Correlation

In audio production, correlation usually refers to the similarity between the left and right stereo channels, expressed as a correlation coefficient between -1 and +1. A correlation meter in a DAW or plugin shows whether the two channels are in phase (+1 = perfectly identical), uncorrelated (0 = wide or independent), or out of phase (-1 = inverted, which can cause cancellation when summed to mono). Correlation helps mixing and mastering engineers detect phase issues, check mono compatibility and gauge stereo width, but it doesn't provide frequency-specific detail nor identify which channel is inverted—so it's one diagnostic among several.

Imagine you have a drum overdub or two microphones on a guitar and you notice a significant level drop when summing to mono. You check the correlation meter and see readings around -0.7 to -1, indicating the channels are partly or fully out of phase and canceling each other. Practically, you can invert the phase on one channel, nudge a small timing offset, or apply corrective delay until the correlation moves toward +1 or 0, then re-check the mono sum; mid/side processing and spectral analysis are additional tools for diagnosing frequency-specific phase issues.