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You may know the reason why but for those that don’t… Bernoulli’s Principle, think thumb over a garden hose. The effect is most pronounced with low frequencies but occurs at all frequencies. We perceive loudness as an increase in pressure.

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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.

Light reflectors (e.g., for bikes) are also made out of many little corners to reflect the light to its source.

For sound, it should be the same with a corner of a room, it ~doubles the sound in the vicinity of the corner, hence a ~3dB increase

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