Phase cancellation happens when two versions of the same sound are combined and their waveforms work against each other: where one pushes, the other pulls. The result is a thin, hollow sound, usually with missing bass. It occurs when one source reaches two microphones, or when the two channels of a stereo file are summed to mono. Check by listening in mono; fix it by flipping the polarity of one signal, by shifting one track in time or by muting the microphone you do not need.
In Short
The short version:
- Two copies of one sound add up when they are aligned and cancel when they are opposed.
- Typical causes: two microphones on one source, microphone bleed, reflections from a nearby wall.
- Symptoms: a thin, hollow sound with weak bass, worse in mono.
- The test: listen in mono.
- The fixes: flip the polarity, align the tracks in time, or use one microphone.
What Is Phase Cancellation?
Sound is a wave: air pressure rising and falling, which a microphone turns into a waveform with peaks and dips. When two waveforms are mixed, they are simply added, moment by moment. If both rise and fall together, the result is twice as big. If one rises exactly while the other falls, they add up to nothing. Physics calls the first case constructive and the second destructive interference; the Handbook for Acoustic Ecology at Simon Fraser University defines both.
In audio the two signals are rarely exact opposites. More often one is a slightly delayed copy of the other, and then the outcome depends on the frequency: some frequencies line up and get louder, others land peak on dip and vanish. That is why phase cancellation usually does not silence a recording but hollows it out.
| How the two signals relate | What happens when they are mixed |
|---|---|
| Identical and aligned | They add up: 6 dB louder, nothing lost |
| Identical, one with inverted polarity | They cancel completely: silence |
| One delayed by a fraction of a millisecond to a few milliseconds | Comb filtering: some frequencies louder, others missing; a thin or hollow sound |
| Unrelated sounds | No cancellation; they simply play together |
Phase and Polarity Are Not the Same
The two words are often used as if they meant one thing. Polarity is which way up the waveform is: inverting it swaps every peak for a dip. Phase is a position in time along the wave, so a phase difference comes from a delay. An inverted signal and a delayed signal can both cause cancellation, but they need different cures: polarity is fixed with a switch, a delay by moving the signal in time.
Where It Comes From
Two Microphones, One Source
Put two microphones on a guitar, one near and one a little further away, and the sound reaches the second one later. Sound travels about 34 centimeters per millisecond, so a difference of 17 centimeters in distance means half a millisecond of delay, enough to cancel frequencies around 1,000 Hz and to color everything above. A special case is a pair of microphones facing each other, such as above and below a snare drum: when the skin moves towards one, it moves away from the other, and the two signals have opposite polarity.
Microphone Bleed in Podcasts
Two people at one table, each with a microphone: every voice is picked up twice, loudly by the near microphone and quietly, a few milliseconds late, by the other. Mixed together, the late copy hollows out the direct one. The closer the two levels, the stronger the effect.
Reflections from a Wall or Desk
Even a single microphone can be affected. A voice that bounces off a nearby wall or a desktop arrives a second time, slightly late, and combines with the direct sound in the microphone itself. This comb filtering is baked into the recording and cannot be removed afterwards.
Stereo Files Played in Mono
Wide stereo effects, and stereo recordings made with two spaced microphones, sound fine on two speakers, where the channels mix only in the air. On a single speaker the channels are added electrically, and whatever is opposed between them cancels. Stereo vs. Mono Audio explains why mono playback is so common.
How to Recognize It
- Listen in mono. If the sound becomes thinner, quieter or loses its bass when both channels are combined, something is canceling. The Stereo to Mono Converter plays the mono sum and shows how well the channels agree.
- Mute one microphone. If the recording sounds fuller with one microphone than with two, the two are fighting each other.
- Read a correlation meter. Values near +1 mean the channels support each other; values below zero mean they cancel. The Phase Inverter has one meter for the whole signal and one for the bass.
How to Fix Phase Cancellation
Start with the simplest remedy and move on only if it does not help.
Step 1: Flip the Polarity
Invert one of the two signals and compare. If the bass returns and the sound becomes fuller, you are done. For a stereo file, open it in the Phase Inverter, invert one channel and listen with the mono check switched on.
Step 2: Align the Tracks in Time
If flipping does not help, the problem is a delay. In an audio editor, zoom in on both tracks and slide the later one earlier until the same peak sits in the same place on both. A few samples can make the difference.
Step 3: Use One Microphone Where Two Fight
When neither works, turn the weaker signal down or mute it. For podcast bleed, a gate that closes each microphone while its speaker is silent removes most of the double pickup; the Noise Gate & De-Esser does this for a single track.
Step 4: Check Again in Mono
After any fix, listen to the mono sum once more. A corrected recording gets louder, so check the peaks before you export; the LUFS & Peak Analyzer shows them.
How to Prevent It While Recording
- The 3-to-1 rule: keep a second microphone at least three times as far from the first as the first is from its source. The bleed is then quiet enough to do little harm.
- Get close: the nearer each microphone is to its own speaker, the smaller the share of bleed.
- Keep away from hard surfaces: a few steps from the wall, and something soft on the desk, weaken the reflections.
- Check before the take: record ten seconds, listen in mono and move a microphone if the sound is hollow.
Check Your Recording for Phase Problems
Flip one channel, switch to mono and see on the meter whether the sound is canceling.
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