This image shows a closeup of two tracks. These are mics that were used to record a guitar amp.
Track (mic) 1 was placed very near the cabinet grill right in front of the speaker.
Track (mic) 2 was placed at the same height but about 6 feet (2 meters) away from the cab.
Due to the room mic (Track 2) being some distance from the cabinet, the sound waves arrive at that mic slightly late. Judging from the time-line in the picture, the Track 2 waveform shows evidence of being late by 4 or 5 milliseconds. (Generally the delay will be roughly about 1 millisecond per foot or per 1-1/2 feet.)
This time difference tends to cause some phase anomalies when the two tracks are played together. In other words, places where the waveform peaks and valleys "disagree" or "oppose" those on other tracks are considered to be a mismatch in phase. These spots may only occur at irregular intervals but they are still sometimes frequent enough to be audible. The recorded sound of the amp can become more thin, hollow, and boxy than the original. This is because certain frequencies become exaggerated while others become canceled out all together, causing the overall sound to be less than pleasing.
On most modern digital recorders, the tracks can be moved manually left or right along the time-line. We can correct this phase problem by simply sliding Track 2 to the left until it lines up visually even with Track 1. You may need to zoom in on the two tracks side-by-side (like my picture) in order to make such a tiny adjustment.
WARNING: Save your song/session before you start moving tracks, just in case you accidentally get things out of whack. You can always restore your files back to the way they were.
After the tracks are in matching alignment, they will likely sound more true-to-life, full, and rich.
This example mp3 is two out-of-phase/out-of-time tracks before the correction.This example mp3 is the same two tracks after the time-line correction has been made.(neither mp3 has been EQ'ed or otherwise altered.)
This correction is better known as time alignment. You can apply the same logic to drum overheads and other distant room mics. Sometimes it makes a huge difference and sometimes it does not. But it is always worth trying if things in your mix seem to sound a little "off."
