Quick update: I did look up boundary effects in my old text late last night. Fundamentally, in addition to being primarily concerned with EM waves and not pressure waves in my day job thus totally blowing the reflection question, where I was confused was in the difference between sound
pressure level (SPL to most of us) and sound
power levels. A subtle but critical distinction. I have a couple of texts that go through this rather thoroughly but the equations can be daunting unless you are comfortable solving multidimensional wave equations. I once was, sort of, but not today. I actually have some papers and programs that do that, but balked at trying to post such dribble here. The folk who understand it probably understand it much better than I, and folk who have not seen it before would find it difficult to follow without pages of explanation. I wanted a simpler presentation.
So instead I looked through a few other books in my home office and hit gold in Dr. Toole's
Sound Reproduction, first edition (I assume the same info is in the third edition but my copy is two floors away and I'm lazy; the second edition is essentially a repackaged copy of the first). He has nice pictures and an explanation in
Chapter 12: Adjacent-Boundary and Loudspeaker-Mounting Effects. Figure 12.1 shows the pressure, intensity, and power radiated in spaces from 4
π (sphere) to
π/2 (corner of the floor or ceiling). <Aside to
@amirm -- is there a way to add the symbol font?>
As you go from no adjacent walls, to one, to two, to three, the pressure level at a given point (radius) goes from 0 dB to 6 dB to 12 dB to 18 dB (+6 dB per boundary). For the same conditions, the power radiated goes from 0 dB to 3 dB to 6 dB to 9 dB (+3 dB per boundary). Note these are absolutes; my assumption for the previous posts was start from 2pi (one wall) so the gain of adding two and three walls (boundaries) was 3 and 6 dB (I am sure there must be a way to mess this up further; I plead lack of attention of excessive tiredness after a 60-hour workweek that is not over yet). This is adjacent-boundary work and does not really matter the frequency.
I don't have time to write more, and have more than proven my lack of expertise in this area (or at least poor memory -- I usually look things up and now you know why!) so instead will direct you to Dr. Toole's book. If you really want the math, there's the
Fundamentals of Acoustics book by Kinsler et. al. I referenced before, along with numerous others. But, I will put in my plug for Floyd's third edition. I have not gotten through it completely yet but it is greatly expanded from the first edition and accessible to anyone with high-school math skills (and you can get away without the math for the most part). And since Dr. Toole contributes here you're better off asking him directly instead of going through me.
Meanwhile, I'm watching the snow pile up after I cleared the walk and drive earlier; it was supposed to be essentially done, but I'm going to have another couple of inches to clear. Blah. At least it's pretty.
