The only operational rule is called "lack of originality". Most things sound the same because the people behind the desk are trying to clone someone else who has a hit instead of actually trying to create a real hit by serving the music instead.
That said, there are a couple of rules of thumb based upon the laws of acoustics and psychoacoustics:
- Because bass frequencies tend to be less "directional" and tend to prefer larger speaker surface area, you'll usually find bass and kick panned nearer the center. Panning them towards the edges does not really take advantage of the stereo spread because of the less directional nature of bass frequencies, and by having them center you get to use both the left and right woofer to push them, virtually doubling your bass speaker size. It's not unusual to find the bass and the kick each panned just a couple of degrees off center in opposite directions just to give them a slight separation and to get them out of the center pan "pocket" sometimes caused by DAW panning laws.
- In songs where the lyrics are of heavy import, the lead vocals tend to be near center panned. I think this is a much looser rule of thumb, however, and tends to be defaulted to by laziest; there are lots of instances and reasons for setting the lead vocals off-center.
None of this is set in stone, however. I've heard some very good mixes where neither the kick or bass were centered, and a LOT of mixes where the lead vocal was partially panned to balance the mix or fit an artistic vision. The best thing to do is to get an IMAGE in your own head of what you want to create, and how it will sound and be panned, and just totally ignore what everyone else is doing as far as pan rules.
G.
