^{4}An 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...