Definition
A mix is mono compatible when, by summing left and right into a single channel (L+R), it does not lose elements, volume or timbre in a significant way. Mono is not a relic of the past: it is what smartphones play (many have a single speaker or two very close together), single Bluetooth speakers, many club sound systems (the lows are often mono, sometimes the entire system), stores, single-channel car radios, smart speakers.
Why something disappears in mono
In mono, what is identical in both channels (the Mid channel, M = L+R) adds up; what is out of phase (the Side channel, S = L−R) cancels. Elements that depend on differences between L and R weaken or vanish:
- stereo wideners that invert the phase of part of the signal;
- the Haas effect (delayed copy on one side): comb filtering;
- very wide chorus and unison;
- very “side” reverbs (the tails shrink: in mono the mix sounds drier);
- stereo recordings with spaced microphones (spaced pair) summed with delays.
Measuring tools
- Phase correlation meter: from +1 (perfect mono) to −1 (total out of phase). A healthy mix generally sits between +0.3 and +1 with excursions; values consistently below 0 indicate problems.
- Goniometer / vectorscope: a narrow vertical shape = mono; wide = stereo; horizontal = out of phase.
- M/S analyzer: compares the spectrum of the Mid and the Side; the Side below 100–150 Hz should be almost absent.
- In FL Studio: Wave Candy (oscilloscope/vectorscope and spectrum), Fruity Stereo Shaper to sum to mono; external: iZotope Insight, Voxengo Correlometer (free), Waves PAZ.
Practical rules
- Lows in mono: kick, bass, sub, 808 in the center and mono below 100–150 Hz. Many people mix with a “mono maker” on the bass bus (Side cut below 120 Hz).
- Main elements in the center and solid: vocal, snare, lead: mono or with stereo that does not depend on phase.
- Widen with different signals (real doubles, two takes, two different synths) instead of phase tricks: real doubles sum in mono without cancelling.
- Haas with caution: very short delays with an HP on the copy, or prefer stereo delays and reverbs.
- Check in mono every so often during the mix, not just at the end. Many engineers do part of the balancing in mono: if the mix works in mono, it opens up better in stereo.
- Listen on a single speaker (even the phone).
Example in A minor
Future bass drop: supersaw on Am9 – Fmaj9 – C – G6 with 9-voice unison and a widener at 150%; in mono the supersaw drops by 6 dB and loses attack. Solution: widener brought back to 100%, unison at 7 voices with a “blend” that strengthens the center voice, a second mono layer (single filtered saw) that holds the chord in the center. In mono the supersaw now drops by 1.5 dB: acceptable. 808 sub on A1 checked: Side absent below 120 Hz.
Mono as a mixing tool
- Balancing in mono: masking problems are more audible; if you solve them in mono, the mix is clearer in stereo.
- EQ in mono: useful for finding collisions between elements in the center.
- Reverb: check that it does not disappear completely in mono (some very wide reverbs do).
Common mistakes
- Stereo widener on the master: risk of cancellations in mono and problems when mastering for vinyl.
- Extreme panning of fundamental elements (hi-hat or guitar only on the left): on mono systems they don't vanish but the balance changes.
- Thinking that “nobody listens in mono anyway”.
Exercise
- Put a correlation meter on the master of your latest mix and note the moments when it falls below 0.
- Listen to the mix in mono on the phone and on a single monitor: make a list of the elements that change the most and fix the cause.
Related topics
“Stereo, panning and image”, “Phase and polarity”, “Unison, detune and supersaw”, “Psychoacoustics in mixing”, “Mixing for social media and smartphones”, “Mastering for clubs”.