A stereo mix must survive summing to mono, so mono compatibility must be checked

Most real listening happens in near-mono conditions: laptop and phone speakers cluster together or are mono, club and PA centre feeds sum to mono, TV and hold systems are mono, listeners sit outside the stereo sweet spot, earbuds get shared, and hearing impairment is common — so much of a mix’s stereo information is lost at playback. Summing stereo to mono raises centre-panned sounds roughly 3 dB relative to edge-panned sounds, so any mix that leans on stereo position for level balance becomes imbalanced in mono. Worse, phase problems cancel: polarity-inverted stereo content — spaced-microphone recordings, phase-based stereo wideners — partially or wholly disappears or comb-filters when summed. Checking mono compatibility remains indispensable; engineers should flip regularly to mono while making mix decisions.

Examples

A lead synth hook panned hard right and a centred bass: summed to mono the centred bass rises ~3 dB relative to the hook, changing their balance. A synth widened by one-sided polarity inversion can vanish entirely in mono; a spaced-pair drum overhead comb-filters when summed.

Assessment

A mix has the lead synth hook panned hard right and the bass centred. What happens to their apparent level relationship in mono, and what should you do? Also name two categories of stereo processing that cause the worst mono problems and explain the mechanism of each.