Acoustic guitar multi-mic phase — why spaced pairs & body+neck cancel in mono
You miked a beautiful acoustic with a spaced pair, or body plus neck, or close plus room. In headphones the image feels wide and alive. Flip the group to mono — body thins, low-mid hollows, or the guitar turns nasal. That’s multi-mic phase (and spacing), not a missing “acoustic body” EQ shelf.
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PHIXER is free stereo phase-repair (VST3/CLAP/AU · Win/Mac/Linux) from Riverflux Audio. Frequency-aware phase rotation for L/R fights — not a polarity button, not a delay compensator, not a widener. On acoustic multi-mics, use only after polarity and mic distance/timing are honest — skip when you want intentional stereo bloom as the sound of the room.
What acoustic multi-mic phase fights sound like
Acoustic guitars are often captured with two or more mics that hear almost the same instrument from different distances and angles: XY coincident pairs, ORTF / spaced pairs, body + 12th-fret / neck mics, close + room, or DI + mic on electro-acoustics. Headphones forgive path-length differences. Mono summing — phones, laptop speakers, social video, some PAs — does not.
Classic symptoms:
- Spaced pair that hollows in mono — lush stereo image; fold to mono and body/low-mid cancel while string detail survives.
- Body + neck mic fight — warm body mic + articulate fretboard mic; mono blend thins the chest of the instrument.
- Close + room cancel low-mid — room adds space in stereo; mono makes the close mic fight the room and lose body.
- XY that still isn’t mono-safe after processing — coincident pairs are more mono-friendly, but stereo EQ, wideners, or a third mic can reintroduce fights.
- Electro-acoustic DI + mic — same dual-path class as bass/guitar DI+mic; mono check reveals cancellation between pickup and air mic. See also DI + mic phase.
Same physics as piano stereo / spaced-pair phase, guitar dual-cab phase, and bass that disappears in mono: similar energy, different path length / polarity / spectrum, then destructive summing. Width tools do not fix that.
Why multi-mic acoustics fight (and why that’s normal)
Sound from the top, body, and strings reaches each capsule at slightly different times. Spaced pairs exaggerate that path-length difference into stereo width — and into comb filtering when summed. Body + neck mics hear different resonances of the same note; they reinforce in stereo and can cancel in mono. Room mics add late energy that is musically useful until it fights the close pair on fold-down.
None of this means multi-miking is wrong. It means mono compatibility is a check, not an afterthought. Name what disappeared (body, low-mid, warmth, pick attack) before more compression, more width, or another mic.
Why mono reveals them
In stereo, opposing-phase energy can still reach each ear. Sum L+R and those bands cancel. Acoustic body and low-mid are exactly where mono is least forgiving. Check: each mic alone → pair in stereo → pair in mono → with/without room. If mono collapses relative to the best single mic, fix the dual path before the master. Workflow: mono-check before/after.
Honest order of fixes
1. Polarity first (Ø / Invert)
Hard 0° / 180° on the secondary mic vs the primary reference (often the body or the left of a pair). Flip; listen in mono. Keep what restores weight without wrecking the stereo image. If one flip fixes it, stop. More: polarity invert vs phase rotation.
2. Mic distance / timing when one path is late
If the secondary mic’s attack sits later on the waveform, nudge (samples or ms) so attacks line up enough that mono stops cancelling body — or move the mic physically next session. Over-aligning into identical spikes can kill the stereo bloom you wanted; align for mono safety, leave intentional room character alone. Details: phase repair vs time-align.
3. Prefer more mono-friendly arrays when you can re-track
XY / coincident pairs are generally more mono-compatible than wide spaced pairs. ORTF is a middle ground. If mono is critical (ads, social, broadcast), bias toward coincident or tighter spacing — then process. This is tracking hygiene, not a plugin step.
4. Frequency-aware phase rotation (PHIXER) when leftovers remain
After polarity and honest timing, some stereo acoustic pairs still fight by band — body dies in mono while string detail survives, or low-mid hollows while highs sparkle. That’s when spectral phase rotation helps: PHIXER rotates phase by frequency so more energy survives mono. It does not invent missing body, replace better mic placement, or “fix” intentional stereo room bloom you want as the sound.
Decision table
| You notice… | First try | Why |
|---|---|---|
| Invert secondary mic and body returns | Polarity — done | Simple 180° mismatch |
| Waveform shows secondary late vs primary attack | Nudge / time-align / re-place mic | Path-length / spacing delay |
| Wide spaced pair collapses hard in mono | Tighter spacing or XY next time; polarity+nudge now | Spacing is the design; mono is the tax |
| Align + polarity help some bands; others still die in mono | PHIXER on stereo acoustic bus | Frequency-dependent residual fight |
| One mic alone sounds better than the pair in mono | Mute the weaker mic for mono-critical uses | Sometimes the honest fix is fewer mics |
| You want intentional stereo room bloom | Don’t “repair” — own the tradeoff | Intentional phase as space |
When frequency-aware phase repair helps on acoustic multi-mics
- You’ve polarity-checked and nudged, and a band (often body / low-mid) still collapses in mono on the stereo pair.
- You printed a stereo acoustic stem (spaced pair, body+neck, close+room printed L/R) and that file itself still fights after basic alignment.
- The pair passes a headphone test but fails phone / mono check after polarity and timing cleanup.
- Each mic sounds fine solo; only the blend cancels.
PHIXER belongs on residual stereo / dual-mic fights — not as the first plugin instead of moving a mic. Related: piano spaced-pair phase.
When NOT to use PHIXER on acoustic multi-mics
- Clear polarity only — if Ø restores mono body, stop.
- Clear timing / spacing only — a secondary a few samples (or more) late is nudge or re-placement, not rotation. See vs time-align.
- One good mic is enough — if mono-critical delivery needs a single close mic, mute the pair rather than over-processing.
- Intentional stereo room bloom — wide spaced pair as the aesthetic. Own the mono tradeoff. Honest list: when not to use PHIXER.
- Using a stereo imager to “fix” cancellation — width tools don’t restore cancelled body. See PHIXER vs imagers.
Short practical checklist
- Solo each mic. Listen for tone problems that aren’t phase.
- Blend. Flip to mono. Note body / low-mid / warmth.
- Try polarity on the secondary. Keep the best combo.
- If the secondary is late, nudge / time-align; re-check mono.
- If a band still cancels on the stereo pair, try PHIXER — then mono-check again.
- Only after mono is solid (or you consciously accept intentional bloom), touch EQ, compression, or width. Imagers last.
- For the next session, consider tighter / coincident arrays if mono is critical.
Order of operations: mic choice / spacing → polarity → timing/nudge → frequency-aware phase repair → creative width. Skipping to an imager or “more room mic” on a cancelling pair is how hollow mono acoustic tracks ship to phones and social video.
Related reading
- Piano stereo / spaced-pair phase — same spaced-pair mono physics
- DI + mic / dual-mic phase — electro-acoustic DI+mic fights
- Guitar dual-cab / multi-mic phase — electric multi-mic cousin
- Why bass disappears in mono — same mono-sum physics
- Phase repair vs time-align — nudge first when one mic is late
- Mono-check workflow — name what disappears before you fix
- Polarity invert vs phase rotation — 180° first
- When not to use PHIXER — skip intentional bloom / wrong symptom
- PHIXER product page · Hub
Get the free fixer
PHIXER — free stereo phase-repair from Riverflux Audio (VST3/CLAP/AU · Win/Mac/Linux). No invented stats: try it on your acoustic stereo pair after the checklist above.