Why it matters
Many listening situations are mono or nearly so: Bluetooth speakers, smartphones with a single speaker, club systems with a single subwoofer, radio, public venues. When a mix is summed to mono (L+R), everything that is the same in both channels is reinforced, while whatever is in opposite phase cancels. A mono-compatible mix keeps its balance, vocal and low end when summed.
Phase correlation
The correlation meter measures how similar the two channels are, from +1 to −1:
| Value | Meaning |
|---|---|
| +1 | Perfect mono (L = R) |
| +0.5/+0.9 | Narrow stereo or a mix with a lot of center content |
| +0.2/+0.5 | Wide but compatible stereo |
| 0 | Independent channels (two uncorrelated signals) |
| below 0 | Prevalence of out-of-phase content: volume is lost in mono |
| −1 | Identical channels with opposite polarity: in mono the signal disappears |
A commercial mix averages between +0.2 and +0.8, with brief lower passages on wide effects. The vectorscope (Lissajous) shows the same information as a shape: a vertical line is mono, a wide cloud is stereo, a horizontal cloud indicates phase opposition.
Typical causes of problems
- Haas effect (a delay of a few ms between channels): comb filter in mono, loss of certain frequencies.
- Psychoacoustic wideners and decorrelators.
- Inverted polarity on one channel of a stereo track (cable, recording, plugin).
- Spaced stereo microphones (AB) with a large distance: cancellations in the bass in mono.
- Synths with heavily detuned unison or with stereo oscillators in opposite phase: bass that vanishes in mono.
- “Wide” 808 and sub samples with stereo in the bass.
- Reverbs with strong decorrelation on the bass.
Verification procedure
- Put a correlation meter and a vectorscope on the master (Wave Candy in Lissajous/Vectorscope mode, Ozone Imager, Fruity dB Meter for levels).
- Listen in mono with a plugin on the master (Fruity Stereo Shaper with the four L/R controls at 50%, or the Stereo separation knob fully to the right) or with the mono button of the interface.
- Listen at low volume in mono: vocal, kick, snare and bass must stay where they are; it is acceptable for reverbs and pads to drop a little.
- Find the responsible track: solo the buses one at a time while watching the correlation meter.
- Analyze the low end: below 150 Hz the correlation must be close to +1.
Solutions
- Mono bass: HP on the Side at 100–150 Hz on the bus or the track (M/S EQ), or a multiband imager at 0% below 120 Hz.
- Haas: longer delays (beyond 25–30 ms they become echoes and the comb filter is less noticeable), or replace with double takes or light chorus.
- Polarity: invert the polarity of the wrong channel (Fruity Stereo Shaper, the invert option on the mixer track).
- Synths: reduce the unison spread, set the sub oscillator to mono, enable the “mono bass” mode if the synth has it (Serum, Vital).
- Spaced microphones: time-align the microphones or use a coincident pair (XY) as the base.
- Reverbs: HP on the returns at 150–300 Hz.
Target values by element
| Element | Correlation |
|---|---|
| Kick, 808, bass, lead vocal | +0.9/+1 |
| Drum bus (with overheads) | +0.5/+0.9 |
| Pads, stereo synths, doubled guitars | +0.1/+0.6 |
| Reverb returns | 0/+0.5 |
| Finished mix | +0.2/+0.8 on average, below 150 Hz +0.9/+1 |
Common mistakes
- Chasing a correlation of +1 to be safe: the mix becomes flat and lacks depth.
- Checking mono compatibility only at the end of the mix: the cause is hidden among many instruments.
- Thinking that headphones reveal phase problems: on headphones the two channels do not sum.
- Widening the bass to make it sound bigger: in clubs and in mono it vanishes.
Starting settings
| Parameter | Value | Why |
|---|---|---|
| Average mix correlation | +0.2/+0.8 | Open, compatible stereo. |
| Correlation below 150 Hz | +0.9/+1 | Stable low end. |
| Side HP | 100–150 Hz | Mono bass. |
| Haas | Beyond 25–30 ms or replaced by double takes | Less comb filtering in mono. |
| Loss in mono | Maximum 1.5–2 LU | Balanced mix even on a single speaker. |
Frequencies to watch
| Range | What to do |
|---|---|
| Below 150 Hz | Correlation close to +1: kick, bass, 808, sub, synth basses. |
| 150 Hz–1 kHz | Cancellations from spaced microphones and Haas: check in mono. |
| 1–8 kHz | Comb filters from wideners: the supporting vocal or synth “sounds like a phone” in mono. |
| Above 8 kHz | Naturally wider content: lower correlation is acceptable. |
Example chain
| Step | Tool | Settings | Why |
|---|---|---|---|
| Analysis | Wave Candy (vectorscope) / Ozone Imager | Correlation and Lissajous | Identify the problems. |
| Mono check | Fruity Stereo Shaper / Stereo separation | Mono sum | Listening in mono. |
| Correction | M/S EQ, polarity inversion, multiband imager | Side HP 100–150 Hz | Mono low end. |
| Verification | Stereo and mono export comparison | Loudness and balance | Difference ≤ 2 LU. |
How to check the result
- Mix correlation (the “correlation” field) between +0.2 and +0.8; below 0 = a phase problem to track down.
- Side/Mid width in the Sub and Bass bands at −20 dB or less.
- Side/Mid width in the Mid and High-mid bands not above −3 dB: beyond that, a risk of cancellations in mono.
- Mono (sum) and stereo exports compared: integrated loudness difference ≤ 2 LU; Sub and Bass bands with a difference ≤ 1 dB.
- L/R balance within ±0.5 dB.