I’ve not heard it described that way before; I would love to hear more of what you mean by this. The way I learned it in my upper level acoustics courses in col

I’ve not heard it described that way before; I would love to hear more of what you mean by this. The way I learned it in my upper level acoustics courses in college was image sources: hard surfaces are effectively mirrors for sound, so just like you see a copy of an object in an actual mirror, you hear a copy of the speaker coming as if from inside the wall. With three surfaces meeting in the corner, you effectively get 8 sound sources instead of just one. For low frequencies these virtual sources constructively interfere and make louder sound. Of course, this is just a more intuitive way of thinking of the effects of boundary conditions to a partial differential equation.