Phase
How wave alignment affects sound — the most-misunderstood concept in audio engineering.
Phase is one of the most-misunderstood concepts in audio. The word gets used loosely — to mean polarity, to mean phase relationship between signals, to mean delay-induced cancellation. They're related but distinct.
Getting phase right is invisible when it works. Getting it wrong destroys mixes — sounds disappear, low-end vanishes, drums sound thin.
Phase vs Polarity
These are different.
Polarity. A signal is either positive-going or inverted (negative-going). Inverting polarity flips the entire waveform vertically. The "phase invert" button on a mic preamp or a plugin is doing this. Not actually phase — polarity.
Phase. A continuous measurement of where a wave is in its cycle, measured in degrees (0–360°) or fractions of a wavelength. Two identical waves can be at different phases, depending on when each started.
A polarity flip is a 180° phase shift across all frequencies. A delay-based phase shift is frequency-dependent — different frequencies are shifted by different amounts.
Comb Filtering
When two copies of the same signal are mixed at different times, frequencies cancel and reinforce in a regular pattern. The frequency response shows a "comb" of peaks and dips. Hence the name.
Comb filtering happens whenever:
- Two mics on the same source pick up the same sound at slightly different times.
- A signal is duplicated and one copy is delayed.
- A reverb or delay is too short.
- A vocal is doubled with very small (sub-30 ms) delays.
The result: the source sounds thin, hollow, or "phasey." The phenomenon is the same whether the delay is 0.5 ms or 50 ms — the cancellations just move to different frequencies.
How to Avoid Phase Problems
Use the 3:1 rule when miking with multiple microphones. If two mics are recording the same source, the second mic should be at least 3 times as far from the source as the first. This minimises level of the same source in the more distant mic, reducing audible comb filtering.
Time-align mics on the same source. Multi-mic'd kick drums (inside + outside), guitar amps (close + distant), and snare drums (top + bottom) all suffer phase issues. Use plugins like Sound Radix Auto-Align or your DAW's track delay function to slide one mic's timing into alignment with another.
Bottom snare polarity. Always invert polarity on the bottom snare mic. The two mics see opposing waveforms because the drum head moves toward one mic while moving away from the other.
Mono-check your mix. A mix that sounds great in stereo but loses elements in mono is suffering from phase problems somewhere. Common culprits: stereo wideners, doubled vocals with tiny offsets, stereo synths with one channel out of polarity.
Phase Issues in Stereo Recordings
When you stereo-mic a source (XY, ORTF, AB, etc.) and the listener collapses to mono, identical content cancels itself. This is more dramatic at higher frequencies where wavelengths are smaller and arrival-time differences create more cancellation.
XY (coincident) techniques are most mono-compatible — both capsules at the same point. Spaced techniques (AB) are wider in stereo but more susceptible to mono cancellation.
Plugins That Detect / Fix Phase
- Plugin metering with goniometer or vectorscope. Shows you the relationship between left and right channels visually. A vertical line means perfectly mono, a horizontal line means out-of-phase across all frequencies.
- Phase rotators. Apply variable phase shift to a signal. Useful for aligning bass, kick, or stereo recordings.
- Auto-Align / SPL Vitalizer. Software that detects phase between tracks and corrects automatically.
Practical Phase Move: The Bass + Sub Layer
Common in modern productions: a deep bass synth layered with a sub sine wave for low-end weight. If their phase relationship is wrong, the sub partially cancels the bass.
Fix:
- Solo bass + sub.
- Listen for low-end weight. Pull each track up; should sound bigger together than either alone.
- If it sounds smaller together than the loudest of the two solo'd, you have a phase problem.
- Either invert polarity on one (try the sub first), or nudge timing. Start with polarity — fastest fix.
- Check mono compatibility throughout.
Common Mistakes
Calling "polarity" "phase invert" everywhere. Habit. The button on most plugins is labelled phase invert but actually inverts polarity. They're not the same; it just doesn't usually matter for that one button.
Ignoring mono check. If your mix loses elements in mono, you have phase problems. Mono compatibility should be checked at every mix milestone, not the end.
Stereo-widening on the way to better headphone mixes. Heavy widening creates dramatic phase relationships that fall apart on speakers, mono playback, and Bluetooth speakers.
Trusting the goniometer alone. A metering tool helps spot problems, but the ear is the final judge. A mix can look "in phase" on a goniometer and still have audible cancellation issues.