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Understanding Audio Phase And Correcting Issues

Has your mix ever sounded “not quite right” but you can’t put your finger on why?

By Daniel Keller March 21, 2016

Image courtesy of Dennis Bos

In The Studio
Phase issues take on a far different magnitude in a recording situation, and can quickly become complicated.

In most recording sessions, we’re dealing with multiple instruments and multiple microphones.

As sound waves of different frequencies reach different microphones at different times, the potential for one microphone’s diaphragm to receive a positive phase while another receives a negative is greatly increased, and the relationship between all of these different waves’ phases can be somewhat unpredictable.

In fact, the more mics in play, the more inevitable some sort of phase issues become.

Let’s look at a simple scenario, like a stereo recording of an acoustic guitar. Most often, two mics will be set up, with one pointed toward the sound hole to pick up the lower frequencies, and the second mic pointed toward the neck and fingerboard to pick up the attack.

Of course, the guitar’s frequency range covers several octaves, which means a wide range of different audio wavelengths. Since the mics are a fixed distance from the source, those different waves will arrive at the mics at different points. Inevitably, one or more harmonics will end up sounding weaker than the rest. Your best practice would involve moving the mics very slightly — even a fraction of an inch can make a difference — until you achieve the best sound to your ears. Another solution would be to use a mid-side miking technique, which you can read about in our Mid Side (MS) Mic Recording Basics article.

The UAD Little Labs IBP plug-in: a quick, helpful tool for adjusting phase.

It’s hardly surprising that the more microphones used in a recording, the more potential for phase problems. In modern music recording, that usually points to the drum kit. Consider even a single snare drum, miked from above and beneath. Since the top and bottom heads of the drum are usually moving in directly opposing motion (when the top drum head is hit, it moves inward, causing the bottom head to move outward), the two mics will inevitably be receiving signals that are directly out of phase. Now factor in the hi-hat mic, a pair of overheads, at least one kick drum mic and one on every tom, not to mention the relationship to ambient mics, and you’ve got a sonic soup that’s ripe for phase problems. That’s why many microphones, as well as mic preamps and consoles, offer a phase flip switch. It’s also why a lot of “old school” recording engineers wax nostalgic about the days when they recorded a kit with only two or three mics!

There are plenty of other “gotchas” that can introduce phase problems into your recordings. A bass track recorded through a Direct Input (DI) box can be too clean-sounding, and many times putting a mic on the amp cabinet and mixing the two sounds can give the track that little extra it needs. It can also introduce just enough out of phase signal to create problems. And certain delay settings, including pre-delays within a reverb patch, can create a delay of your original signal that ends up beingr out of phase enough to mess things up. And sometimes a certain piece of outboard gear, or even a mic cable, can be wired out of phase.

Correcting Phase Issues
So what’s the fix for phase problems? As with most things, the answer is “it depends.” Assuming you identify a phase problem during the recording process, a fix is as easy as moving a mic or flipping the phase on a mic or its input channel.

When attempting to capture ambience, there’s also a quick cheat: the 3:1 Rule of Mic Placement. Put simply, when using two microphones to record a source, try placing the second mic three times the distance from the first mic, as the first mic is from the source. So if the first mic is one foot from a source, the second mic should be placed three feet from the second mic. Using this simple 3:1 rule can minimize phase problems created by the time delay between mics.

Of course, if the problem doesn’t show itself until you’re mixing down, it’s often possible to pull the tracks up in your DAW, zoom in close on their waveforms, and slightly nudge one track just a bit. You’d be amazed what a difference just moving a track by one or two milliseconds can make. There are also some very effective phase alignment plug-ins on the market that can really clean things up — and even serve as great creative tools — one of which is the Little Labs IBP Phase Alignment Tool Plug-In, available on the UAD Powered Plug-Ins platform.

Sum It Up
We’ve only scratched the surface, but the bottom line is that phase issues are a fact of life, and practically unavoidable.

The first order of business is to identify the problem. Most phase problems will not show themselves in stereo, and will only appear when you collapse your mix into a single summed channel. That’s why it’s critically important, as you build your mixes, to check them regularly in mono. Don’t wait until you’ve got a completed mix to sum it into mono. Check the basic tracks, especially drums and bass, early on in the process when the arrangement and the mix are less dense and fewer things are going on. And check it again every time you add a few more instruments, or change a track’s EQ, or add reverb.

As with many things, the sooner you catch a phase problem, the easier it will be to fix. Happy mixing!

Daniel Keller is a musician, engineer and producer. Since 2002 he has been president and CEO of Get It In Writing, a public relations and marketing firm focused on audio and multimedia professionals and their toys. Despite being immersed in professional audio his entire adult life, he still refuses to grow up. This article is courtesy of Universal Audio.


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About Daniel

Daniel Keller
Daniel Keller

Chief Executive Officer, Get It In Writing Inc
 
Daniel Keller is a musician, engineer and producer. Since 2002 he has been president and CEO of Get It In Writing, a public relations and marketing firm focused on audio and multimedia professionals and their toys. Despite being immersed in professional audio his entire adult life, he still refuses to grow up.

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