Phase describes where a periodic waveform sits in time. When the same sound is handled by two microphones or on two tracks, whether the peaks and troughs of the waveforms line up or are offset changes how the level, the low end, the definition, and the width are heard.
How the waveforms line up
If the peaks and troughs of two identical waveforms line up, adding them together makes the sound stronger. If they are offset, some frequencies add and others cancel. A 180-degree offset is described as a relationship of opposite polarity, but real recordings can have complex phase relationships that include time offsets and changes that differ by frequency.
Where it comes up
Phase makes itself felt when you record drums with several microphones, mic a guitar amp with more than one microphone, duplicate a sound and delay it, or use stereo widening effects. The causes are the distance between microphones, the difference in arrival time from the source, plugin processing, and differences between the left and right channels.
Why the sound gets thin
When you mix similar signals and certain frequencies cancel, you get changes such as reduced low end, blurred definition, and a thin, swishy quality. This is sometimes described as sounding like a comb filter. Even when it is not obvious in stereo, the problem can become much bigger when the mix is summed to mono.
The order for checking and fixing
First listen to each side on its own, then to both together, then switch to mono and check what changes. Moving the microphone, nudging the timing of the recorded material slightly, trying a polarity flip, and reducing unnecessary overlap are all possible fixes. Do not decide from the waveform display alone; always judge with your ears.
The trade-off with width
Width created from differences between left and right can involve phase differences. If you give priority to the stereo impression alone, you can lose mono compatibility. After creating width, check in mono and adjust so that the core sounds of the song survive. When you need to convert material for different uses, you can use the audio format converter tool.
