1
Mono Compatibility
A lot of real-world playback sums stereo to mono, phone speakers, many Bluetooth speakers, club and PA systems. When that happens, anything out of phase between the left and right channels partially or fully cancels: instruments thin out or disappear.
So a master must be checked in mono. A stereo correlation meter dipping below zero flags the danger; wide stereo effects are the usual culprits.
In practice: Flip the master to mono and a wide-processed guitar nearly vanishes, its out-of-phase stereo content is cancelling. Narrow it or fix it in the mix.
Check the master in mono, out-of-phase content cancels when summed.
2
Mono Compatibility and Why It Matters
Every mix should be checked in mono before delivery. When stereo sums to mono, out-of-phase content cancels, elements that rely on stereo width can vanish entirely. Common casualties: stereo-widened acoustic guitar, heavily chorused effects, wide-panned reverb returns.
Mono playback is everywhere, phones, Bluetooth speakers, PA systems. If an instrument disappears in mono, it has a phase problem to fix before the mix ships.
In practice: An X-Y stereo guitar panned hard L/R that goes thin on the mono button: partial phase cancellation, narrow the panning or use mid-side EQ so the mono channel keeps the body.
Check every mix in mono, out-of-phase content cancels, and mono playback is everywhere.
3
Stereo Width and Mono Summing Artifacts
Stereo widening adds "side" (L-R difference) information. In mono, the side cancels completely, because L+R eliminates L-R. So over-widened reverbs lose their spatial quality, wide-panned double-tracked guitars develop comb filtering, and anything living mostly in the side channel disappears.
The Haas effect, a delayed panned copy, is the worst offender: wide in stereo, comb-filtered in mono.
In practice: A Haas-widened guitar gorgeous in stereo but thin in mono: the 15 ms delay comb-filters when summed, use a mono-safe widener or narrow it.
Stereo "side" content cancels in mono, over-widening and Haas tricks comb-filter or vanish.
4
Stereo Width & Imaging
Stereo width is widened (or narrowed) via mid/side balance, stereo imagers, or M/S EQ. The mono bass rule: everything below 100-200 Hz should be mono to prevent phase cancellation on club PAs and Bluetooth speakers.
Widen the top end (5-15 kHz) on the side channel for air, but use a goniometer/correlation meter to ensure the signal does not go too far out of phase (correlation below 0 means partial mono cancellation).
In practice: A narrow close-miked mix that needs more spacious imaging without changing the mix: a stereo widener at 110-120% on the master, plus a 1-2 dB side-channel boost above 5 kHz, mono-check the bass.
Stereo width in mastering: mono below 100-200 Hz, widen above 5 kHz on the side, verify with a correlation meter.
5
Mid-Side EQ: Width and Focus Control
M/S EQ gives you two especially powerful moves. Cutting low frequencies on the Side channel below ~200 Hz collapses the bass to mono, making the low end tight and focused. Boosting a high shelf on the Side above ~8 kHz adds width and "air" without brightening the center vocal.
Because you EQ Mid and Side separately, you fix width and focus problems that channel EQ cannot reach.
In practice: A wide stereo guitar with phasey bass that buries the vocal: HPF the Side at 150-200 Hz for mono bass, cut the Side at 1-3 kHz to free the vocal, boost the Side above 8 kHz for air.
Cut Side lows for mono bass; boost Side highs for air, width control channel EQ can't do.
6
Mid-Side Compression: Controlling the Center
Compressing only the Mid channel of a stereo signal controls the dynamics of the center-panned elements, vocal, bass, kick: without squashing the stereo width of the panned instruments and room ambiance.
Standard stereo compression cannot do this: it reacts to whichever channel is loudest, so taming the vocal would also clamp the stereo guitars. Mid-only compression separates the two concerns.
In practice: A vocal-driven mix where the vocal needs taming but the stereo guitars should stay open: 1-2 dB of compression on the Mid channel only.
Mid-only compression tames center elements (vocal, bass) while leaving stereo width dynamic.
7
M/S Processing on the Master Bus
M/S on the master bus is for the cases where the center of the mix and the sides have genuinely different problems. A regular L/R EQ treats both channels the same, so cutting harshness in the vocal also dulls the wide guitars; brightening the guitars also adds bite to the vocal. M/S splits the stereo signal into "everything center-panned" (mid) and "everything panned away from center" (side), and lets you EQ, compress, or saturate each independently.
It is not a widener and not a loudness tool. Boosting the side will partly cancel in mono, so use it surgically, always check mono, and keep it for problems that cannot be fixed in the L/R domain.
In practice: A folk master where the center vocal is harsh and the side guitars are dull: a 2-4 kHz cut on the mid tames the vocal without dulling the guitars; a high-shelf boost on the side opens the guitars without adding bite to the vocal.
Use master-bus M/S when the center and sides need opposite treatment, and always check mono.
8
Phase & Polarity
Two related but distinct ideas. A polarity flip simply inverts a waveform, multiplies it by -1, the same at every frequency (the console's "Ø" button). A phase shift is a frequency-dependent time offset, what you get when two mics sit at different distances from one source.
They matter because when signals combine out of step, frequencies cancel and the sound thins. The 3:1 rule keeps multi-mic phase problems quiet: space the two mics at least three times as far apart as either one sits from its own source. Six inches off the guitar means eighteen inches to the next mic. It is a rule about the gap BETWEEN mics, not about moving the second one further from the band.
In practice: A guitar mic sounds full alone but thin once nearby mics open, their delayed bleed is phase-cancelling it. Flip polarity to test, or time-align the tracks.
Polarity flips the whole wave; phase is a per-frequency time offset.
These lessons are the teach side of knowledge cards from the Studio Mix deck. In the daily plan each one is followed by a quiz, spaced over weeks, so it stays known rather than read once.