Mid-side (M/S) recording is a powerful technique used in audio engineering to achieve greater control over stereo sound images. It allows producers and engineers to manipulate the stereo width and balance more precisely than traditional stereo recording methods, offering a level of flexibility that is indispensable in modern mixing and mastering workflows. Whether you are recording a vocalist, an acoustic guitar, or an entire orchestra, understanding M/S techniques can elevate your stereo production from good to exceptional.

Understanding Mid-Side Recording

The M/S technique involves two microphones: one positioned to capture the mid signal, which contains the mono information directly in front of the recording setup, and another capturing the side signal, which records the stereo information from the sides. The magic happens when these two signals are combined during mixing — they provide a flexible stereo image that can be adjusted dynamically without losing mono compatibility.

First developed in the 1930s by EMI engineer Alan Blumlein, M/S recording was originally intended to improve the compatibility of stereo recordings with mono playback systems. In the decades since, it has become a standard tool for recording engineers across all genres. Unlike X/Y or A/B stereo techniques, where the stereo image is fixed at the time of recording, M/S gives you post-capture control over the width and balance of the stereo field. This means you can widen a sound that felt too narrow, or narrow an overly wide recording, all without re-tracking.

Setting Up Mid-Side Recording

To implement M/S recording, follow these steps:

  • Place the mid microphone directly facing the sound source, typically a cardioid or omnidirectional mic.
  • Position the side microphone perpendicular to the mid mic, often using a figure-eight pattern to capture the stereo information.
  • Ensure the microphones are aligned correctly and at an appropriate distance to avoid phase issues.

While the basic setup is straightforward, getting the best results requires careful attention to microphone selection and physical placement.

Microphone Selection

The mid microphone should be a high-quality cardioid or, in some cases, a small-diaphragm omnidirectional capsule. The cardioid pattern is popular because it isolates the center image well, providing a clean mono, which is critical for mono-compatible broadcasts and club systems. The side microphone must be a figure-eight (bidirectional) capsule — this is essential because it picks up sound equally from the left and right sides while rejecting sound from the front and rear. A matched pair of microphones is not required, but the two microphones must be carefully calibrated in level and phase to avoid artifacts.

Common mid-side microphone combinations include:

  • Small-diaphragm condenser cardioid + figure-eight (e.g., Neumann KM 184 paired with a Schoeps MK 8)
  • Large-diaphragm condenser cardioid + ribbon figure-eight (e.g., AKG C414 in cardioid mode + Beyerdynamic M 130)
  • Dual-capsule microphones (e.g., Sennheiser MKH 418-S or Audio-Technica BP4029) that contain both capsules in a single housing — ideal for portable recording.

Positioning and Phase Alignment

The two microphones should be placed as close together as possible — ideally with their diaphragms intersecting vertically. In practice, the side microphone is often mounted directly above or below the mid microphone, with their capsules aligned along the same vertical axis. This minimizes time-of-arrival differences between the capsules, reducing comb filtering and phase cancellation. The distance from the sound source should be chosen based on the desired acoustic perspective: for intimate vocals, work as close as 6–12 inches; for roomy instruments or ensembles, back off to several feet.

After recording, check the phase relationship between mid and side by toggling polarity on the side channel. Ideally, the mid signal should be in phase with the side signal when the side channel is panned fully left or right. Unwanted phase shifts often manifest as a loss of low-end or a hollow center image. If you encounter issues, try flipping polarity on the side channel or physically adjusting the slight angle of one microphone.

Decoding and Processing M/S Signals

During mixing, the mid and side signals are decoded to produce a stereo image. The process involves:

  • Adding the mid signal to the sum of the mid and side signals to create the left channel.
  • Subtracting the side signal from the mid to create the right channel.
  • Adjusting the level of the side microphone to control stereo width.

This technique allows for dynamic control of the stereo field, making it easier to focus on specific elements or widen the overall soundstage.

The Math Behind M/S

Understanding the simple algebra is helpful:

  • Left channel = Mid + Side
  • Right channel = Mid − Side

When the levels of the mid and side signals are equal, the stereo image is full and wide. If you reduce the side level, the stereo image narrows. By increasing the side level, you can push the stereo image beyond the width of the original recording, creating an exaggerated spatial effect. Conversely, if you remove the side signal altogether (mute the side mic), the sound collapses to pure mono — demonstrating total mono compatibility.

M/S Encoding/Decoding in Your DAW

Most modern digital audio workstations (DAWs) include a dedicated M/S encoder/decoder plugin. Alternatively, you can use a simple routing:

  1. Record the mid microphone to a mono track.
  2. Record the side microphone to a separate mono track.
  3. Duplicate the side track and invert the polarity of one copy (using a utility gain plugin or a phase invert button).
  4. Pan the mid track center, the non‑inverted side track hard left, and the inverted side track hard right.
  5. Adjust the level of the side channels to control width.

If your DAW supports dual‑mono or stereo linking, you can also route the mid and side into a stereo auxiliary track with two inserted M/S‑coded plugins. The advantage of using dedicated M/S plugins (like Brainworx bx_digital V3 or Waves S1 Imager) is that they often include convenient controls for width, balance, and even mid/side equalization.

Controlling Stereo Width in a Mix

One of the greatest strengths of M/S processing is the ability to change width after the recording. For instance, if a guitar part recorded in a small room sounds too narrow, you can boost the side channel to add spatial depth. If a vocal recorded in a live room has too much room reverb, you can reduce the side signal to dry it up without affecting the direct vocal in the center. This is especially useful in post‑production for film and broadcast, where audio must often be rebalanced to match picture edits.

Warning: Over‑widening can cause out‑of‑phase artifacts on mono playback systems. Always check your mix in mono after adjusting the side level. A good practice is to keep the side level at or below the mid level unless a special effect is desired.

Advantages and Disadvantages of Mid-Side Recording

Implementing M/S recording offers several benefits:

  • Enhanced stereo control during mixing: Adjust width and balance without re‑recording.
  • Maintains mono compatibility: The mid signal is pure mono, so the mix collapses cleanly.
  • Flexibility to adjust stereo width for different mediums: A single M/S recording can serve a radio mix (narrow) and a streaming mix (wide) by simply changing the side level.
  • Improved spatial imaging: The figure‑eight side mic captures extremely clean panoramic information with strong left/right separation.
  • Eliminates the need for microphone stands on stage: For live broadcast, a single M/S microphone can be mounted on a boom, reducing clutter.

However, there are limitations:

  • Requires a figure‑eight microphone: Not all studios have one, and high‑quality bidirectional capsules can be expensive.
  • Side microphone picks up rear‑side noise: Room reflections and ambient noise behind the mic array are recorded and must be managed.
  • Phase‑sensitive setup: Even slight misalignment of the capsules can lead to comb filtering.
  • Increased noise floor: Because the side channel is a difference signal, any noise from the side mic is added to both left and right channels, potentially raising the noise floor.

Practical Applications of the M/S Technique

The M/S method is widely used in music production, film, and broadcast. It is particularly useful for recording vocals, ensembles, and ambient sounds where precise stereo placement enhances the listening experience.

Vocals

For a single vocalist, M/S recording offers the ability to capture both a focused center image (mid) and the natural room ambience (side). During mixing, you can tighten the vocal by reducing the side level, or add a sense of space by boosting it. Many engineers also use M/S on backing vocal arrays: place the M/S pair in front of the group and you can later widen the background harmonies while keeping the lead centered.

Acoustic Instruments

Acoustic guitar, piano, and small ensembles benefit enormously from M/S. For example, an acoustic guitar can be recorded with a close cardioid mic (mid) and a distant figure‑eight (side) to capture both the direct string body and the room resonance. The result is a natural, spacious sound that can be tweaked in post‑production to fit any arrangement.

Ambient and Room Sound

M/S is the go‑to technique for recording room tones, ambience, and environmental sounds. The mid channel captures the direct sound, while the side channel captures the left/right spatial information of the space. For film location recording, M/S allows the sound recordist to adjust the width of ambience to match the shot framing — without needing a stereo boom.

Film and Broadcast

In film and broadcast, M/S recording is used for dialogue, Foley, and surround sound. M/S easily decodes to a stereo left/right feed, and the mid channel can be used directly for center‑channel dialogue. Many high‑end location microphones (like the Sennheiser MKH 418‑S) are purpose‑built M/S rigs.

Advanced M/S Techniques

Once you’re comfortable with basic M/S, you can explore more advanced methods.

Blumlein Pair

A Blumlein pair uses two figure‑eight microphones crossed at 90 degrees. This creates an M/S system where both microphones are side microphones, but one acts as the mid when the signals are summed. The result is an extremely wide, natural stereo image with excellent depth. Blumlein is beloved for classical and orchestral recordings but can be noisy in uncontrolled environments because of the bidirectional rear pickup.

Double M/S (Double Mid-Side)

Double M/S adds a second figure‑eight microphone facing the opposite direction (usually backward). This captures rear‑side information, enabling the stereo image to be manipulated in three dimensions. Double M/S is used in immersive audio formats like Dolby Atmos to create a 360‑degree sound field from a compact microphone array.

Using M/S for Mastering

M/S processing isn’t just for recording — it’s a staple in mastering. By splitting a stereo mix into mid and side signals, mastering engineers can EQ or compress the center (vocals, bass, kick) independently from the sides (cymbals, reverbs, wide pads). This allows for precise balancing of the stereo image without damaging the mono compatibility. Popular mastering plugins like iZotope Ozone and FabFilter Pro‑Q 3 include M/S modes.

Common Mistakes to Avoid

Even experienced engineers sometimes trip up when using M/S. Watch out for these pitfalls:

  • Improper microphone orientation: The side microphone’s null must face the sound source. If the figure‑eight’s front lobe points toward the source, you’ll get a phase‑cancelled mess. Label your microphones clearly on site.
  • Over‑widening in the mix: Cranking the side level too high creates an unnatural, hollow center and can cause phase cancellation when summed to mono. Aim for a side level that is roughly equal to the mid level or less.
  • Ignoring phase on the side channel: Always listen to the decoded stereo with the side channel soloed (inverted polarity if needed) to ensure the sides are in phase.
  • Using a low‑quality figure‑eight mic: Noise or coloration in the side channel is doubled when summed into both left and right. Invest in a clean figure‑eight or use a high‑quality ribbon if possible.
  • Forgetting to decode before playback: If you listen to a raw mid‑side recording without decoding, it will sound thin and unnatural. Use a dedicated plugin or the manual routing described above.

Conclusion

By mastering mid‑side recording techniques, audio professionals can achieve more natural and immersive stereo recordings, giving them greater creative control in the mixing process. The ability to adjust stereo width after the fact — without sacrificing mono compatibility — is a superpower that every recordist should have in their toolkit. Start by experimenting with a simple cardioid + figure‑eight pair on an acoustic instrument or vocal, and soon you’ll find M/S indispensable for everything from intimate singer‑songwriter projects to massive orchestral sessions.

For further reading, check out Sound On Sound’s classic M/S recording tutorial, Wikipedia on Mid/Side audio, and The Recording Revolution’s practical guide. Practice, listen critically, and soon you’ll hear the difference M/S makes — a stereo image that’s wide, stable, and entirely under your control.