Definitions
- Polarity: the sign of the waveform. Inverting the polarity flips the wave upside down (peaks become troughs) equally at all frequencies. The “Ø” button on many consoles is a polarity inverter, even though it is improperly called “phase”.
- Phase: the position within a wave's cycle; between two signals, the phase difference depends on the delay and on the frequency. A constant delay produces different phase shifts at each frequency.
Interference and comb filtering
Two similar signals summed:
- in phase → they reinforce each other (+6 dB if identical);
- out of phase → they cancel;
- with a delay → some frequencies are reinforced and others cancelled at regular intervals: comb filter. The first notch falls at 1 ÷ (2 × delay): 1 ms → 500 Hz, then 1.5 kHz, 2.5 kHz... 0.5 ms → 1 kHz. The sound becomes “hollow”, “metallic”, and loses lows.
Where the problem comes from
- Multi-miked drums: kick in/kick out, snare top/bottom (the bottom mic almost always needs a polarity flip), overheads relative to the close mics (the sound reaches the overheads a few ms later: 1 m of distance ≈ 2.9 ms).
- Bass DI + amp: the miked amp arrives a few ms late (also due to the latency of the amp/simulator).
- Guitar with two microphones.
- Layering sampled kicks and snares: the samples start at different points.
- Artificial stereo (Haas, copied and delayed doubles) that sums in mono.
- Minimum-phase EQ in parallel with the unprocessed signal (parallel EQ): phase shifts around the modified frequencies.
How to fix it
- Listen in mono and try inverting the polarity on one of the tracks: choose the version with more lows and more body.
- Align in time: zoom in on the transients and shift by samples/milliseconds (or with the track's “time offset” field). Automatic tools: Sound Radix Auto-Align, MeldaProduction MAutoAlign; phase rotators like Little Labs IBP or Voxengo PHA-979 for fine adjustments.
- Phase rotation for the lows: when the “right” delay for the highs is wrong for the lows, a phase rotator adjusted by ear in the 60–200 Hz zone.
- Don't align at all costs the overheads: a small delay gives space. Above all, align the kick and snare with each other.
- Layers: align the attacks of the samples, check the polarity of each layer, filter to reduce overlap.
Phase and EQ
- Minimum-phase EQs (most of them, analog and digital) introduce phase shifts around the modified bands: normal and often musical.
- Linear-phase EQs do not alter the relative phase between frequencies but introduce latency and pre-ringing (an echo before transients) with steep cuts in the lows; useful in mastering and on parallel buses, less so on drums.
In FL Studio
- Every mixer insert has a polarity inversion option and an L/R channel swap option (in the track properties or the track menu).
- Fruity Stereo Shaper allows per-channel polarity inversion, delay between L and R, and M/S control.
- Time offset for channels (Channel settings → Misc → Time offset) or move clips with snap turned off; for fine alignment, zoom in on the Playlist.
- Plugin delay compensation (PDC) must be on to avoid phase shifts between tracks processed in parallel.
Example in A minor
Electric bass playing A1 – F1 – C2 – G1 recorded DI and with a microphone on the amp: in mono the bass loses body. Zooming in shows a 3.2 ms delay on the miked track: advance it by 3.2 ms; then try polarity inversion: with normal polarity the lows at 55 Hz are fuller, keep it. If a small hole remains at 100–150 Hz, a phase rotator on the amp track fixes it.
Common mistakes
- Inverting polarity “by eye” without listening.
- Aligning everything to the sample and losing the natural space of the ambient mics.
- Forgetting the comb filtering of artificial stereo doubles.
Exercise
- Duplicate a drum track, delay it by 0.5, 1, 3, 10 ms and listen to the sum in mono: identify the first comb-filter notch with an analyzer.
- Invert the polarity of one layer of your layered kick: what happens to the lows?
Related topics
“Mono compatibility”, “Layering sounds: sub, body, top”, “Psychoacoustics in mixing” (Haas effect), “Stereo, panning and image”.