Mid-Side EQ Ultimate Guide (Tips, Strategies & Applications)

• What is mid-side EQ?
• Discover its potential uses in mastering and mixing.
• Avoid the problems M/S EQ can introduce.

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I’m sure you’re drawn to this article because you’ve heard that mid-side EQ has the potential to “save your mix”, and you’re probably curious about how to integrate it into your production.

Well, firstly I’m going to have to clarify a few misconceptions around what its purpose is and when to use it.

Mid-side processing is something that is more commonly employed within the mastering domain, when a mastering engineer is handed an audio file that contains some phasing issues or there’s a lack of clarity due to too much stereo spread, especially in the lows and mids.

However, It can be a useful tool in mixing and especially in sound design. Before we jump into those possibilities, let’s address the technical aspects.

What Is Mid-Side EQ?

Mid-side EQ equalizes the mid and side components of a signal independently. This gives us greater control over the stereo image and has other benefits as well.

The mid-channel (M) can be thought of as what is absolutely identical in the left and right channels of a stereo signal. The stereo, or side (S) signal is what is different between the 2 stereo channels.

Formulaically, this is represented as Mid = L + R, Side = L – R. Another way to help cement the concept, is to split the left and right channels of a stereo mix, invert the phase of one channel, and sum it to mono. When doing this, the mid-channel becomes canceled out, leaving only the difference between the left and right channels. Therefore, M/S processors separate the side and mid signals and rejoin them later to regain the stereo image again.

An important note to make here is that Mid/Side-splitting does not offer a perfect separation of the mid and the sides, there is always some overlap.

If, for example, we have a vocal line panned hard left in a mix and this line has an amplitude of X, the central/mono channel will look like this: Mid = L + R = X + 0 = X, and the side channel will be, Side = L – R = X – 0 = X.

Furthermore, if we had a guitar panned hard right, with an amplitude of Y, it would look like this: Mid = L + R = 0 + Y = Y, however, the side would be L – R = 0 – Y = -Y.

So, yes… you guessed it…. we get a phase inversion. Audio on the far left will be found with the same amplitude in the Mid and Side signals but will be phase-flipped in the Side signal.

This aspect of Mid Side processing is very often overlooked, and really should be taken into account when altering the Mid Side EQ balance.

I know it may seem confusing, but what it really boils down to is that any mid-side processing should be undertaken with a light hand. A general rule of thumb is to make adjustments of less than +/- 4dB in any region. It’s also a good idea to user lower Q values when dealing with M/S EQ.

So What Are The Uses of Mid Side EQ?

M/S EQ is most useful when you want to alter the tone of the spatial qualities of a sound – including what sits in the middle. This might be the case for a mastering engineer who only has access to a single stereo mixdown, but M/S EQ is also useful in the mixing stage, as well as being an important sound design tool in general.

Just to get this out of the way, an M/S EQ is not a substitute for the good old panning knob. Creating a 3D mix requires placement within the stereo field (1st dimension, or X-axis).

The 2nd dimension is pitch/frequency (apparent height = Y-axis) and the 3rd is the time field (delay/pre-delay = Z-axis). This placement in the stereo field cannot be achieved by an M/S EQ.

Mid Side EQ in Mastering

Mid side tools can be used by a mastering engineer when there is too much spatial spread in the lows and mids, or not enough spread in the high-mids and highs.

To return to the opening statement of this article, that M/S EQ can save your mix, I’d like to remind readers that Mastering is not about repairing errors in mixes.

I find this mindset catches many ME’s and distracts them from what they are actually employed to do, focus attention on the beauty and vibe of the song. Oftentimes this may require that distractions from that beauty (“errors”), have to be addressed so that the art can really be showcased.

But the true art of a ME is enhancing the vibe of the mix, not “fixing” it. For this reason, robots will never surpass a great mastering engineer (and to find out why check out this article).

As with any EQ work in mastering, soft touches are your best friend. M/S EQ has the ability to significantly, and drastically, change the track.

On top of the capacity to introduce further phase issues, usually, instruments and sounds are not panned hard to the sides. So M/S processing affects the sounds in unpredictable ways.

Effectively you would be EQ-ing with 2 different processes and then attempting to return them together for a stereo signal. This may be a cool effect, but also it may deviate from the artist’s intention significantly.

You need to make sure that your EQ improves the track by bringing out the vibe, not just making it different.

In truth, I rarely reach for an M/S EQ when mastering, however, I do use stereo imagers (like Izotope’s Ozone Imager) on every track, even if only for metering. Really maybe 1 in 10 tracks will benefit from M/S EQ.

But the times that it is advantageous is when there is too much stereo energy in the low mids (100-500Hz), or not enough in the tops (5 kHz+). I tend to reduce width in the subs (< 80 Hz) with a multi-band imager as well.

So, let’s say you’re working on a mix and it feels ‘soupy’ in the low mids. Attenuating a few dB’s in the problem range on the sides will enhance clarity and focus to that range.

Conversely, if you have a track that sounds thin in that range, you could add width to beef it up without adding new harmonic content (saturating).

I’ll also reach for mid-side EQ if I feel like there isn’t enough high-end ‘space’, ‘air’, or ‘brilliance’. In this case, I may add a few dB to the tops to enhance the sense of 3D space.

Phasing issues in the higher frequencies tend to be much harder to recognize as the waveform is moving very fast, thus presenting fewer phasing problems. So in this range, you can get slightly more dramatic, however, instrument mix balance needs to be your guide as to when it’s too much. So if your EQ choices are just making the track louder and affecting the balance of the instruments, you need to back off.

If there is too much ‘mud’ in your mix, chances are that the reverbs and delays are not EQ-ed independently from the instrument channels themselves. I highly recommend using return tracks (or buses) for reverbs and delays with an EQ placed after to cut low frequencies.

If you’re an Ableton user, there are some free templates available here that will illustrate how to set up independently EQ-ed return tracks for reverbs and delays. This trick will save both yourself and your mastering engineer a headache and give you far cleaner and more focused mixes.

Mid Side EQ in Mixing

Mid Side EQ in the mixing phase of production can be really helpful in creating space in the mix for other elements. Consider this scenario:

I have a vocal that really seems to grab the “ess-es” (5-9 kHz) when it is in the mix, but solo-ed doesn’t seem so sibilant.

Possible answer: The cymbal and hats sum with that vocal in the 5-9 kHz range, bringing up that range in the mix.

Possible solution: take your stereo cymbal (assuming it’s in stereo – like stereo overheads) and use an M/S EQ to pull down that region in the center, whilst leaving the sides intact.

As you can see here, the grey area is the cymbal, red is the vocal (notice that painful sibilance at 8 kHz), the green line is the mid eq curve, and the blue line is the side eq curve.

Taking this to the next level, you would employ a dynamic eq which is side-chained to the sibilances of the vocal to duck that same range of the mid-channel, leaving it intact when the vocal 8 kHz is below your set threshold.

(For a roundup of the best de-esser plugins, check out 7 Best De-Esser Plugins For Taming Harshness.)

Mid Side EQ in Sound Design

M/S EQ is a great sound design tool that can really enhance the spatial qualities of audio. Using mid-side EQ gives you a level of control over the stereo field that you would not otherwise get with just simple panning controls. For example, if you’re using a wide chorus effect that has no control for reducing the width, you can apply a broad cut to the most problematic frequencies in the side channel to rein things in.

Alternatively, sweetening up the high frequencies of the side channel can make sounds much wider without throwing your stereo image completely out of balance.

As with all things sound design, informed experimentation is key. The best way to learn to use a mid-side EQ is to play around with one and listen to how it affects the sound. This will help you make better EQ choices the next time you’re using M/S EQ for your sound design.

Tools Of The Trade

Here are a few of the top M/S EQ plugins available to have a look at and compare. Brainworx’s bx_digital is pretty much the industry standard for mastering engineers, however, Fabfilter, Izotope, and Slate Digital all have very good offerings as well.

If you’re particularly concerned about phase “smearing” and other artifacts created by parametric EQ, you can look at linear phase equalizers.

Linear phase EQs tend to be CPU intensive and can introduce latency, so they are rarely used in mixing.

However, if linear phase EQs are used in the mastering phase of production, and undertaken with light use, the pre-ringing artifacts are minimized. To learn more about linear phase EQ, check out Linear Phase EQ Vs Minimum Phase EQ (Pros & Cons).

Summing Up

There is a huge amount of information on Mid Side EQ-ing available online, via YouTube for example. However, it’s not the snake oil of mix ‘repair’ that it is often touted to be.

A well-mixed track would see limited benefit from the use of M/S EQ, and in most cases would likely be detrimental to the overall final product. So if you’re reaching for M/S EQ to “fix” issues in mastering, it’s best to go back to the mix and deal with the problem at the source.

M/S EQ will work nicely on busses to help add cohesion to multiple, separate elements. It’s also an amazing sound design tool that can really help to shape the overall depth of your sounds. It gives you much better control over the stereo field than just simple panning controls.

There’s a chance your DAW’s stock EQ has a mid/side mode, so check to see if this is the case. Otherwise, you can check out one of the aforementioned plugins such as Fabfilter Pro-Q or Brainworx bx_digital to get you started.