D-
Thanks for the clarification on S/N which I should have remembered but had forgotten about. I suppose that's because unless the noise floor is audible that aspect is not perceptually apparent, whereas the increase in direct sound pickup relative to reflected sound is immediately obvious at all levels.
And thanks for the warning.. I didn't mean to suggest trying to mount them so as to face the boundary in my attempt at providing some historical background of how it has been done and why. I imagine that unless otherwise compensated for, mounting a capsule in such a way could produce a significant HF peak relative to the gap spacing and capsule surface area. Might that be the source of the high frequency response problems in those designs?
C-
Yes, I mean the microphone diaphragm facing sideways in relation to the surface on which it is mounted.
[edit- like the csm-BLC image 3rd from the top in the post above]Assuming the use of omnis, if both microphones of a stereo pair are mounted on the same surface it acts as a spaced A-B configuration. If mounting each microphone on a separate boundary "plate", those plates can be faced in different directions and like directional microphones will begin to produce level-differences as well as time-of-arrival differences. That directionality will manifest only within the effective bandwidth of the boundary-effect region, determined by the area of the boundary at the low frequency end of that range and the spacing between diaphragm and boundary surface at the high frequency end of things.
A CD case cover is too small to be effective below the kHz region. It would act like the pressure modification attachments used on some omnis to change their presence range and higher frequency response and directionality. Those range from circular disks to spherical attachments. Some of the wooden circular disk ones I've seen are close to the diameter of a CD.
Generally an area of a few square feet at minimum are needed to be effective down through the lower midrange frequencies.
For stereo configurations, other than A-B mounting to a single surface, mounting to squares or rectangular pieces of polycarbonate (Plexiglas) is common, and the two can be arranged in a wedge shape, which achieves a similar angle/spacing relationship to a stereo pair of directional microphones.
Below are a couple examples of how Pierre Sprey of Mapleshade records small ensembles this way-