What is Mono Compatibility?
Mono compatibility is how well a stereo mix holds up when the left and right channels are combined into one. A mono compatible mix keeps its balance; a mix with phase problems loses instruments or level when summed.
A lot of music is still heard in mono or close to it: phone speakers, some smart speakers, club and venue systems, and audio in some video and broadcast chains. If parts of the mix only exist as differences between the channels, they partly or fully cancel when the channels are added together.
Common causes are very wide stereo effects, out of phase microphones on the same source, and widening plugins that flip the phase of one side. Low frequencies are the most important to keep mono compatible, because the bass and kick carry so much of the energy.
The simple check is to switch your monitoring to mono now and then while mixing, and listen for anything that disappears or changes tone.
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