Stereo and spatial processing

Put on headphones and press play. A pad, spread wide across your head, big and enveloping. Now imagine it coming out of a phone speaker, a Bluetooth box or a club system, many of which play everything in mono: left and right added together into one channel.

Before you touch anything, predict: when the two sides are added together, will the pad get louder, quieter, or stay about the same?

With Mix at full, Width 0 removes everything that differs between left and right, which is precisely what a mono speaker does. So the lab doubles as a mono switch. What survives is the part of the sound that both sides share. Everything else was living in the difference, and the difference cancels.

Mid and side

Here's the idea that explains all of stereo processing. Any stereo sound can be described in two ways. One is left and right. The other is mid and side:

• Mid is what the two channels have in common: their average, (L + R) ÷ 2.
• Side is how they differ: half their difference, (L − R) ÷ 2.

Left	Right	Mid	Side	What you hear in mono
+1	+1	+1	0	Everything: the sound is centred
+1	0	+0.5	+0.5	Half: panned hard left, but still there
+1	−1	0	+1	Nothing: the two sides cancel

That last row is the disappearing pad. If the left channel goes up exactly when the right goes down, they're out of phase, and adding them gives silence, even though each one alone sounds perfectly loud.

Real pads aren't usually that extreme. They're a mix: some of the sound shared between the sides, some of it different. The wider they sound, the more is in the side, and the more there is to lose.

What a widener really does

A mid/side widener turns the side up or down and leaves the mid alone. That's the Width control in the lab: at 0.5 the pad is as recorded, below that the side shrinks, above it the side grows.

Notice what that means. Turning Width up makes the pad sound bigger on headphones and does nothing at all to the mono version, because the mono version is the mid, and the mid never changed. Turning Width down doesn't rescue the mono version either. It just makes the stereo version sound more like the mono one.

You might try narrowing a sound that vanishes in mono until it comes back. It won't come back, because it was never in the middle. To fix a sound that disappears in mono, change the sound: add a centred layer (a mono copy of the pad, a simpler version of the chord), or choose a stereo source whose two sides share more. Then use width for the extra shimmer on top.

One practical note: keep Mix at full when you use Width 0 as a mono check. At lower Mix, some of the original side leaks back in.

The same logic shows why a mid/side widener can't make a mono sound wide. If left and right are identical, the side is zero, and turning zero up is still zero. Width has to come from somewhere real: two different takes, a stereo room, a chorus or a delay.

Panning puts a sound in a place

The oldest stereo tool is the pan knob. It sends a mono sound to both sides at different levels, so it appears somewhere between the speakers. Early stereo records were bold about it: plenty of sixties mixes put the drums in one speaker and the vocals in the other, which sounds startling on headphones today.

Panning is the safe way to spread a mix. A panned sound is the same signal in both channels, just at different levels, so it never cancels in mono. It only gets a little quieter or louder as it moves away from the centre, depending on the pan law your software uses.

The short-delay trick, and its price

A popular way to widen a mono part is to delay one side by a few milliseconds. Your ear hears the earlier side first and the sound seems to spread out (it's called the Haas effect). On headphones it can be very convincing.

In mono, though, the sound and its delayed copy add together, and you've built a comb filter: some frequencies reinforce, others cancel.

This is the same comb filter a flanger sweeps on purpose for its jet-plane sound. Here, frozen in place, it just makes the part thin and boxy on any mono system. A correlation meter, if you have one, will often warn you: readings near +1 mean the sides mostly agree; readings that dip below zero mean a lot will cancel. Treat it as a reason to listen, not a verdict.

Same idea, different place: the hollow sound of a short delay in mono is exactly what a flanger moves up and down on purpose. See Chorus, flanger and phaser.

Keep the low end in the middle

Bass is where mono problems hurt most. A kick or bass line that's spread wide can lose its weight on a club system that sums the lows, and vinyl cutting engineers have long kept the bass centred, because big differences between the channels at low frequencies make the cutting stylus move vertically and can make the record skip.

The common habit is a mono, centred low end, with width saved for the upper layers: the bass's grit and harmonics, the pads, the reverb. It isn't a law, but it's the reason the kick and bass in most records hit just as hard on a phone as on headphones.

Make it yours

Take a wide pad and a bass line of your own. Listen to the pad with the lab's Width at 0 (Mix at full): what's left? If the chord's body disappears, add a centred layer, a mono copy or a simpler voicing, and then bring the width back for shimmer on top. Keep the bass in the middle.

Save the arrangement as a project, the pad, its centred layer and the bass together, rather than a widener preset on its own. The width setting only works because of the source underneath it, and next time you'll want both.

Quick check

1. Left and right are exactly equal and opposite. What does a mono speaker play? Silence. The mid, (L + R) ÷ 2, is zero, even though each channel alone is loud.
2. Your pad vanishes in mono, so you turn Width down. Does the mono version get stronger? No. Width only scales the side; the mono version is the mid, which didn't change. Add something to the centre instead, like a mono layer or a different voicing.
3. You widen a mono guitar by delaying one side by 10 ms. What happens on a phone speaker? The two copies add up and comb-filter: a row of notches across the guitar's range, so it sounds thin and hollow. A real double-tracked part or a chorus usually survives better.

Next steps

• Use it: Depth and space in a mix. Width is one side of space; depth is the other.
• Sideways: Low end. Keep the kick and bass solid while the top of the mix spreads out.