Stereo Image Processors in Mixing and Mastering

What Are Stereo Image Processors?

Stereo image processors are plugins that let you control the stereo width of your mixes or masters. At the most basic level, they do two things: make your audio wider or make it narrower. That sounds simple. In practice, the decisions you make with a stereo imager can make or break the mix.

A well-placed widener adds a sense of space, scale, and excitement to a track. A badly-placed one causes phase problems and destroys the low end. Your music then sounds worse the moment someone plays it on a phone speaker or a club PA that folds the stereo image down to mono.

This is why engineers treat stereo image processors as a last-resort tool rather than a default habit. You use them when the mix actually needs it, and you stop the moment you have gone far enough.

Why Stereo Width Matters (And Where It Goes Wrong)

Before we talk about specific plugins, let’s talk about what can go wrong when you widen a track.

The way most stereo wideners work is by increasing the difference between the left and right channels. That creates a sense of width. The problem is that when left and right get too different, their signals start cancelling each other out when played back in mono. This is called phase cancellation. The symptoms are ugly: loss of low end, loss of vocals, loss of clarity. In the worst cases, entire elements disappear when your track plays on a speaker that sums to mono.

Phone speakers sum to mono. Many Bluetooth speakers sum to mono. Club systems often sum the sub range to mono. TV speakers sum to mono. If your mix falls apart the moment it hits one of these, you have a stereo image problem.

The tool that keeps you safe from this is a correlation meter. When the meter reads on the positive side (close to +1), your mix is mono-compatible. Negative values mean phase cancellation is already happening. Always have a correlation meter open when you are working on the stereo image, and keep an eye on it the whole time.

The Stereo Image Processors I Use in My Sessions

Here are the plugins I reach for when I need to shape the width of a mix or master. This is what lives in my own sessions, not a “best of” list.

Pro Tools Stereo Width (Stock)

The Stereo Width plugin that comes with Pro Tools was the first widener I ever used, and it is still solid today. What I like about it is that you can affect the stereo width of highs, mids, and lows independently. You can also pick between a few different settings and see if any of them work for your material. For engineers starting out, this is a great first stop. It is already on your system and it teaches you what stereo manipulation actually sounds like.

Brainworx bx_control V2

The Brainworx bx_control V2 is a powerful utility plugin with multiple functions, and stereo width is one of them. One feature I use often is Mono Maker, which forces everything below a certain frequency back to mono. I usually apply it to the sub range on masters. This keeps the low end tight and centred while the rest of the mix breathes wider.

Mono Maker is also a great diagnostic tool. If summing your low end to mono makes the mix sound better, that tells you there was phase junk down there you were not hearing. Fix it upstream.

bx_control V2 also has a built-in correlation meter. That is handy when you want to widen and monitor phase at the same time without opening a separate plugin.

Waves Center

The Waves Center plugin is different from most wideners. Instead of just adjusting left-right balance, it gives you independent control over the center and the sides of the stereo image. You can push the side content wider or pull it in. You can also increase or reduce what is sitting dead centre.

The Waves Center is a tool where your source material dictates everything. On some mixes, pushing the sides up opens the track beautifully. On others, the same move makes the whole thing feel hollow and weak. Listen carefully and A/B constantly.

iZotope Ozone Imager 2

The Ozone Imager 2 from iZotope is another widening plugin I reach for. It works both as part of the full Ozone suite and as a standalone. It is clean, visually clear, and the width control is predictable, which matters when you are making small adjustments on a master. If you want a deeper look at the Ozone, I covered the Maximizer in my iZotope Ozone 9 Maximizer review.

Waves S1 Imager

The S1 Imager from Waves is a classic, and it still delivers good results when used with restraint. It is on the older side in terms of interface, but the algorithm holds up well. If you already own a Waves bundle, chances are you have it.

Leapwing Audio Stage One

Leapwing Audio Stage One is my current go-to for stereo image processing. It goes beyond width alone. With Stage One you can control width and depth. You can spread the mono information. You can also adjust the center balance if you have received an unbalanced mix.

That last feature is a quiet lifesaver. Sometimes a client sends you a master with the vocal sitting slightly to one side, or the kick not quite centred. Stage One lets you pull it back without having to go back to the mix. For mastering engineers dealing with imperfect source material, that is gold.

Stage One is what I now reach for first when I need to do serious stereo image work.

How to Use Stereo Image Processors Without Wrecking Your Mix

Here are the rules I follow every time I touch a stereo imager.

1. Always Have a Correlation Meter Open

Non-negotiable. If you do not have a correlation meter open somewhere on your bus, you are flying blind. Keep the reading on the positive side (close to +1 is ideal, and you should never sit in the negative zone for long). The moment the meter dips negative, you have pushed too far. Pull back.

2. Keep the Low End Mono

Sub frequencies and phase cancellation are a terrible combination. On most masters, I force everything below roughly 100-150 Hz back to mono using bx_control’s Mono Maker or a similar tool. This keeps the low end tight and powerful, and it is what every well-mastered record does.

3. Less Is More

Stereo imagers are seductive. They make mixes feel bigger, more exciting, more separated. But that does not always mean better. A wide mix with no focal point is worse than a tighter mix where the vocal and the groove are clearly in charge. Ask yourself constantly: is this wider version actually more musical, or just more impressive-sounding for about 30 seconds?

4. Bypass Often

A/B the stereo processor against the bypassed signal at matched volume. If the processed version is clearly better, keep it. If it is the same or worse, take it off. This is the same rule I apply to exciters and saturators, and it applies here for the same reason: these tools are easy to abuse.

5. Check in Mono

At some point in the session, sum your mix to mono and listen. If major elements disappear or lose punch, you have a phase problem that will hurt real-world playback. Fix it before you deliver.

6. Trust Your Monitoring

Stereo width decisions are only as good as what you hear. If your room is giving you bad stereo information, you will make bad stereo decisions. My studio listening environment guide covers how to set up monitors and calibration so you can actually trust the stereo image you are working with.

When You Do Not Need a Stereo Image Processor

One of the best pieces of advice I can give you: sometimes you do not need a widener at all.

If the mix that came to you already has a strong, well-balanced stereo image, adding a widener on top often makes it worse. The sense of space was built during the mixing stage through panning, delay, reverb, and chorus effects on individual elements. Trying to artificially widen the whole thing again at the master stage is like adding salt to food that is already seasoned.

A good habit is to listen to the mix in full before you reach for any stereo tool. If the stereo image feels right, move on. If it feels narrow or off-balance, then it is time to open a widener. But the default should be “probably not needed” rather than “always useful.”

Common Mistakes When Using Stereo Image Processors

• Not using a correlation meter. This is the number one mistake. You cannot fix what you cannot see.
• Widening the sub range. Wide lows cause phase problems on almost every playback system. Keep the bottom mono.
• Pushing width as a rescue for a dull mix. If the mix feels lifeless, width is not the fix. Look at EQ, compression, and saturation first. A wider dull mix is still a dull mix.
• Never checking mono compatibility. If you have not listened to your mix in mono, you do not actually know what it sounds like on half the devices out there.
• Comparing processed vs processed. A/B your widened version against the original dry signal, not against an earlier widened version, or your ears drift further and further wide.
• Using a widener to chase a commercial sound. Commercial records do not all sound wide. Many of them are remarkably mono-compatible. Width is a tool, not a target.

Why Stereo Image Processors Are Secret Weapons

Here is the honest truth. Stereo image processors are one of the tools that separate amateur mixes and masters from professional ones, but only when used with restraint. Engineers who understand them use them sparingly and intentionally. Engineers who do not either avoid them entirely or overuse them to the point where the mix falls apart the moment it leaves the studio.

The middle ground, the place where these plugins actually earn their place in your chain, is where mixing and mastering engineers quietly live. It is worth learning the terrain.

Key Takeaways

• Stereo image processors let you control how wide or narrow your mix or master feels. Used well, they add space and scale. Used badly, they cause phase cancellation and mono collapse.
• Always have a correlation meter open when you touch a stereo imager, and keep the reading on the positive side.
• Keep the low end (roughly below 100-150 Hz) in mono to protect your mix from phase problems on real-world playback.
• The plugins I use regularly: Pro Tools Stereo Width, Brainworx bx_control V2, Waves Center, Ozone Imager 2, Waves S1 Imager, and Leapwing Stage One (my current favourite).
• Sometimes the right call is to not use a widener at all. If the mix already feels balanced, leave it alone.

Related Articles

• Harmonic Exciters and Saturators in Mixing and Mastering. The other “exciting” category of plugins that can ruin a master if overused. Same principles apply: subtle use and constant bypass checks.
• How to Use EQ in Mixing. Proper EQ decisions fix a lot of the problems people try to solve with stereo wideners.
• How to Use Compression in Mixing. Dynamic control sits at the heart of every good mix. Width decisions work better on a well-controlled mix.
• How to Set Up Your Studio Listening Environment. Stereo decisions are only as good as your monitoring. This guide covers monitor placement and calibration.
• The Mixing & Mastering Cheat Sheet. A free one-page reference of the checks and moves I use on every session.

Ready to Go Deeper?

Knowing what a stereo image processor does is one thing. The real skill is knowing when to use one, which plugin fits the material, and how to spot the moment you have gone too far. That kind of judgment comes from sessions, not from blog posts. I put together three programs to cover the whole journey, depending on where you are right now.

About the Author

Adrian Milea is a mixing and mastering engineer, music producer, and educator with over 25 years in the industry. His work has been released on Armada Music and featured in DJ Mag. Through adrianmilea.com and his online courses, he teaches the technical and business sides of making records that compete at a professional level.

Until next time, Stay Cool and Make Great Music!
-Adrian Milea