What it is
A stereo L/R signal can be rewritten as Mid (M = (L+R)/2, what is the same in both channels: the center part) and Side (S = (L−R)/2, what is different: the lateral part). A mid/side EQ encodes L/R into M/S, applies different curves to the two components and decodes back to L/R. It lets you change the tone of the center (vocal, kick, snare, bass) without touching the sides (guitars, pads, reverbs, overheads) and vice versa.
The operation is perfectly reversible: with a neutral EQ, M/S → L/R returns the original signal. However, every change also alters the stereo image: raising the highs of the Side widens the top end, cutting the bass of the Side narrows the low end.
Main uses
In the mix (stereo group bus)
- Guitar or pad bus: −2/−3 dB at 2–4 kHz on the Mid to leave room for the vocal in the center, keeping the sides bright.
- Overheads and room: HP on the Side at 150–250 Hz (room bass to the center, more punch), shelf +1/+2 dB at 10 kHz on the Side for wider cymbals.
- Reverb returns: HP on the Mid at 300–500 Hz and a slight cut at 2–4 kHz on the Mid so as not to cover the vocal in the center; the sides stay full.
- Stereo synths and samples: HP on the Side at 100–200 Hz to make the bass mono (essential for clubs and vinyl).
In mastering
- Mono bass: HP on the Side at 80–150 Hz, slope 12–24 dB/oct. It makes the low end stable in mono, on single-subwoofer systems and in lacquer cutting.
- More present vocal: +0.5/+1.5 dB at 1–3 kHz on the Mid, Q 0.7–1.
- Airier mix: shelf +1/+2 dB at 8–12 kHz on the Side. More effective and less harsh than boosting the highs of everything.
- Mud on the sides: −1/−2 dB at 200–400 Hz on the Side when overlapping guitars and keyboards swell the low mids.
- Sibilance in the center: de-esser or dynamic EQ on the Mid only at 5–8 kHz, so the cymbals on the sides stay intact.
Parameters and tools
- In FabFilter Pro-Q every band has the Stereo / Left / Right / Mid / Side mode. Ozone EQ, Waves Center and Brainworx bx_digital have dedicated M/S sections.
- In FL Studio the Fruity Parametric EQ 2 works only in L/R: for an M/S EQ use an external plugin or Patcher with two Fruity Stereo Shaper instances (M/S encode, two EQs, decode).
- Solo Mid / Solo Side: listen to each component before working on it. The Side of a pop mix usually contains reverbs, guitars, backing vocals and cymbals; if you hear the vocal or kick in the Side, there is a phase problem or a stereo effect on the source.
- Mono check: the Side disappears in mono. Anything you boost on the Side is lost in mono listening.
Typical Side/Mid ratio values
| Band | Side relative to Mid in a commercial mix |
|---|---|
| Below 100 Hz | −20 dB or less (practically mono) |
| 100–250 Hz | −15/−10 dB |
| 250 Hz–2 kHz | −10/−6 dB |
| 2–8 kHz | −8/−4 dB |
| Above 8 kHz | −6/−2 dB (the sides are richer in highs) |
Values closer to 0 dB indicate a very wide image, with a risk of loss in mono.
Procedure in mastering
- Listen to Mid and Side separately for 20–30 seconds each.
- Correct the Mid first (vocal, kick, snare are here) with cuts and boosts ≤ 1.5 dB.
- Then the Side: bass HP, low-mid cuts, possibly some air.
- Compare in stereo and in mono at matched volume: in mono the track must not change character.
- Check the correlation: it must not drop below +0.2 on average after the processing.
Common mistakes
- Boosting the highs of the Side heavily: a fake-wide mix that turns dark and flat in mono.
- Side HP too high (300–500 Hz) on the master: loss of warmth and of the natural width of the guitars.
- Strong boosts on the Mid that “push” the vocal but also the kick and snare: check what is in the center before boosting.
- M/S EQ on mono tracks: on mono tracks the Side is empty, so the Side EQ does nothing.
- Forgetting that linear-phase and minimum-phase EQs behave differently in M/S: with strong minimum-phase boosts on one component only, the image can shift across frequency (see “Linear-phase and minimum-phase EQ”).
Starting settings
| Parameter | Value | Why |
|---|---|---|
| Side HP on the master | 80–150 Hz, 12–24 dB/oct | Mono, compatible low end. |
| Mid boost | Maximum +1.5 dB, Q 0.7–1 | The Mid contains vocal, kick and snare: every boost moves them all. |
| Side high shelf | +1/+2 dB at 8–12 kHz | Air without hardening the center. |
| Overhead Side HP | 150–250 Hz | Drum bass in the center, more punch. |
| Reverb, Mid | HP 300–500 Hz, −2 dB at 2–4 kHz | The tail does not cover the vocal in the center. |
| Correlation after processing | Average above +0.2 | No problems in mono. |
Frequencies to watch
| Range | What to do |
|---|---|
| Below 80–150 Hz (Side) | High-pass on the Side: mono bass, stable in clubs, mono playback and vinyl. |
| 200–400 Hz (Side) | −1/−2 dB if guitars and keyboards on the sides swell the low mids. |
| 1–3 kHz (Mid) | +0.5/+1.5 dB for a more present vocal; −2/−3 dB on the Mid of guitar and pad buses to free up the vocal. |
| 5–8 kHz (Mid) | De-essing in the center only, the cymbals on the sides stay intact. |
| 8–12 kHz (Side) | Shelf +1/+2 dB for air and width. |
Example chain
| Step | Tool | Settings | Why |
|---|---|---|---|
| Listening | Plugin with Mid/Side solo (Pro-Q, Ozone Imager, bx_control) | Solo M, solo S | Understand what is in each component. |
| Mid EQ | FabFilter Pro-Q / Ozone EQ | Cuts and boosts ≤ 1.5 dB | Center of the mix. |
| Side EQ | Same EQ, bands in Side mode | HP 80–150 Hz, air 8–12 kHz | Sides of the mix. |
| M/S dynamics (optional) | Compressor or de-esser in Mid mode | 1–2 dB | Control of the vocal in the center. |
| Measurement | Correlation meter and vectorscope (Wave Candy, Ozone Imager) | Correlation and width | Mono compatibility check. |
How to check the result
- Side/Mid width in the Sub and Bass bands: −20 dB or less after the HP on the Side.
- Side/Mid width in Air (8–16 kHz) no higher than −2 dB: above that, the high-end image is too wide.
- Average mix correlation between +0.2 and +0.8; no long stretch below 0.
- Integrated loudness in mono (L+R sum) no more than 1.5–2 LU below the stereo one.
- High-mid band (2–4 kHz): after the boost on the Mid, relative energy must not rise by more than 1–1.5 dB.