My caveman-level understanding of boundary mounting a mic is that (assuming the mic is properly mounted so it's close enough to the boundary) it reinforces the sound coming towards the boundary by reflecting it back to the mic. So what I'm wondering about is how the angle between the boundary surface and the direction of the sound source comes into play. It would seem to me that when the boundary surface is 90* perpendicular to the sound source, the sound arriving directly from the source is reinforced to the greatest degree possible, and when the boundary surface is parallel with the direction of the sound source there would the least amount of reinforcement. If that's correct, then the amount of boundary surface effect would increase as the angle of the surface is turned back closer to 90* perpendicular, right?
With respect to the above I'm basically ignoring reflected sounds from other surfaces in the room. But when those are taken into account, they would also be reinforced by the boundary, right? So, if the boundary surface is parallel to the direct sound source, there would be minimal boundary effect with respect to the direct source, but maximum effect with respect to sound reflected off a side wall, right?
I realize my understanding of all this is "like chimps at their first fire." The information I've been able to glean from Googling hasn't quite cleared up the things I ask about above though (obviously).