and the improvement in the size of acoustic space created by the reduction in standing wave zones between the listener and the front speaker system. When the standing waves are not present, the recreated acoustic space expands towards the listener to the point that complete envelopment occurs, image precision improves as does depth layering and separation in space of instruments in the sound stage.
I first heard of the idea of using an 180 degree out of phase sub-woofer in the rear of the room to cancel out standing waves from my friend Stan Warren over 25years ago. His idea involved a sub-woofer with phase correcting servo control like the one he designed for RH Labs sub-woofers and delay provided by an analogue bucket brigade line, which was the only available technology at the time, to implement the necessary delay of the signal to the rear woofer.
His explanation for the perceived improvement in sound staging was that the bass wavelengths act as a carrier wave for the the mid and high frequencies. When the complex phase relationships that exist in the Mid/Hf signal that is carried by the bass are degraded or destroyed by the presence standing waves in the room, then the spaciousness and precision of the imaging inherent program material will also be lost.
This can be demonstrated quite dramatically by simply turning off the out of phase woofers in the rear of the room. The re-created space and 3 dimensional image will collapse towards the front of the room and be trapped on the front wall.
When Stan approached lawyer friend with the idea of patenting this concept he was told to put his plans in a paper bag and place them on a top shelf of his closet. The reason being that he could not afford to defend the patent in court when infringing products appeared based on his published patent.
When I told Stan about Celistinos' work he said that as simple an idea as it was, it was remarkable that someone didn't discover and put the concept into to the test before now, especially with the ready availability of inexpensive digital chips to implement the delay with.
Even one woofer in the rear with delayed output is sufficient to realize a marked improvement in spaciousness and bass accuracy even if it does not result in + or - 3dB bass response in the entire listening room.
Paper C which starts on page 50 of Celestinos' work discusses mesurements of the equalization system in the real rooms.
Paper D starting on page 70 outlines his Controlled Acoustically Bass System(CABS), A method to achieve uniform sound field distribution at low frequencies in rectangular rooms. This is where the rubber meets the road. How CABS is implemented and the measured results in two real world rooms are presented. The science underlying the approach easily understandable as explained in this part of the thesis. The graphs are very valuable.
Scotty
