Fix out-of-phase audio
Voice vanishing on a phone or one speaker? fix.fm checks if one side is flipped upside down and turns it back. Free, in your browser, nothing uploaded.
Updated
Never uploaded. Everything runs in your browser.
In short
Out-of-phase audio has one stereo side upside down, so the two sides cancel when played in mono on a phone, a smart speaker or the radio, and the voice nearly vanishes. A swapped XLR cable, a plugin left on invert or mis-paired mics cause it. fix.fm measures how alike the sides move and flips the wrong one back. On our sample the correlation went from -1 to +1.
What's happening
Out of phase, or reversed polarity, means one side of a stereo file is upside down: every push on the left is a pull on the right. It nearly always comes from the gear. A miswired XLR cable with pins 2 and 3 swapped, a plugin left on invert, two mics paired the wrong way round or some USB interfaces can all do it.
On headphones it sounds odd and hollow. Played in mono, on a phone, a smart speaker, one Bluetooth speaker or the radio, the two sides cancel and the voice nearly disappears.
fix.fm measures how alike the two sides move, from +1 to -1. Its warnings start at -0.3; it calls it moderate at -0.6 and severe at -0.85. The fix turns one side back the right way up.
In Audacity, split the stereo track, select one side and use Effect, Special, Invert.
How far the out-of-phase fix goes
A voice with one side flipped read severe at a correlation of -1 and after the fix fine at +1, the clean file sample for sample. Flipped music folded to mono lost 16 dB and after the fix 0 dB. A very wide mix at +0.19 was left alone. Measured on with the samples described in how we test.
The out-of-phase fix looks for one thing: a stereo file where one side is the other upside down. The cases below are a clean reading and a piece of synthesised music with one side flipped in software, so we know exactly how they sounded before and can check the fixer gives that back.
One clean reading and one piece of synthesised music, with one side flipped in software, then run through this page's fixer at full amount. The stereo clips are matched to -23 LUFS. Use headphones for those; the two mono clips play what a phone does.
Clean reading
The reference. The same voice on both sides, so it sits in the middle of your head.
Voice, right side flipped
On headphones the voice feels hollow and hard to place. Play it on a phone speaker and it all but disappears.
Voice, after the fix
Back in the middle. This is the clean reading again, sample for sample.
Music, right side flipped
Synthesised music with one side upside down. The bass and the tune in the middle sound vague and spread out.
Music, after the fix
The bass and the tune come back to the middle.
Flipped music, as a phone plays it
The two sides added together, at the same volume as the stereo clip. The bass and the tune cancel, and mostly the wide pad and the hits are left.
Fixed music, as a phone plays it
The same fold to mono after the fix, at the same volume. Everything is there.
Very wide music
The music with its sides pushed 5 times further apart. Wide, but nothing is upside down.
Very wide music, after the fix
Identical to the clip above. The check reads it as fine, so the fix leaves it alone.
Public-domain LibriVox voice and music synthesised by fix.fm. The flips are made in software rather than with a miswired cable. So the clean version really is what was there before.
| Case | The check | Played in mono | Each side |
|---|---|---|---|
| Clean reading | +1.00 fine | 0 dB | untouched |
| Voice, one side flipped | -1.00 severe to +1.00 fine | under -60 dB to 0 dB | one flipped back |
| Music, one side flipped | -0.95 severe to +0.95 fine | -16 dB to 0 dB | one flipped back |
| Very wide music | +0.19 fine to +0.19 fine | -2 dB to -2 dB | untouched |
The voice read -1.00, severe: added together, the two sides cancel completely. After the fix it reads +1.00, fine, and the file is the clean reading again, sample for sample.
The music read -0.95, and in mono it lost 16 dB, most of it the bass and the tune in the middle. After the fix it reads +0.95 and loses 0 dB in mono.
The very wide music reads +0.19: low, because its sides carry quite different sounds, but not negative, because nothing is flipped. The check calls it fine, and the fix leaves it exactly as it was. Low correlation is width, not a flipped side.
Flipping is all or nothing, so any AMOUNT above 0 turns the whole side over. Turning it halfway would only make that side quieter.
What this does not prove: these flips are made in software, so each flipped side is exactly the original turned upside down. Two mics at different distances give a time offset instead, where the sides cancel at some pitches and add at others. Flipping a side doesn't fix that; lining the tracks up in your editor does. How we test has the pass marks behind the check.
How it works here
- 01
Compares the two sides
It measures how alike left and right move, half a second at a time, with the loud parts counting most. A wide mix isn't mistaken for a flipped side.
- 02
Turns one side back
It flips the out-of-phase side the right way up. Each side keeps its own sound, and on a phone or one speaker the voice comes back.
- 03
Stays on your computer
Your file is never uploaded. The out-of-phase fix runs in this browser tab.
Questions
Why does my song disappear in mono?
When one side is upside down, adding the two sides together cancels everything they share. In most songs that's the vocal, the bass and the kick drum, which sit in the middle, so a phone or a mono speaker plays mostly the reverb and the wide parts. Turn one side back and the middle returns.
Is this the same as a phase problem between two mics?
No. Two mics at different distances hear the same sound a moment apart, which is a time offset, not a flipped side. This page only fixes a side that is upside down. For a time offset, slide one track earlier or later in your editor until the waveforms line up, or use an alignment plugin. Next time, keep the second mic at least three times as far from the first mic as the first is from the voice.
Will this change the sound on headphones?
A little, for the better. The odd, hollow feeling goes and the voice sits in the middle again. Nothing else changes: each side keeps its own sound, and one of them is just the right way up.
What is correlation?
A number from +1 to -1 for how alike the two sides move. +1 means the same sound on both, 0 means unrelated sounds, like very wide stereo or two different mics, and -1 means one side is the other upside down. fix.fm only flags clearly negative readings, from -0.3 down, so a wide mix isn't called out of phase.
How do I fix out-of-phase audio in Audacity?
Open the track's menu and choose Split Stereo Track. Select the right-hand track, then choose Effect, Special, Invert. Open the top track's menu and choose Make Stereo Track to join them again. fix.fm does the same thing here, and tells you first whether a side is really flipped.
Is my audio uploaded anywhere?
No. The file is read and fixed inside this browser tab and never leaves your computer.
Understand the next step
Related fixes
Often turns up with Out of phase- FX-06One earVoice only on the left or right
- FX-13DC offsetWaveform sitting off centre
- FX-05Too quietLevel to podcast volume
Working on a song? Fix music checks it for this and every other problem a mix can have, in one pass.