What disappears in mono and why
Mono playback commonly sums left and right. Components that are similar reinforce; components with opposing polarity or certain phase relationships can attenuate. The result may be a quieter ambience, a hollow instrument or, in an extreme case, a nearly missing signal.
That does not mean every difference between channels is wrong. Natural stereo recordings and deliberate effects rely on inter-channel differences. The question is whether the important musical information remains useful after the intended downmix.
Read correlation with context
A stereo correlation meter often ranges from positive similarity through zero toward negative opposition. Sustained negative readings deserve investigation, while brief movement around zero can be normal for spacious material.
Because one number compresses frequency and time, inspect which section and which band causes the change. A wide high-frequency reverb may be acceptable while a negatively correlated bass can create unstable low-end translation.
Use a repeatable mono check
Monitor a proper left-plus-right mono sum and compensate its level so louder does not automatically sound better. Check vocal intelligibility, kick and bass relationship, snare impact and any hook carried by stereo effects.
Also audition one speaker rather than only routing identical mono to two speakers; two-speaker mono still creates room interactions. Headphones help expose details, but a loudspeaker check shows acoustic translation.
Fix locally before narrowing globally
If one synth vanishes, revisit its unison, delay or stereoizer. If low frequencies wander, make the relevant source or band more coherent. Reversing polarity may solve a true polarity inversion but will not correct arbitrary time or phase offsets across frequency.
Global width reduction is a valid mastering choice when the stereo image is broadly unstable, but it also changes every healthy element. Make the smallest change that restores reliable translation and compare the result in stereo again.
METHOD
How this guide was prepared
The workflow uses standard sum-to-mono and inter-channel inspection. ITU-R BS.775 is referenced for the broader principle of compatibility across stereo and monophonic reproduction, not as a song-mastering target.
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PRIMARY SOURCES