A polarity flip turns a signal upside down and leaves its timing alone. A phase shift moves a signal through its cycle, and because a low note’s cycle is longer than a high note’s, the same shift lands differently on every frequency.
In a session, the Ø button fixes polarity and nothing else. If two tracks still sound hollow with the button in either position, the problem is timing, and you fix it by moving one track until the two line up.
At a glance
| Property | Polarity | Phase |
|---|---|---|
| What it describes | Which way up the waveform is | Where a wave is in its cycle |
| Values | Normal or flipped | Any angle from 0° to 360°, at each frequency |
| Does anything move in time? | No | Yes, when the shift comes from a delay |
| Same at every frequency? | Yes, 180° at all of them | No |
| The usual control | The Ø button or a utility plug-in | Moving the track, a delay, or a phase-alignment plug-in |
| Two identical copies, added | Silence when one is flipped | A comb of notches when one is late |
| Where problems come from | Snare bottom mics, wiring, figure-8 mics, layered samples | Mics at different distances, reflections, Haas delays, filters |
What is polarity?
A recording is a list of numbers that tell a speaker cone how far to push out or pull in at each moment. Polarity is which way round those numbers are. Flip it and every positive sample becomes negative and every negative one positive, so the cone pulls wherever it used to push, by the same amount and at the same instant.
The flip is the whole of it. There’s no setting between normal and flipped, and nothing happens to the timing, which is why the switch adds no latency. On its own a flipped track sounds the same as before. Against a second copy of the same sound, such as another mic or a layered sample, it decides whether the two add up or cancel. The polarity lesson goes through where flips come from and how to check for them.
What is phase?
Phase is a position in a cycle. One cycle of a wave is a push and a pull, and phase counts how far through that cycle the wave is in degrees, from 0° at the start to 360° when it gets back there. A 100 Hz wave takes 10 ms to go round once, so 1 ms into the cycle it’s at 36°, and 5 ms in it’s halfway, at 180°.
Like polarity, the phase of a single sound tells your ears very little. What you hear is the phase of one copy compared with another: in step they add up, half a cycle apart they cancel, and anything in between gives something in between. The phase lesson starts from the beginning and covers stereo and mono as well.
The usual way two copies end up out of phase is a delay. A second mic a little further from the drum hears it a little later, and the lab’s Delay R control does the same to the right channel.
Why do people mix up phase and polarity?
The labels are part of it. Mixing consoles have put a phase or phase reverse button on each channel for decades, usually marked Ø, and DAWs kept the name: Logic’s Gain plug-in and Pro Tools’ Trim plug-in both call it Phase Invert. Every one of those buttons flips polarity.
The other reason is the sine wave. A sine turned upside down is exactly the same picture as a sine pushed half a cycle along, so on a single test tone a flip and a 180° phase shift are impossible to tell apart. Most diagrams of phase use a sine wave, and on that picture the two words really do mean the same thing.
Real sounds are made of many frequencies at once, and that’s where the two come apart. You can see it in the lab. Flip the right channel of the guitar and the pink wave on the 110 Hz screen turns upside down without sliding anywhere; delay it instead and the pink wave slides along.
A flip vs a delay
A delay is a fixed amount of time, and every frequency has a different cycle length, so the same delay is a different number of degrees at each frequency. A 1 ms delay is 36° at 100 Hz, 90° at 250 Hz, 180° at 500 Hz and a full 360° at 1 kHz, where the late copy is one whole cycle behind and lines up again. A flip is 180° at all of them.
That difference decides what you hear when the two copies are added together. The table gives the level of two equal copies added in mono, with 0 dB meaning they add up as if in step.
| Frequency | Flipped | 1 ms late | Flipped and 1 ms late |
|---|---|---|---|
| 100 Hz | Cancels | 36°, −0.4 dB | 216°, −10 dB |
| 250 Hz | Cancels | 90°, −3 dB | 270°, −3 dB |
| 500 Hz | Cancels | 180°, cancels | 360°, 0 dB |
| 1 kHz | Cancels | 360°, 0 dB | 540°, cancels |
| 1.5 kHz | Cancels | 540°, cancels | 720°, 0 dB |
The flipped column is the same all the way down: two identical copies, one upside down, add up to nothing. The delayed column is a comb filter, with notches where the delay is an odd number of half cycles and full level where it’s a whole number of cycles. In the lab, the response screen draws this from your settings and labels every notch.
When a delay looks like a flip
Pick a frequency and you can always find a delay that puts it at 180°: half of its cycle. For 110 Hz that’s 1 ÷ (2 × 110), which is about 4.5 ms. On a 110 Hz sine wave, a delay of exactly that length and a polarity flip give the same result, and summed in mono both cancel the tone.
Put the same 4.5 ms delay on the guitar and it no longer behaves like a flip. 110 Hz still cancels, along with every 220 Hz above it, while the frequencies halfway between, such as 220 Hz and 440 Hz, are whole cycles behind and add up. The guitar comes through hollow instead of disappearing. In a room, 4.5 ms is the gap you’d get from a second mic just over 1.5 m further away than the first.
Phase shift without a delay
A delay isn’t the only thing that shifts phase. An allpass filter lets every frequency through at the same level and changes its phase by an amount that depends on the frequency, along a smooth curve. The first-order one in the figure, set to 300 Hz, shifts 300 Hz by 90° and gets closer to 180° the higher you go. A phaser mixes a sound with a chain of these, and that’s where its moving notches come from.
Ordinary EQ shifts phase too. A filter or EQ band changes the phase of the frequencies around it as part of changing their level, so a low cut on one of two mics can knock the pair slightly out of phase in the low end even when their timing is spot on. Linear-phase EQ avoids that by delaying the whole signal, at the cost of latency.
Some phase-alignment tools are built on allpass filters. Little Labs’ IBP and Waves’ InPhase shift the phase of one track against another, up to 180°, around a frequency you choose, so you can bring the low end of a DI and an amp mic into line without moving either in time. Sound Radix’s Auto-Align works on the timing instead: it measures the delay between two mics and moves one to match. The Ø button does neither, and only ever has two positions.
Can a polarity flip fix a delay?
Only at one frequency and its odd multiples. Back on the guitar with the 4.5 ms delay, flip the late side and 110 Hz comes back into step, since half a cycle plus an upside-down wave brings it round to 360°. The notches haven’t gone, though. They’ve moved to 220 Hz and every 220 Hz above, with a new one at 0 Hz that drains the bass from under the guitar.
Adding a flip to any delay does the same: every notch moves to where a peak was, and one lands at the very bottom of the spectrum. How much that helps depends on the gap. With a tenth of a millisecond the first notch is up around 5 kHz, so the flip mainly decides whether the low end adds up or cancels. A millisecond or more leaves a comb through the middle of the sound whichever way the switch is set, and the only fix is to slide one track in time until the delay is gone.
Telling them apart in a session
Say two tracks of the same sound don’t combine well, and you need to know whether it’s polarity, timing or both. The table sorts the usual symptoms.
| What you see or hear | Polarity | Timing (phase) |
|---|---|---|
| Zoomed-in waveforms | One is a mirror image of the other | The same shape, starting a little later |
| Added together | Thin all over, worst in the low end | Hollow or boxy, with some frequencies missing |
| Flip one track | Fixes it | Changes the colour, but it stays hollow |
| Nudge one track in time | Never fixes it | Fixes it once the hits line up |
| Correlation meter, on a stereo pair | Pushed below zero | Depends on the sound and the gap |
A correlation meter is most use for spotting a flip between the left and right of a stereo track, since that pushes the reading below zero. A delay can land the needle almost anywhere, so for timing trust the mono check and your ears.
How to fix polarity and phase problems
Use this on any pair of tracks that recorded the same source: kick in and out, snare top and bottom, DI and amp, a close mic and the overheads.
- Listen to the pair in monoMute everything else, set the two tracks to mix levels and put a mono switch on the master, so their cancellations are as loud as they’ll ever be.
- Try the polarity switchFlip the second track with its Ø button and compare. Keep the setting with more low end and a higher combined level on the meter.
- Look at the timingZoom in on a loud hit in both tracks. If the later one starts more than a few samples after the first, drag it earlier until the first big swings line up, then listen again.
- Check polarity againOnce the timing is close, compare the switch in both positions a second time, since the best setting can change after a move.
- Bring the rest backUnmute the other mics and listen in stereo and mono, because a fix for one pair can upset another.
The drum phase alignment lesson takes a whole kit through the same process, and the comb filtering lesson covers gaps you can’t remove, such as a mic picking up a reflection off the floor.
Reading the lab
The lab runs the loop through a delay and a polarity switch on the right channel, then the Mono button, which adds left and right together. Width, pan and the copy controls are locked on this page so it stays on the two things being compared. With the acoustic guitar selected the two channels start out identical, so pressing Mono is the same as mixing two identical mics.
Watch the two cycles at 110 Hz, with the left channel in teal and the right in pink. A flip turns the pink wave upside down where it stands, and a delay slides it along, with the gap in degrees written above. With Mono on, both waves show the summed signal instead, so judge the shapes in stereo. The wide screen shows what mono would do to the tone whether Mono is on or not, measured by sending a click through your settings, and the panel on the right explains what you’re hearing using numbers measured from the loop.
Common mistakes
Calling every cancellation a phase problem
It sounds like hair-splitting, but the word points you at the fix. A polarity problem takes one click to solve. A timing problem needs a track moved, and no amount of clicking the Ø button will get you there.
Using the flip to fix a delay
If both positions of the switch sound hollow, there’s a gap in time between the tracks. The flip only trades one set of notches for another, as the seventh step in the lab shows.
Treating 180° as one thing
A 180° shift at one frequency comes from a delay or a filter, and a 180° shift at every frequency is a polarity flip. When someone says a track is 180° out, ask which they mean before you reach for a fix.
Forgetting the filters
A low cut or an EQ boost on only one of two mics shifts its phase near that frequency. If a pair sounded fine before you EQ’d it, try the same EQ on both, or EQ the pair together on a bus.
Train your ears
The quiz under the lab plays a loop with the screens covered and asks one of two questions. Some rounds ask whether the sound is in phase or has one side flipped; listen for a centre that has gone vague and bass that sounds lighter. Others ask whether it would survive mono, and those hide short delays as well as flips, so listen for a hollow, boxy tone. Every round is matched in loudness and plays in stereo.
Questions people ask
Is polarity the same as phase?
No. Polarity is which way up a waveform is, and a flip changes it at every frequency at the same instant. Phase is where a wave is in its cycle, and a delay changes it by a different amount at every frequency. The two only look the same on a single sine wave.
Is flipping polarity a 180° phase shift?
At every frequency at once, yes, which is a good way to describe it. What no delay can do is give 180° everywhere: a delay reaches 180° at one frequency and its odd multiples, and lands at other angles everywhere else.
Should I flip polarity or move the track?
Flip first, because it takes a second. Listen to the pair in mono, keep whichever setting has more low end, then nudge the later track earlier if it still sounds hollow. A gap of more than about a millisecond needs moving whatever the switch says.
Why is the polarity button called phase?
Consoles labelled it phase or phase reverse for decades and the name stuck. Logic’s Gain plug-in and Pro Tools’ Trim plug-in still call it Phase Invert. Any button that turns the waveform upside down without moving it in time is a polarity switch.
Does EQ change phase?
Most EQs do. A normal EQ band shifts the phase of the frequencies around it, which is part of how it changes their level. Linear-phase EQs avoid that by delaying the whole signal equally, which adds latency and can smear sharp transients.
Which sounds worse in mono, a flip or a delay?
A flip between two identical channels is worse, since the shared sound cancels completely. A delay leaves a comb of notches, which can still spoil a mix but keeps most of it audible. On real stereo mixes either can make the centre thin.