Audio phase is one of those phenomena that can be tricky to wrap your head around. Once you’ve got a grip on it, though, you’ll be able to take your productions to a whole new level. In other words, it pays to know about phase. Let’s dig into this concept further and explore its implications in the studio.
What Does Phase Refer to in Audio Production?
Audio phase indicates a point in time in a given sound wave. Sound waves are constructed of three main components: amplitude, wavelength, and frequency. The amplitude refers to the loudness of the wave at a particular point in time; for a perfectly symmetrical and repetitive sound wave, wavelength measures the distance between two equal amplitudes along the cycle; and frequency (a.k.a. “pitch”) is the number of times per second the sound wave repeats itself along that cycle. The phase of said sound wave tells us where exactly along this cycle we’re looking. In audio production, the relationship between two or more sound waves is what really matters; the absolute phase of a singular sound wave isn’t all that relevant for reasons we’ll discuss next.
When Waves Collide
Audio mixing is all about combining distinct but cohesive elements so that every component can be heard as intended by the artist, producer, and engineer. As such, you’ll be juggling countless sound waves that each vary in frequency, amplitude, harmonic overtones, and more. What’s bound to happen is that some waves will go in and out of phase with one another at different points in time. When two signals are “in phase” with one another, their amplitudes (i.e., peaks and troughs) coincide.
To keep things simple, you can imagine two perfectly symmetrical and repetitive sound waves, one in the left channel and one in the right. When both halves are perfectly aligned, their amplitudes are identical across time, meaning you would hear the same sound on both sides. Bring those channels together, and you’ve got what’s known as “constructive interference” because the combination of these in-phase waves doubles the resultant amplitude. Conversely, if these channels were perfectly “out of phase” (i.e., the lowest amplitude of one channel’s wave occurs when the other channel’s wave is at its loudest), their peaks and troughs would cancel each other out. This is called “destructive interference” or “phase cancellation.”
Phase in Practice
The most common origin of phase issues is when more than one microphone records one instrument or audio source. The previous sound wave example dealt with two channels of the same waveform (i.e., identical frequency). If you’re recording an instrument with two separate microphones, the incoming fundamental frequencies (i.e., the notes played) will be the same in each channel.
However, because each microphone is in a unique spatial position, different overtones will enter each mic at a different time. As a result, each channel’s sound waves will be similar in some respects but different in others. Some frequencies may get boosted, diminished, or virtually canceled depending on the phase relationship between the two channels. As you might imagine, introducing another microphone or more into the fold makes things even more complicated, increasing the odds of phase issues.
When it comes to recording drums, then, phase problems often run rampant. After all, most modern drum recordings involve a minimum of 5 mics (or as many as 20) to capture each component, the entire kit, and room reflections. It also doesn’t help that cymbals resonate at high frequencies or that kick and snare drums often require two mics to capture their lower and higher frequencies. If you’re not strategic with your mic placement/configuration, your initial drum recording can become a mess and nearly impossible to mix into submission.
Fortunately, the handy “phase flip” switch featured on several modern microphones can quickly resolve phase cancellation problems, whether you’re tracking drums, guitar, or anything else.
Reflecting on Phase
Multiple microphones aren’t the only culprits of phase problems. You can run into phase issues even if you’re only recording into a single channel, especially if your recording space isn’t properly treated. Sound waves easily bounce off acoustically reflective surfaces. These reverberations essentially duplicate the initial sound, sending back another, quieter, and tonally distinct version of it after a certain amount of time, depending on your proximity to the surface, the room’s size, and shape, etc.
If the timing of these reflections is just right, destructive or constructive interference may occur when captured by your microphone, altering the resulting tone and volume. Using delay and reverb effects intentionally can also cause phase problems. To make matters more complicated, you might hear phase problems when playing back a recording, even if your recordings themselves don’t have phase issues. This issue can occur if your speakers themselves are “out of phase,” i.e., wired with improper polarity.
How to Handle Phase Fiascos
With so many potential sources for phase problems, it’s crucial to arm yourself with knowledge, tricks, and tools that help you identify and resolve such difficulties.
For starters, a good rule of thumb to remember is the 3:1 ratio for mic placement.
This method applies when working with two mics; the second mic should be placed three times further from the initial mic as the initial mic is from the audio source being recorded. If one mic is six inches from the guitar’s soundhole, you should set up the second mic 18 inches (1.5 feet) away from the other mic. This trick doesn’t always work, and some adjustments may be needed, but it’s a good starting point for minimizing phase issues when recording with two microphones.
Another critical method for identifying and resolving phase problems is mixing in mono. It might seem counterintuitive to mix in mono, considering most tracks will be listened to in stereo. However, some instances of phase cancellation can go unnoticed when listening back in stereo—bringing your tracks into mono at various intervals while mixing can reveal phase problems you might have otherwise missed. Simply put, mixing in mono can help you further contextualize your mix as a whole and help you clear up any muddiness before returning to stereo sound.
On top of proper mix mic placement and mixing in mono, you can also use various plug-ins to correct phase interference and easily visualize what’s going on in these moments. Fortunately, there’s no shortage of phase-fixing implements available these days. Some prime examples include Waves’ InPhase plug-in, Little Labs’ In-Between Phase, and Eventide Precision Time Align. And if you get good enough at recognizing phase issues with your ear, simply moving tracks a tiny bit to the left or right can also resolve phase issues. This trick won’t always work, especially if your track sticks strictly to the grid.
Using Phase to Your Advantage
So far, we’ve mainly discussed phase interferences as problems to be solved. In truth, phase cancellation isn’t inherently a bad thing but merely an acoustical artifact that you can manipulate in several ways. Sure, steering clear or correcting phase cancellation is one way to deal with this phenomenon.
However, if you know what you’re doing and what you want from your mix, you can use phase cancellation as another mixing tool. For example, manipulating the phase relationship between guitar tracks can allow you to shape the resultant track’s tone (the same goes for the tone of any instrument or vocal), akin to a filter EQ. Specific devices (i.e., Neve Portico 5016 and Phazer by Radial) contain phase-shifting circuits that let you pick and choose the frequencies you want to boost while still canceling out for unique tone-shaping possibilities.