⁴An intuitive way to consider this is that the closer loudspeaker is at 0^∘on a cosine wave when the more distant loudspeaker is at 90^∘on the same cosine wave (because it’s one quarter of a period late). cos(0^∘) + cos(90^∘) = 1 + 0 = 1, so you might as well turn off one of your loudspeakers if you’re just thinking about combined output level...