Why should I check the mix in mono?

Because summing both channels cancels any out-of-phase content and it disappears. If your mix loses the bass or a guitar goes dull in mono, you have a phase problem that stereo was hiding. On top of that, plenty of real listening systems are mono or close to it: bluetooth speakers, a phone, a bar's PA.

Should low frequencies always be mono?

It's a widespread convention and there's a reason: low frequencies carry little directional information and a lot of energy, so a wide low end spends headroom and risks cancellation with no perceptual gain. That said, it isn't a law — there are excellent records with some width down low. The prudent default is centring below roughly 100-150 Hz unless you have a specific reason.

What is mid/side processing?

A way of splitting the signal into what's common to both channels (mid, the centre) and what differs between them (side, the edges), so you can process each separately. It lets you, for example, brighten only the sides or compress only the centre. It's powerful, which is exactly why it's easy to overuse.

Why does my mix sound narrower after mastering?

Usually the limiter. Reducing peaks also compresses the differences between channels, and the sense of width rests on precisely those differences. If you need a lot of limiting to reach your target level, width is one of the first things you pay with.

Are stereo widener plugins any use?

They are, but many achieve width by introducing phase shift, and that's exactly what cancels in mono. If you use one, always check the result summed to mono. The safest ways to gain width are genuine panning of distinct elements and double takes, not a processor over a mono source.

Mixing10 min read

Stereo image: panning, width and why you check in mono

Placing each element across the width, what mid/side processing is, when widening hurts, and what a mono check reveals.

Width isn't an effect: it's a layout

Just as the spectrum gets divided by frequency, the stereo field gets divided by width. And it works for the same reason: two elements at the same point compete, two separated elements coexist without touching. The difference is that almost everyone works on the frequency layout and very few people plan the horizontal one, which gets left to whatever the presets did.

The useful starting point is deciding what goes in the centre and why. The centre is the highest-energy position and the one that survives on any system, so it's reserved for what holds the song up: kick, bass, snare and lead vocal. Everything else is a candidate for moving, and the further it goes from the centre, the more room it leaves.

Where to put things

Centre: kick, bass, snare and lead vocal. What defines the song and can't be lost on any system.
Semi-lateral (30-60%): keys, rhythm guitars, backing vocals. There they free up the centre without becoming detached.
Extremes: guitar doubles, decorative percussion, effects. Elements that break nothing if they vanish.
Overheads and room mics: as wide as you like, they're the drums' sense of space.
Doubled vocals: one on each side sounds far wider — and more natural — than a widener over a single take.

A warning about doubles: for them to sound genuinely wide they have to be separate performances, not a copy of the same audio nudged a few milliseconds. Copying and delaying produces comb filtering and, in mono, cancellation — the hollow metallic sound that gives the trick away.

The mono check

It's the habit that prevents the most problems and the one fewest people have. Put a mono button on the master and use it every few minutes. The point isn't for the mix to sound the same — it won't — but for nothing to disappear. If summing loses the bass, or a guitar drops in level, or the vocal recedes, there's out-of-phase content that stereo was masking.

There are three usual causes: two microphones on the same source at different distances, a widener that works by phase shift, and duplicated layers offset in time. All three get fixed at the source, not with more processing on top.

Mid/side without overdoing it

Mid/side processing separates what sounds identical in both channels from what differs, and lets you treat them apart. The uses that pay off are few and specific: centring the low end by filtering the side channel below 120 Hz, adding air only to the sides to widen without touching the vocal, or compressing only the mid to steady the foundation without squeezing the room.

What doesn't pay off is pushing the sides until it «sounds big». You pay for that feeling in mono and at the mastering stage, where the limiter meets a signal with no solid centre left. If you're at that point, it's worth reading the mastering guide before pushing further. And to train your perception of position and width — the part you can't learn by reading — there's Stereo Space.

Frequently asked questions

Why should I check the mix in mono?: Because summing both channels cancels any out-of-phase content and it disappears. If your mix loses the bass or a guitar goes dull in mono, you have a phase problem that stereo was hiding. On top of that, plenty of real listening systems are mono or close to it: bluetooth speakers, a phone, a bar's PA.
Should low frequencies always be mono?: It's a widespread convention and there's a reason: low frequencies carry little directional information and a lot of energy, so a wide low end spends headroom and risks cancellation with no perceptual gain. That said, it isn't a law — there are excellent records with some width down low. The prudent default is centring below roughly 100-150 Hz unless you have a specific reason.
What is mid/side processing?: A way of splitting the signal into what's common to both channels (mid, the centre) and what differs between them (side, the edges), so you can process each separately. It lets you, for example, brighten only the sides or compress only the centre. It's powerful, which is exactly why it's easy to overuse.
Why does my mix sound narrower after mastering?: Usually the limiter. Reducing peaks also compresses the differences between channels, and the sense of width rests on precisely those differences. If you need a lot of limiting to reach your target level, width is one of the first things you pay with.
Are stereo widener plugins any use?: They are, but many achieve width by introducing phase shift, and that's exactly what cancels in mono. If you use one, always check the result summed to mono. The safest ways to gain width are genuine panning of distinct elements and double takes, not a processor over a mono source.