Why phase matters in recording and mixing
Phase describes where a sound wave is in its cycle. Two identical signals that are aligned (in phase) reinforce each other, summing to a louder signal. Two identical signals that are exactly opposite (180 degrees out of phase) cancel completely, resulting in silence. Most phase problems occur during recording when multiple microphones capture the same source from different distances: the sound travels different path lengths, arriving at each mic slightly delayed relative to the others.
A kick drum recorded with both a top mic and a bottom mic is vulnerable to phase cancellation in the low-mid range if the two mics are not phase-aligned. Guitars recorded with a condenser and a dynamic mic from different positions will suffer phase issues. Even stereo pairs can exhibit phase problems if not positioned carefully. The solution is aligning the timing of the signals, usually by delaying the nearer mic to match the farther one.
Detecting and fixing phase problems
A visual phase alignment tool (spectral analyzer or phase correlation meter) shows the relationship between two signals. A mostly centered dot indicates good phase relationship; a dot extending left (negative correlation) indicates problems. The classic test is to sum two potentially phase-related sources to mono: if the sum is noticeably quieter or thinner than either signal alone, phase cancellation is present.
Fixing phase problems requires either time-alignment (delaying one signal) or polarity inversion (flipping the signal 180 degrees). Most DAWs include phase-alignment tools. A 1ms delay is roughly equivalent to 12 inches of air travel, so a kick mic 2 feet farther from the source than another needs approximately a 24ms delay to align. Once aligned, the combined signal will be louder and fuller. Phase awareness during mic placement (measuring distances and positioning symmetrically) prevents most problems at the source.