Phase alignment and polarity

Polarity and phase: two different things

• Polarity: the sign of the waveform. Inverting it means flipping it upside down (what was positive becomes negative), identically at all frequencies. It is a switch: normal or inverted (the Ø button on consoles).
• Phase / timing: the time offset of one signal relative to another. A 1 ms delay shifts a 500 Hz frequency by half a cycle (cancellation at 500 Hz, reinforcement at 1 kHz, cancellation again at 1.5 kHz...): this is the comb filter. Sound reaches different microphones at different times (sound travels about 34 cm in 1 ms).

When two microphones pick up the same source, or when you sum two versions of the same signal (DI and amp, sample layers), the result depends on polarity and timing. If they are not aligned, you lose low end, attack and definition.

Typical cases

Kick in / kick out

The inside microphone (inside the drum, near the beater) receives the hit earlier and with opposite polarity compared to the outside microphone (in front of the resonant head), because the two heads move in opposite directions relative to the microphones. Procedure: invert the polarity of the outside microphone (or of the inside one), then move the outside track forward by 1–3 ms until the sum has more low end. A distance of 30–60 cm between the microphones = 0.9–1.8 ms.

Snare top / bottom

The bottom microphone sees the snare wires moving in the opposite direction: invert the polarity of the bottom microphone (standard procedure). Then check the alignment with the overheads.

Drums and overheads

The overheads receive the kick and snare 2–5 ms later than the close microphones. Two approaches:

• Align the overheads to the snare (move them back in time by 2–4 ms): a tighter, punchier drum kit.
• Leave the natural delay and check only the polarity: more depth and sense of room.

In both cases the overheads must first be put in phase with each other (equal distance from the snare, measured with a tape measure or a cable).

DI bass + amp (or DI + reamp/plugin)

The miked amp signal arrives 2–8 ms after the DI (amp chain + microphone distance; amp simulation plugins add variable latency). Move the amp forward until the waveforms coincide and check the polarity: with the right combination the bass has more fundamental. Same approach for DI guitar + amp.

Acoustic guitar with two microphones, piano with a close pair

Different distances from the source = different delays. Align the farther microphone to the nearer one, or choose the main microphone and use the other at a low level.

Sample layers (kick, 808, snare)

Two overlapping kick samples can cancel each other in the low end. Zoom in on the waveform: the first half-cycles must go in the same direction; shift the sample by a few samples or invert it. Tune the layers to the same note.

How to align

1. Listening: solo the two microphones at a similar level, invert the polarity of one and choose the version with more low end and more body.
2. Waveform: zoom in to individual samples on the first transient; move the delayed track until the peaks coincide. In FL Studio you can move the audio in the Playlist with the grid turned off (Alt + drag) or use the audio channel's offset control.
3. Fine delay plugins: Fruity Stereo Shaper (delay between channels), Fruity Delay 3 with 100% dry and delay in samples, or dedicated plugins: Sound Radix Auto-Align, UAD Little Labs IBP, Melda MAutoAlign, Voxengo PHA-979. Auto-Align calculates the delay by itself.
4. Variable phase alignment (all-pass): when delay alone is not enough (different relationships at different frequencies), a phase rotator (IBP, PHA-979) rotates the phase in a frequency region. Useful on DI + amp and kick in/out.
5. Check: sum the two signals and listen in mono; the result must have more low end and more attack than either microphone alone.

Table of typical delays

Common mistakes

• Inverting polarity at random: it must be chosen by listening to the sum, not out of habit.
• Aligning the room mics too: they lose their depth function.
• Using EQ to fix a phase cancellation: the hole is not filled, you just raise everything else.
• Moving one channel only of a stereo microphone pair (overheads): the stereo image shifts.
• Forgetting the latency of amp simulation plugins on the processed branch.