Audio Recording

I received an inquiry from Ivan relating to “phase cancellation” problem in audio recording:

From Ivan:
Can you please explain what is a “phase cancellation” problem?
I think I have this problem in a song. When I check my mix in mono, my guitar tracks (left and right) disappear. Is this phase cancellation ?
If so, how can I fix that?
Thanks

Emerson reply:

Phase cancellation is a serious issue in recording/mixing/mastering. Your problem is indeed phase cancellation. It is because your left and right guitars are “out of phase” with each other (like 180 degrees). You will NOT notice this problem when played in stereo but the problem will seriously affects when your audio signal is converted to mono.

An out of phase can be best explained graphically, supposing you have a stereo sound wave below:

If the top peaks on the left channel correspond exactly or even “near” perfectly with the top peaks on the right channel, and then you can say that the audio stereo sound file is “in-phase”.

If you convert a stereo to mono, the left and right channels are “sum up”, so if they are in-phrase you can basically sum their amplitudes and there is no cancellation due to mismatch.

Example = + 2dB (left channel) + (+2dB right channel) ~ 4dB summation, so there is no weakening of audio results. A “+” symbol indicates the phase. Since both right and left channel are positive, then they are in-phase with each other such as shown in the above screenshot.

What will happen if the left and right channels of the stereo are “out of phase”? See screenshot below:

The top peaks of the left channel corresponds to bottom on the right, they are now “out of phase”. Of course, this sounds normal in stereo. When the signal reverses polarity, the signs will change so a signal of -5dB will become 5dB and vice versa.

If out of phrase stereo is converted to mono: +2dB (left channel) + (-2dB right channel) ~ 0dB or almost no volume when played in mono because the wave are “cancelled”. This describes the “phase cancellation” problem. In actual scenario, they are not perfectly out of phase, so you can still hear mono signals but they are VERY weak unlike what they should sound in stereo.

How to prevent this?
1.) Do not use long delays between left and right channel. Chances are, these long delays can cause phase related issues. Be careful with the effects you use, make sure they are not introducing phase related problems.
2.) Check with your audio recording software the phase between the left and right signals. You can zoom in to make sure the left and right channels and matched /in phase.
3.) Check your speaker wiring. Make sure that left and right wires are in-phase with each other. Refer to your studio monitor manual.
4.) Always check your mix in mono. You can basically convert a stereo to mono and then play it to spot some phase cancellation issues.
5.) Mike properly and monitor the phase of the resulting wave. You might be tempted to use 2 mikes to record a guitar track simultaneously (one using DI for the left channel and one using microphone to get sound from the guitar amplifier – recorded to right channel).

This might be good for great stereo sound (delays are nice). But be careful, these delays can cause phase mismatch and introduce phase cancellation problems resulting to poor mono mix version of your audio track.