AudioScope
What Disappears When Stereo Folds to Mono?
Hear a two-tone stereo signal fold to mono and lose its higher tone.
What to listen for
A plays the original left channel in both speakers so both tones are audible. B plays the arithmetic average of left and right. The shared 440 Hz tone stays and the opposite-polarity 880 Hz tone cancels. The comparison works on one speaker.