Phase Cancellation: Everything You Need To Know!

Phase Cancellation: Everything You Need To Know!

Phase cancellation occurs when two or more audio signals are combined, and their waveforms are out of alignment in such a way that certain frequencies cancel each other out. This happens because sound waves are made up of pressure variations that move through the air, and when these pressure variations are exactly opposite (inverted) at the same time, they cancel each other out.

In simpler terms: when two similar sound waves are 180 degrees out of phase (opposite in polarity), their positive and negative pressure values cancel each other out, leading to a reduction or total loss of sound at those specific frequencies.

How Phase Cancellation Works

Sound waves are composed of compressions (high-pressure points) and rarefactions (low-pressure points). If two waves are in phase, their compressions and rarefactions align, resulting in constructive interference, which amplifies the sound.

However, if two waves are out of phase, their compressions and rarefactions oppose each other. When combined, the high-pressure point of one wave coincides with the low-pressure point of the other, leading to destructive interference—and this causes phase cancellation.

Common Causes of Phase Cancellation

Phase cancellation typically occurs in the following situations:

1. Multiple Microphones on a Sound Source

When using multiple microphones to record the same sound source (e.g., drums, guitar amps, vocals), phase cancellation is often the result of slightly different microphone positions. If two microphones capture the same sound from slightly different angles, the sound waves may reach each mic at slightly different times, causing them to be out of phase.

  • Example: If you use two microphones to record a guitar amplifier, one close to the speaker and one further away, the sound will hit each mic at slightly different times, causing phase issues, especially in low frequencies.

2. Stereo Microphone Techniques

In stereo recording techniques (like XY, AB, or ORTF), microphones are positioned at different angles, which can introduce phase discrepancies if the distance or angle isn't correct.

3. Inverted Polarity

Sometimes phase cancellation occurs because one of the channels is inverted (its polarity is flipped 180 degrees). If a signal that was recorded or processed in this way is combined with the original, phase cancellation will occur.

  • Example: If the same sound is recorded in two channels but one of them has been accidentally inverted, when combined, parts of the sound (particularly low-end frequencies) may cancel out, making the track sound thin or hollow.

4. Layering Tracks or Samples

When you layer multiple tracks or samples on top of each other, phase issues can arise, especially if the samples are from different sources (e.g., a drum kit where you have kick drum, snare, and overheads that weren't recorded with phase coherence). This can cause certain frequencies to cancel out, making your mix sound unclear.

5. Panning and Stereo Imaging

If two identical sounds are panned to opposite sides of the stereo field, phase cancellation can occur if they are out of alignment in time. This is especially problematic in the mid/side configuration, where the center and side channels have opposite phase relationships.


Identifying Phase Cancellation

You can often identify phase cancellation when:

  1. The sound is thin or hollow: If you notice that a track, especially in the low frequencies, sounds less full or lacks impact, there might be phase cancellation occurring.

  2. The sound disappears or weakens: When two tracks are perfectly out of phase, the result could be that entire elements disappear, like certain instruments or frequencies.

  3. Low-end issues: Phase cancellation often affects the low end (bass frequencies), making them sound weak or non-existent, since low frequencies have longer wavelengths and are more susceptible to phase issues.

How to Fix Phase Cancellation

  1. Adjust the Timing/Position of Microphones: If you're recording multiple microphones, adjusting the mic placement can help reduce phase problems. The 3:1 rule suggests that for every inch of distance between mics, the distance between the sources (e.g., snare and overhead) should be three times as great. This helps ensure that the signals reach the mics at slightly different times, reducing phase issues.

  2. Use Phase Alignment Plugins: Tools like Auto-Align (Sound Radix) and Waves InPhase can automatically detect phase problems and adjust the timing of tracks to improve phase coherence. These tools can be incredibly helpful when working with multi-mic setups.

  3. Invert the Polarity: If one of your tracks is inverted, simply flipping the phase (polarity) can sometimes resolve phase cancellation. Most DAWs have a button or switch on the track's mixer or channel strip to invert polarity.

  4. Move Tracks Slightly: In some cases, nudging the timing of one track (even by a few milliseconds) can resolve phase issues. For example, if you have a drum kit with close mics and overheads, slight time adjustments can align the signals better.

  5. Use EQ: In some cases, the phase issue may only affect certain frequencies (typically in the low end). Applying EQ to cut or boost certain frequencies can reduce the impact of phase cancellation.

  6. Mono Compatibility Check: If you’re working with stereo or multi-track mixes, always check how your mix sounds in mono. If it sounds thin or hollow in mono, that’s a strong indication that phase cancellation is occurring.


Conclusion

Phase cancellation is a phenomenon that occurs when two or more sound sources are out of phase with each other, causing certain frequencies to cancel out and resulting in a thin or hollow sound. It's common in multi-mic setups, layered tracks, or when polarity inversion occurs. By using tools like phase alignment plugins, adjusting microphone positions, or flipping the phase of tracks, you can resolve phase issues and ensure a clearer, more coherent mix.

By the Stealify Team! 

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