Phase vs Polarity: Understanding Comb Filtering

TL;DR

Polarity = instant inversion (+/–). Phase = temporal shift frequency-dependent. Comb filtering occurs when two similar signals are shifted. Inverting polarity can help, but only a fine delay fixes the cause.

1) Understanding the Difference

1.1 Polarity

Polarity is an instant inversion of the signal. It's a sign change (+ becomes –, and vice versa) that affects all frequencies the same way.

Polarity inversion is a binary tool: either the signal is inverted, or it's not. There's no intermediate degree.

When to use it:
- Two mics capture the same source but one is wired backwards
- A mic is placed on the other side of a membrane (e.g., kick drum, internal/external)
- Wiring or XLR connection issue

1.2 Phase

Phase is a time delay between two signals. Unlike polarity, phase affects each frequency differently according to its wavelength.

A 180° phase shift at 1 kHz doesn't have the same effect as at 500 Hz or 2 kHz. This is why we talk about "comb filtering": some frequencies cancel, others add up.

When it occurs:
- Two mics at different distances from the same source
- Drum mics (overheads + individual mics)
- Multi-mic on the same source (e.g., guitar, multiple mics on a speaker)

2) Comb Filtering Symptoms

2.1 Audible Signs

• Hollow sound, lack of body
• Irregular, unstable highs
• Variations when slightly moving a mic
• Lack of coherence in low-mids

2.2 Quick Diagnosis

Listen in mono to reveal cancellations. Phase problems are often masked in stereo.

Simple test:
1. Put your mix in mono
2. If the sound becomes hollow or loses body → probable phase problem
3. Test polarity inversion on one of the mics
4. If it improves but isn't perfect → temporal phase problem

3) Practical Solutions

3.1 Placement: The 3:1 Rule

The best solution is preventive: respect the 3:1 rule from placement.

Reminder: distance between two mics ≥ 3× the distance of the closest mic to the source.

For more details, see our article: Proper Microphone Placement: Simple Rules and Pitfalls.

3.2 Fine Delay for Realignment

If placement isn't optimal, use a fine delay (in milliseconds) on one of the sources to realign transients.

Method:
1. Measure the distance difference between the two mics
2. Calculate the delay: ~2.9 ms per meter (speed of sound ≈ 343 m/s)
3. Apply delay on the closest mic
4. Adjust by ear listening to low-mid return

The low-mid movement is a good indicator: when body returns, alignment is better.

3.3 Polarity Inversion

Polarity inversion can improve the situation, but it doesn't fix the cause (time delay).

When to use it:
- Quick test for diagnosis
- If the problem comes from reversed wiring
- As a complement to delay, for fine-tuning

Important: Polarity inversion doesn't fix temporal phase. If the problem persists after inversion, it's a temporal phase problem requiring delay.

4) Practical Cases

4.1 Drums: Overheads + Individual Mics

Problem: Overheads also capture the snare, creating a delay with the snare mic.

Solution:
1. Respect the 3:1 rule
2. If necessary, fine delay on the snare mic (a few ms)
3. Test polarity inversion if sound is still hollow

4.2 Kick Drum: Internal + External

Problem: Both mics are on either side of the head, creating a natural delay.

Solution:
1. Polarity inversion on one of the two mics (often necessary)
2. Fine delay if distances are very different
3. Adjust by ear listening to punch and body

4.3 Guitar: Multiple Mics on a Speaker

Problem: Two mics at different distances from the cone.

Solution:
1. Respect the 3:1 rule
2. Fine delay on the closest mic
3. Adjust to get the best sum

5) Diagnostic Checklist

Before correcting, diagnose:

• ✅ Listen in mono to reveal problems
• ✅ Polarity inversion test (phase button on console)
• ✅ Check distances (3:1 rule respected?)
• ✅ Listen to low-mids: does body return?
• ✅ Fine adjustment with delay if necessary

Did You Know?

• A 1 ms delay at 1 kHz = 360° phase shift (one complete cycle)
• Comb filtering creates periodic cancellations: some frequencies cancel, others add up
• In live sound, phase problems are often masked by room ambience, but remain audible at mix position

Additional Resources

• Proper Microphone Placement: Simple Rules and Pitfalls - Preventive placement with 3:1 rule
• System Time Alignment - Align subs, fills and delays
• Live Gain Staging - Optimize levels after phase correction