Mono Compatibility
Mono compatibility describes how well a stereo master survives when the left and right channels are summed to mono.
Many playback situations still collapse stereo partially or completely: phones, smart speakers, club systems, broadcast paths, public spaces, and accessibility devices. A master can sound wide and impressive in headphones but lose vocals, bass, or important effects when summed.
Why Width Can Break
Stereo width often comes from differences between left and right channels. Some differences are healthy: panned instruments, room reflections, or natural stereo recording. Other differences create phase cancellation, where left and right partly cancel each other out when summed. This happens when the two channels are very dissimilar, or "decorrelated".
When left and right are summed, opposite-polarity or highly decorrelated content can cancel. The result can be:
- Vocals become weaker.
- Bass loses focus.
- Reverb or effects disappear.
- High frequencies feel hollow.
- The whole master becomes smaller than expected.
Correlation
Correlation is a single number, from +1 to -1, that estimates how similar the left and right channels are. Meters display it so you can watch it while you work.
| Correlation | Practical reading |
|---|---|
| Near +1 | Very similar channels; mono-safe but possibly narrow. |
| Around 0 | Wide or decorrelated material; check mono carefully. |
| Below 0 | Strong phase opposition; mono cancellation risk. |
Correlation is not a full answer. Some wide mixes are safe, and some narrow mixes still have local phase problems. Always listen in mono when width decisions matter.
Mastering Tradeoff
Stereo widening can make a master feel larger, but it should not weaken the center. Kick, bass, lead vocal, and snare usually need a stable mono foundation. Low frequencies are especially sensitive; widening the low end can make playback inconsistent.
Use width as a finishing move, not a way to fix arrangement problems.
Verification Order
A/B in stereo first to verify that the added width is musically useful. Then switch to mono and check that lead vocal, kick, bass, and snare are still anchored. Look at correlation last: even when the meter looks safe, back off if the central elements weaken in mono.
Widening tends to flatter on first listen, so it is important to compare short, loudness-matched phrases. If the side energy increased while the center merely feels smaller, that is a balance shift in disguise — not an actual mastering improvement.
In libsonare
The mastering demo shows phase/correlation-style information and exposes stereo width in Quick and Studio modes. Use the width control conservatively, then compare against the reference and check the correlation reading.
Implementation notes
Correlation is a practical indicator computed from a short range of the left and right channels. Values near +1 mean the channels are very similar, values around 0 mean more independent side material, and negative values mean stronger opposite-polarity content.
The demo's stereo width control is meant for finishing adjustments that preserve the center. Widening that weakens low end or the main center elements is likely to fail mono playback, so the control should be used conservatively.
Related: Reference Track, Dynamic Range, Mastering