Mixing: What Is Phase & Why Is It So Important To Get Right?

The phase coherence of individual channels in a mix is vital to a strong mix, but it's an often overlooked aspect of mixing. If you've landed on this article, you're likely wondering how and why phase is such an important part of mixing. I'm here to tell you.

In this article, we'll discuss phase in more detail, its importance in the mix and how to improve the phase relationships in our mixes for optimal results.

If you prefer video format, please check out my video on the topic of phase below:

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What Is Phase?

In periodic waveforms (like audio signals), phase refers to the position of a point in time on a wave cycle, measured in degrees. The beginning of a waveform starts at 0º and repeats every 360º.

In a sine wave, we generally start at an amplitude of 0 at 0º. We work our way up to the maximum amplitude at 90º and go back down to 0 amplitude at 180º on the way to the maximum negative (or minimum) amplitude at 270º. We then return to 0 at 360º, where the cycle repeats.

This cycling and phase can be visualized in the following graph:

Inverting the phase of a cyclical sine wave would flip this around so that the maximum negative (minimum) amplitude would be at 90º and the maximum amplitude would be at 270º.

An inverted sine wave would look like this:

If we duplicate a signal and play both copies back, we'll get a doubling of the signal level (+6 dB):

However, if we flip the phase of one copy, the two will cancel each other out entirely, giving no output signal:

While real-world audio signals are rarely cyclical (even with synthesizer signals), we can still think of them in terms of phase, though more so as a concept than hard science.

Rather, in the real world, we won't have the same exact audio on two tracks unless we copy the audio exactly. Audio signals are complex, and recording two perfectly in-phase audio tracks is nearly impossible. This is true of multi-miked recordings, double-tracked instruments and layered samples.

So when we're discussing phase in the context of mixing, we're really talking about the overall alignment or similarity between the channels' waveforms. This is true of individual tracks in the session and also of the multiple output channels in multi-channel mixes (the left and right channels of a stereo mix, for example).

The more two signals/waveforms match up in terms of amplitude and time, the more we can say they are “in phase” with each other.

When two or more signals are more in phase with each other, they'll combine together into a healthy, constructive output. When the signals are more out-of-phase with each other, we'll run into issues of destructive interference that will ultimately thin out the output signal.

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Why Are Phase Relationships Important In The Mix?

I've already alluded to this, but ensuring proper phase relationships in the mix is a big part of getting a strong, powerful mix.

Having good phase coherence between our tracks allows them to work together in the mix rather than against each other (cancelling each other out).

For instance, if we have a snare top and a snare bottom mic track in the mix, they'll likely be largely out-of-phase during the snare hits. This is because they're capturing the same source and are pointed toward each other.

Suppose the initial snare transients show up as negative amplitude in the top mic track and positive amplitude in the bottom mic. In this case, these two waveforms will mostly cancel each other out even if they aren't perfectly aligned. Flipping the phase of one of these mics during recording or flipping the phase of the track during mixing will allow these two snare tracks to work synergistically to produce a stronger, more present snare in the mix.

Low-end frequencies are also susceptible to thinning out if the phase relationships aren't on point. For example, aligning multiple kick samples so their transients and the low-end of their waveforms (the longer wavelengths) are as “in-phase” as possible will yield a stronger result than having them largely out of phase with each other.

Beyond the individual tracks, we should be concerned about the mono compatibility of our stereo mixes and how “in-phase” or “out-of-phase” the left and right channels are with each other.

Of course, it's these differences between the left and right channels that give us the stereo effect. If the left and right channels were identical, we'd effectively have mono audio (where the same signal is sent to all speakers).

Rather, mono compatibility is the practice of ensuring unsubstantial changes in frequency content as a multi-channel (typically stereo) recording is summed to mono. Mono compatibility is important because some playback systems, such as PA systems, smartphones and Bluetooth speakers, playback audio in mono.

So we must walk a fine line here. We want the width and directionality that stereo mixing has to offer, thanks to the differences between the left and right stereo channels. However, we don't want to push the left and right channels too far “out-of-phase” and cause severe phase cancellation when the mix is summed to mono.

Phase correlation meters are decent tools for showing us how the left and right channels line up phase-wise. Modern digital audio workstations worth their salt will come with a stock phase correlation metering plugin.

Phase correlation meters span continuously from -1 to +1, or from 180º to 0º. They can be put on stereo tracks or the stereo mix bus to meter the phase relationship between the left and right stereo waveforms.

At +1, we have a 100% correlation between the channels (they are exactly the same).

At 0, we have the “widest permissible left/right divergence” or the widest permissible stereo image.

Having the mix bus correlation meter moving between 0 and 1 is ideal. Smaller variations mean smaller differences in width.

At -1, our left and right channels are completely out of phase and will completely cancel each other out.

Mix bus correlation meter values between -1 and 0 mean that significant phase issues are present that will interfere with the stereo audio and definitely with the summed-to-mono audio.

It's best to hover between 0 and +1. However, we may want our mixes a bit closer to +1 to ensure better phase relationships between the left and right channels and, therefore, better mono compatibility.

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How To Improve The Phase Relationships Between Tracks In A Mix

We understand phase and the term's usage in mixing, along with the importance of proper phase relationships between the channels of our mix, including the individual audio tracks and the channels of stereo and other multi-channel mixes.

Now let's turn our attention to the practical steps we can take to help improve the phase relationships between the tracks in our mixes!

First, if we're also tasked with recording, we should opt to set up our recording to capture our audio as “in-phase” as possible. This typically means careful mic placement and polarity flipping where necessary.

To be more specific about the mixing process, we'll be discussing the following strategies and tips in this section:

• Polarity flipping
• Audio nudging
• Sample/time stretching
• Phase rotation

Polarity Flipping

Polarity flipping or, erroneously, “phase flipping” (as a matter of semantics) is typically achieved via a simple button or toggle and acts to flip the polarity of a waveform. The positive amplitudes become negative and vice versa.

Polarity flipping/inverting is an easy way to fix audio waveforms that are nearly completely out of phase with one another. Flipping one track will align it with the other.

Perhaps the most common situation that calls for polarity flipping is when two microphones face each other to capture the same source. I mentioned the case of having a top and bottom snare drum microphone earlier, as it's one of the more common instances where polarity flipping is warranted.

However, we can also have instances where mics are spaced apart in such a way that they capture their source in poor phase. While these issues are best addressed at the recording stage, we may need to address them in the mix.

We may also benefit from polarity inverting when mixing or producing with samples. If two samples are playing together and are largely out-of-phase, we can opt to flip the polarity of one versus the other.

When deciding which track to flip the polarity of, consider the other tracks in the mix and choose the one that best aligns with every other waveform taken into consideration. For example, flip the phase of the bottom snare mic (pointing upward) rather than the top snare mic (which points downward along with the tom mics, overheads and cymbal spot mics).

Audio Nudging

In most cases, a simple polarity flip won't entirely solve our phase woes. Rather, it's often the case that we should get more surgical in our phase adjustments to help the mix.

Audio nudging, on the macro scale, will alter the timing of tracks in the mix. Although this may be necessary, it's not what we're discussing here.

Rather, I'm writing about nudging the audio clips of individual tracks ever so slightly so that they align better with the other tracks in the mix.

A great example here is aligning the overhead drum mics (let's assume a right and left mic) to the top snare mic. In this instance, we have three mics, all effectively capturing the sound of the snare drum but at varying distances from the snare drum.

By nudging each overhead track so that the snare transients align between all three mics, we can improve the overall clarity and punch of the snare drum as we bring the different signals closer in phase.

Sample/Time Stretching

In some particular cases, it may be necessary to time stretch or compress a certain track to fit in phase with the other tracks of the mix. This is generally reserved for samples, which are consistent (and can be consistently altered) and triggered (versus having to time stretch a full recorded audio clip).

This strategy is rarely used, but I figured I should mention it as it can be effective, again, in very particular instances. For example, getting two or more problematic kick drum samples to work together synergistically in the mix.

Phase Rotation

Phase rotation is a powerful tool for phase alignment. It works by shifting the phase of each individual frequency by the same amount rather than the entire waveform.

Phase rotation technically distorts the waveform (it changes the waveform shape) without changing the perceived sound or level of the audio (as would be the case with a typical distortion effect).

As we shape the waveform, we can form the audio signal to better align with the other tracks in the mix, phase-wise.

When using phase rotation, we'll have to depend on our critical listening skills to hear how the effect is making a difference for the better. If we find that it helps, we can always bounce down the affected audio signal and go on to read the bounced waveform against the other tracks' waveforms in the mix.

In addition to helping with phase alignment, phase rotation can help us make asymmetrical waveforms more symmetrical. Increasing symmetry lowers peak levels (of the positive or negative peaks) without altering the perceived loudness or sound of the signal. By making the signal more symmetrical, phase rotation also increases the overall headroom of the track and often the mix as a whole.

The Phase Rotate by x42 is a great phase rotator plugin option.

Other Tips For Improving Phase Relationships In The Mix

I'll state once again that phase issues disproportionately affect the longer waveforms of low-end frequencies. While phase issues are often negligible in the high-end (and practically impossible to deal with, anyway), we should pay special attention to our bass elements.

This is why high-pass filtering noise and unmusical low-end information can help tremendously in improving the low-end solidarity of the mix. First off, we're removing noise. Second off, we're greatly reducing the number of potential phase relationships in the low-end that can really mess with the sub-bass and bass frequencies.

I should add, too, that while high-pass filters are excellent tools for ridding of low-end noise in tracks, EQ does have a side effect of frequency-dependent phase shifting.

Higher-order filters, which are steeper or narrower, have more reactive components (i.e., capacitors) and cause greater phase shift.

So the steeper or narrower the EQ move, the greater the phase shift.

Typically, high-pass filters cause a positive phase shift at and below the corner frequency.

Low-pass filters cause a negative phase shift at and above the corner frequency.

High-shelf boosts and low-shelf cuts cause a positive phase shift at their corner frequency, while high-shelf cuts and low-shelf boosts cause a negative phase shift at their corner frequency.

Bell-type EQ bands cause both positive and negative phase shifts around the centre frequency.

So be aware of the frequency-dependent phase shifting caused by EQ (especially with aggressive EQ moves) and mix accordingly.

If you find yourself having issues with phase-shift in EQ (it's usually not too bad), you can always opt to use a linear phase EQ that's designed to produce no phase shifting, though at the expense of increased CPU load and the potential for pre-ringing artifacts.