Phase describes where a periodic waveform is in its cycle at a given time. The relationship in time between two or more waveforms with identical or harmonically related periods gives us a measurement of their phase difference.
Phase relationships are extremely important during all stages of audio production, as out-of-phase waveforms, whether they spring up during recording or throughout the later stages of production, will wreak absolute havoc on your mixes. These out-of-phase waveforms result in an unpleasant phenomenon that's known as phase cancellation.
In this post, we'll do a deep dive into phase cancellation, what causes it, how you can identify it, and — ultimately — what you can do about it.
Phase vs. Polarity — There's a difference
Before we dive into phase and phase cancellation, we should first discuss the concept of polarity, as it's very easy to confuse the two. In a nutshell, phase is a function of time, while polarity is a function of positive and negative changes.
Polarity really comes into play during the recording process, as you'll want to ensure that your loudspeaker moves in the same direction as the microphone membrane while it captures a sound source. During the initial attack of a sound, your mic creates a positive voltage, passes it through a cable and preamp and into your recording device, then finally out of a speaker.
If you've maintained correct polarity throughout your entire signal chain, your speaker will move in a positive direction with every initial attack of your sound source. If you haven't maintained correct polarity, your signal will do the opposite, which can cause destructive interference when combined with other signals with correct polarity.
Signals can be anywhere from zero to 180 degrees out of polarity, with zero being in correct polarity and 180 being in reverse polarity, which will result in complete signal cancellation.
While you may not notice incorrect polarity with a single sound source, it will most certainly cause issues in a multitrack recording.
The easiest way to understand polarity is to run identical signals through two speakers. Then, flip the polarity of one of the speakers 180 degrees by reversing its positive and negative wires.
Doing this will cause the two signals to cancel each other out.
When a microphone is 180 degrees out of polarity, you'll get a similar result to that of reversing the polarity on a loudspeaker. That's why most mixing consoles, DAWs, and mic preamps include a polarity reverse function.
To reiterate: polarity is a function of positive and negative changes and phase is a function of time relationships. While these two terms are sometimes used interchangeably, they are not the same thing and must be addressed separately.
Why Does Phase Cancellation Happen?
Audio waveforms move in periodic cycles, in that they proceed through regular cycles or repetitions. As we noted earlier in this post, phase describes where the periodic waveform is in its cycle at a given time.
Phase is measured in degrees, with 360 degrees being one completed cycle.
When two waveforms are mixed together, especially identical or similar waveforms (i.e., two microphones capturing the same sound source), the two sounds should commence at exactly the same moment.
If one of the waveforms is delayed with respect to the other, they will be out of phase with one another. This will result in frequency cancellations, which produce a hollow sound and comb filtering effects.
How much cancellation, and at which frequencies it occurs depends on the waveforms involved and how far out of phase they are. Signals can be zero (in phase) or 180 degrees (completely out of phase) with one another, as well as any point in between.
Anytime complex audio signals are combined, you can expect some degree of phase cancellation. That said, if two identical signals are 180 degrees out of phase with one another, they will completely cancel one another out when combined.
Because folding your stereo or multichannel mix down to mono involves combining complex audio signals, you can almost count on some level of extra phase cancellation (relative to the stereo mix) to occur. This is why it's important to check your mix in mono periodically while mixing.
Doing so will ensure that whatever phase cancellation occurs doesn't make your mix fall apart while playing on a mono Bluetooth speaker and other similar devices.
How to Identify Phase Cancellation
We've all heard the expression "Mix with your ear, not with your eyes." While this is generally stated with good intentions, mixing with your eyes (using meters) is crucial to getting a good sounding mix.
After all, what are meters for? Even old-school engineers in the golden age of analog used them — they're a vital part of audio engineering.
When it comes down to identifying phase cancellation, the pro engineer's tool of choice is the mighty correlation meter. And what makes it mighty?
Simply put: your mix can come completely unglued if you don't use one!
That's why our LEVELS plug-in includes a top-shelf correlation meter in its Stereo Field section. The correlation meter, which is located on the left side of the plug-in's circular display, displays the degree of similarity between your left and right channels.
If the meter reads near +1, your mix is balanced and free of serious phase issues. If the pointer hovers past the central point towards -1 (which will also cause the Stereo Field icon to turn red), you have problems that need to be addressed, as your mix likely won't translate well when played back in mono.
Our EXPOSE application also includes a stellar phase correlation heat map to help you understand the degree of similarity between your left and right channels. EXPOSE allows you to analyse the bounces file in just a few seconds, allowing you to catch any phase issue before releasing the song to the world.
Like the correlation meter in LEVELS, EXPOSE's phase correlation heat map will read near +1 if your mix is free of significant phase problems, and it will point towards -1 if you have issues.
How to Fix Phase Cancellation (Or Avoid it Altogether)
To avoid phase cancellation in your mix, it's crucial that you follow proper recording techniques and to pay attention to the phase relationships between your tracks.
The most common reason for phase cancellation occurs at the recording stage, when you're capturing a single source with multiple microphones (a fully mic'd drum kit is a frequent culprit).
This is because, as we discussed earlier, blending identical or similar waveforms will cancel each other out unless they're captured at precisely the same moment. Even slight delays in one or more of the waveforms will result in ugly comb filtering effects.
An effective way to minimize phase issues while recording is to use the 3:1 rule. To utilize the 3:1 rule, place the second mic three times the distance from the first mic that the first mic is from the source.
For example, if your first mic is five feet from your audio source, the second mic should be 15 feet away from the first mic.
Other common sources of phase cancellation are synth patches with built-in effects. One way to solve this issue is to use only a single channel of the patch (i.e., use only the left channel).
Overlapping frequencies within the different elements of your mix can also cause phase issues. The easiest way to solve these issues is to use an EQ to attenuate the offending frequencies in one or more of the elements to minimize the unwanted sonic artifacts.
If you're at the mixing stage and you can't re-record a sound source, there are numerous phase alignment plug-ins available, such as MeldaProduction MAutoAlign, Waves InPhase, and Solid State Logic Native X-Phase.
You can also simply nudge one of the problem tracks a few milliseconds forward or back. You'll be shocked at what effect even a small adjustment can have on the phase of your mix.
Regardless of which correction method you use, be sure to monitor your mix in mono and listen for the sweet spot where the audio stops phasing.
If audio engineers have an arch enemy, its name just might be "phase cancellation."
If you follow the guidelines laid out in this post, you'll find yourself well on the way to achieving a solid-sounding mix, free of phase issues, that translates perfectly when you fold it down to mono. And, if your mix sounds great in mono, it's much more likely to sound great on any playback system.
Keep reading our blog for more helpful mixing tips and techniques!