Guitar or vocal pan refers to an audio track mixing process called panning. The term is based on the word "panoramic," referring to the wide scenic images used in both film and photography. The camera is panned from left to right across a scene. Similarly, audio tracks are spread or panned out over the stereo field. Guitar and vocal tracks should be panned differently due to the unique nature of each instrument.
The purpose of panning is to create an audio mix that echoes the experience of a live performance. Each track is typically given a location that corresponds with where it might stand on a stage. Drums and vocals tend to be centered, while guitarists are to one side or another. If you imagine the physical location of each performer, it will help you determine where the track should be located in the mix.
There are three main locations on the stereo spectrum: left, right and center. Tracks can be panned anywhere in between those locations. When a track is moved toward one direction or another, the track's volume is reduced in the opposite speaker, making it seem louder in the direction that it's moved toward. Panning strongly to one side leaves no trace of it in the other speaker, and the track may fade back a bit. Placing tracks halfway between center and left or right leaves some audio still playing in the other speaker and gives that track a bit more emphasis.
Guitar tracks are almost always panned to one side or another to avoid clashing with centered tracks such as vocals and drums. It's best to keep your mix balanced so that there isn't more audio in one speaker than another. Often when there are multiple guitars, they will be set off to opposite sides: one left and one right. If you find you have guitar tracks that don't have another instrument to balance them out, you can also duplicate or double that track and pan each one on each side.
Vocals are often panned directly in the center of a mix because they are focal points of most genres of music. If a mix has both lead vocals and backing vocals, then only the lead vocal would be panned center. The backing vocals need to be located somewhere different so that they don't interfere with that lead track. At the same time, they should be spaced appropriately depending on how much you want them to blend in with the rest of the audio or stand out during key moments of the song.
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