Mid/Side: Sum, Difference, and the Stereo Field

What Is Mid/Side?

A standard stereo signal has a left channel and a right channel. Mid/side is a different way to describe the exact same information:

• Mid (M) = everything that’s the same in both channels. The center image. Mathematically: (L + R)
• Side (S) = everything that’s different between the channels. The width. Mathematically: (L - R)

Mid is what both ears agree on. Side is what they disagree on. Every stereo signal contains both, in varying proportions.

M/S as a Recording Technique

Two mics, two roles:

• Mid mic: Cardioid, pointed straight at the source. Captures the direct sound — the center of the image.
• Side mic: Figure-8, pointed sideways (perpendicular to the mid mic). Captures the room ambience arriving from left and right. The front of the figure-8 hears the left side; the back hears the right side.

We teach mid/side not just because it sounds good — though it does — but because understanding it forces you to wrap your head around how the fragile elements of the stereo field intersect. The technique is mono-friendly by design, the width is continuously adjustable after the fact, your mid mic isn’t off-axis, and your two mics don’t need to be matched. It’s also an efficient way to capture something like an array of three horn players with just two microphones.

Decoding

To turn these two signals into a usable stereo recording:

1. Duplicate the side mic track
2. Flip the polarity of the duplicate
3. Pan the original side hard left, the flipped copy hard right
4. The mid mic stays centered

Now you have three faders: Mid (center), Side-L, Side-R. The magic of M/S recording is that the width is continuously adjustable after the fact. Turn up the side faders for a wider image. Turn them down for a narrower one. Mute them entirely and you’re in mono. No other stereo technique gives you this kind of post-recording control over width.

Sum and Difference

The math behind M/S is the same math behind balanced audio and common-mode rejection (Chapter 3):

• Sum (L + R): Everything common to both channels survives. Everything different cancels.
• Difference (L - R): Everything different between channels survives. Everything common cancels.

When you collapse a stereo mix to mono, you’re hearing the sum — the mid. Anything that only existed in the side channel vanishes. This is why mono compatibility matters. If you pan a reverb return hard left and right with nothing in common between the channels, it disappears in mono. Phones, earbuds worn one-sided, club PA systems summed to mono, laptop speakers — all of these are mono or near-mono playback situations.

Center Cancel

Flip the polarity of one channel and sum to mono. The center — everything identical in L and R — cancels to silence. What’s left is the side information: room reverb, hard-panned instruments, stereo effects. This is how karaoke machines remove lead vocals (or try to). It works because most vocals are panned dead center.

Correlation and Phase Metering

A correlation meter tells you how similar your left and right channels are:

• +1 = perfectly correlated. Identical signals. Pure mono.
• 0 = uncorrelated. Completely independent signals. Maximum perceived width.
• -1 = perfectly anti-correlated. One channel is the exact inverse of the other. This cancels to silence in mono.

Most music sits between +0.3 and +0.8. If your correlation meter hovers near 0 or dips negative, your mix has phase problems and will lose energy when summed to mono. Check regularly.

The Goniometer

Pronounced go-nee-AH-meh-ter.

A visual representation of the stereo field. The vertical axis is mid (center), the horizontal axis is side (width). A healthy stereo signal looks like a football shape — taller than wide, with some spread. A pure mono signal is a vertical line. A pure side signal is a horizontal line. If you see a circle, you have equal mid and side energy — which usually means the mix is wider than it should be.

M/S Processing in Mixing

You don’t need M/S microphones to use M/S processing. Any stereo signal can be split into mid and side components, processed independently, and recombined:

• EQ the sides differently from the mid. Brighten the sides for air and width. Cut low frequencies from the sides to keep the bass image centered and solid. This is a standard mastering move.
• Compress the mid and side independently. Tighter compression on the mid for punch and focus. Lighter compression on the sides to preserve the sense of space.
• Adjust width. Simply change the balance between the mid and side levels. This is the most transparent way to widen or narrow a stereo mix.

Pan Law

When a mono signal is panned to center, it plays equally from both speakers — which means the same energy hits both ears. This makes it perceptually louder than when it’s panned to one side. Pan law is the automatic level compensation your DAW applies to account for this. Different DAWs use different pan laws (-3 dB, -4.5 dB, -6 dB). It affects how loudness changes as you move a signal across the stereo field. Most people never change the default, and that’s fine — but knowing it exists explains why panning something to center can feel like it gets louder.

What to Practice

1. Decode a stereo track into M/S. Take any finished stereo mix. Duplicate the track. On the duplicate, flip the polarity of one channel and sum to mono. What you hear is the side information — everything the center cancel removed. Compare it to the original.
2. Try M/S EQ. Take a stereo mix bus and insert an M/S EQ plugin (most modern EQs have an M/S mode). Cut the lows from the side channel below 200 Hz. Notice how the bass tightens without losing overall width. Now boost the highs on the side channel. Hear the air open up.
3. Watch the correlation meter. Pull up a correlation meter on your mix bus while you work. Notice what happens to the reading when you pan things wide, when you add stereo reverb, when you sum to mono. Get a feel for what healthy correlation looks like.
4. Set up M/S mics (if you have them). A cardioid and a figure-8 mic. Position them as described above. Record a source and decode using the three-track method. Experiment with the side level — go from pure mono to very wide. Hear how the room changes.