How do you use phase inversion and delay to correct comb filtering in your mix?

1 What is phase inversion?

Phase inversion is the process of flipping the polarity of a signal, so that the positive and negative peaks are reversed. This can be done with a switch, a plugin, or a cable. Phase inversion can help you cancel out unwanted frequencies or enhance desired ones, depending on the phase relationship between the signals. For example, if you have two microphones on a snare drum, one on the top and one on the bottom, you can use phase inversion on the bottom mic to remove the low frequencies that are out of phase with the top mic. This can make the snare sound more crisp and punchy.

2 What is delay?

Delay is the process of adding a small amount of time to a signal, so that it arrives later than another signal. This can be done with a knob, a plugin, or a distance. Delay can help you align the signals that are out of phase due to different microphone positions or speaker placements. For example, if you have two microphones on a guitar cabinet, one close and one far, you can use delay on the far mic to match the arrival time of the close mic. This can make the guitar sound more full and coherent.

3 How to use phase inversion and delay to correct comb filtering

The first step to correct comb filtering is to identify the source and the extent of the problem. You can do this by soloing the tracks or channels that are causing the comb filtering and using a spectrum analyzer or your ears to detect the frequency dips and peaks. You can also use a phase correlation meter or a plugin like Auto-Align to measure the phase relationship between the signals.

The second step is to apply phase inversion and delay to the appropriate tracks or channels. You can start by using phase inversion on one of the signals and checking if the comb filtering is reduced or increased. If it is reduced, you can fine-tune the phase alignment by using delay on the same signal and adjusting it until you find the sweet spot where the signals are in phase. If it is increased, you can switch back to the original polarity and use delay on the other signal instead. You can repeat this process for each pair of signals that are causing comb filtering, until you achieve a smooth and consistent frequency response.

4 How to avoid comb filtering in the first place

To avoid comb filtering, use proper microphone techniques and monitor placement. Generally, you should use fewer microphones placed closer to the sound source to reduce phase differences and reflections. Directional microphones and filters can be used to reject unwanted sounds and capture different frequency ranges and characteristics. Additionally, headphones or in-ear monitors should be used to prevent feedback and interference from the speakers. Lastly, acoustic treatment and isolation can improve sound quality and reduce room modes and reflections.

5 How to enhance your mix with creative phase inversion and delay

Comb filtering is usually unwanted, but you can also use phase inversion and delay creatively to enhance your mix. For example, to generate a wide stereo image, use phase inversion on a duplicate track and pan them hard left and right. To create a sense of depth and space, use delay on a duplicate track and pan them opposite the original track. You can also apply delay with modulation or automation on a track to create a sense of movement and variation. Additionally, blending phase inversion on a parallel track with the original track can create contrast or a special effect.