48: What is Phase (and How to Deal With it)

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Have you ever tried recording something using two microphones?

Occasionally you’ll find that the sound of each microphone sounds good alone but that the tone changes when they’re combined. Where did the low end go? Why does it sound so thin?

If you’ve run into this, you may have stumbled onto the infamous “out of phase” issue.

Don’t worry! In this episode, Vadim and Ben explain the concept of phase and how to make sure you get it right in your recordings. They also discuss why getting phase “perfect” may not always be a good thing.

In this episode you will learn:

  • The basics of phase relationships in audio through real examples
  • The situations in which phase relationships may be a concern
  • Multiple audio and visual techniques for checking phase without buying any special tools
  • How phase relationships can have a negative impact on your recordings
  • About examples of cases when “out of phase” can actually be a good thing
  • To hear the differences between in-phase and out of phase audio files using examples acoustic guitars, drums, electric guitars and bass.
  • And more!

What is phase?

When we think and talk about phase we need to understand that phase is relative. There needs to be at least two waveforms present for there to be a phase relationship. When we add two waveforms that are exactly the same and perfectly aligned with each other the source will sound louder. This is because of “constructive interference”. The peaks of the waves add up to make bigger peaks while the troughs add up to make lower troughs. The result is a waveform that’s twice as loud.

What is phase cancellation?

When we flip the “phase” of one of the two waveforms, the peaks and valleys of the two waveforms will cancel each other out. In other words there will be silence. This cancelling of the wave forms is called phase cancellation (or “destructive interference”). Consequently as we start delaying the second copy of the inverted waveform, we are effectively changing the phase relationship. As the phase relationships between two waveforms change we’ll start getting an effect called comb filtering – which means that different parts of the frequency spectrum will start disappearing and others will be emphasized. As the amount of delay between the two waveforms changes, different frequencies will be impacted constructively and destructively until we reach a full 180 degree shift that equates to opposite waveforms effectively causing silence.

When do we need to think about phase?

So now that we understand phase, when should we care about it? Most importantly this applies to scenarios where you may need to balance multiple mics looking at the same source (drum kit mics, double mic’ing acoustic guitar, etc.) The distance that two or more mics are from a source will affect their phase relationship. Because of the comb filtering and frequency effects described above, this can cause tonal changes in your recordings.

What can cause phase issues?

Phase cancellation and comb filtering can be caused by a variety of things but there are a few common ones. First off, having multiple mics at different distances from the source can cause phase issues. Next, inverted or backwards mic and speaker wiring can cause weird phase relationships that affect what you hear coming out of a speaker system. Finally, reflections in a room picked up by your mic can cause phase cancellation between the direct signal and the reflected signal.

How to check for phase issues

It’s important to note that when recording a source with two different mics, flipping phase on one won’t necessarily cancel the other – even if they’re perfectly in phase because the waveforms picked up by the two mics won’t be identical and only identical waveforms can have a complete cancellation effect. However, phase issues will still affect the tone of the blended signal. One of the key areas to check for phase issues is on the low end. If your individual recordings sound deep and full but suddenly sound weak and thin when played together this might be and indicator of a phase issue. You can check the two tracks playing together and flip the phase between them and try to decide which one is better. You can also use a spectrum analyzer to see if the phase flip is affecting certain frequencies. As a result you will be able to find the best phase relationship. Additionally, you can check the width of what you are hearing. If you catch the sound coming mainly from the sides instead of the center, that may also be a clear indicator of a phase issue. Finally, just zooming in on the waveforms and seeing if the peaks are hitting at the same time is another way

How to correct phase issues

So now that we’ve identified the phase issues, how do we fix them? First, you can start by checking and fixing the distances of your mics. Having mics at equal distance from a sound source will usually result in the best phase relationship for the blended signals. Secondly you can try flipping the phase of your tracks in your DAW to see which relationship works best. Finally, you can try zooming super close on your waveform on a transient and see if the peaks are hitting at the same place. If they aren’t, in theory you can just align the waveforms manually effectively putting your recordings back in phase. That’s simple enough right? Well…not quite.

Should we ALWAYS correct phase issues?

Despite having the only recently available option of zooming in on waveforms and manually aligning them, there are some reasons why we may not want to.

  1. Phase discrepancy is an important factor in how we perceive the space of a recording. If you record something with a close microphone and a far microphone, the phase difference between the two recordings is what tells our brains that there is a spatial component. It allows us to “simulate” the recording room in our brains. Phase aligning those two microphones will actually sound weird.
  2. You’ll end up wasting a lot of time. Many phase issues, if not adding desired character, can actually be fairly benign and driving ourselves crazy with aligning every little peak can be extremely tedious. B

For much more, including real phase examples, check out the episode!