What Is Phase Correction?
I will try to explain phase correction without getting into boring theory stuff what phase is and how to correct it while recording.
But before I do, I want to emphasize that it is one of the most important things you should keep an eye (and ear) on while you record an instrument with two or more microphones (or line out and a microphone) because it will be the difference between a thin demo sounding recording and a thick, punchy professional sounding recording.
If you just have a sound card and no pre-amps, good miking techniques and good phase alignment of the microphones can make up for that and you can still come out with a punchy recording. Some people may not even tell the difference.
In words anyone can understand, phase is the direction of the wave form. Whether it’s going up or down. For example:
(Zooming in on a wave form in your digital audio recording application)
The Phase is going up.
The Phase is going down.
When Is Phase Correction Needed?
So in a multi-track recording application, when you record an instrument with two or more inputs (line or microphones or combined), you should always check that the phase of each input is correctly aligned and going in the same direction (up or down). Why? Because when they’re not, frequencies are cancelling each other out (especially low frequencies), It’s an audio phenomenon. And that’s why your recording doesn’t sound punchy.
Here is an example below of what a correct phase alignment of two microphones should look like in a multi-track recording application:
Correct Phase alignment.
Incorrect Phase alignment.
Keep in mind that the phase will not always be completely aligned throughout the whole song. So just find a big “spike” in the audio form and use it as a reference. Also, you can really hear the difference. Slide one track a bit to the left or the right and leave it where it sounds punchier and fuller.
So how do you apply phase correction and correct it in different kinds of instruments? Here are some tips:
For drums, let’s say you’re recording with the typical 8 microphone setup, have the drummer record at the end of the song (after the cymbals fade out) one firm hard kick on the kick drum. This will create a big “spike” (which you can delete later) on all of the 8 tracks. Use it to make sure the phase is going in the same direction (up or down) on all microphones and that they are aligned.
If the phase is inverted in one or more microphones, most audio editing software have an “invert” option for phase correction in the effects section or add an EQ plug-in that has a button that looks like this: ∅ And press it. That is the symbol for phase switching. Also some hardware equipment like pre-amps or compressor/limiters have a phase button to invert the phase on the incoming signal.
When recording guitars or a bass with lets say the line out jack in the amplifier and one or more microphones in the front of the amplifier box, at the end of the song have the musician mute the strings with one hand on the frets and slap the strings with the other hand over the pick-ups. This will create a “spike” that will easily allow you to see the phase of all inputs in your multi-track recording software application. Use it as a reference for phase correction.
With other instruments such as trumpets, accordions, violins, etc. A trick for phase correction is to just stand in front of the microphones and clap your hands once loudly and use that as your reference. Remember, the phase doesn’t need to look identical, just going in the same direction and pretty much aligned. Always do what sounds better. When applying phase correction always remember that your ears should have the final word not your eyes.