Someone hit me up on the SNR Facebook page (link at right below the email subscription box) and asked me about panning when mixing live. Alfred Brown is a recording engineer in the Buffalo, NY area and has been adapting his studio techniques to live mixing. Here's my thoughts on the matter.
When I started out as a fledgling sound guy I didn't always have the option to pan. On the tiny rigs I was using in those days sometimes I had to give up half the house mix so there could be a monitor on stage. When I got a little bigger I would often mix in stereo, but again, occasionally give it up in favor of being able to pan the bass instruments to one side and kind of fake an aux fed sub when I didn't have an aux free. As time went on I got into many a strange venue. In a wider room or in certain outdoor applications I would mix mono because not everyone in the audience was getting information from both speakers.
After a while I just got lazy and mixed everything in mono. But one night, just for kicks I set up to run stereo for a bar band. It was like the whole mix just came alive. It was a pretty small place, about 200 cap, pretty narrow but also pretty long. So I was doing more than just drum and vocal support. With a packed house the guitars could easily get lost, and when the crowd was jumpin they really loved their bass. I had been used to mixing in there and just having everything turn to mush. I figured it was just the shape of the room and the noise of 200 extremely drunk and noisy patrons.
The simple act of panning the drums and splitting out the guitar mics and backup vocals made just a ton of difference. I didn't get all extreme and hard pan anything, but just creating that little bit of separation started to clear up a lot of weird phase issues that were happening because I had always sent the same signal to each stack in the past. The drums got cleaner, guitars sounded fatter, the lead singer wasn't fighting to be heard, and when the whole band sang together it was heavenly.
Carrying that lesson forward I had the opportunity to mix on a small Vertec rig at a music festival. This was an ideal situation, lots of power and a controlled audience area. Every listener had a pretty good shot from each hang so I could slide things off to the sides. I wanted to make sure that no listener missed out on anything though. So I had both guitarists double miced, which is something I often do anyway so I can create a sound by addition and cancellation rather than EQ (that's a whole 'nother post). People hearing a good balance of both hangs got this nice wide stereo image of each guitar. People far off to the sides got predominately one mic from each guitar but still had a smidgen of the other. Not quite as fat sounding but still a pretty good representation of their sound as interpreted by yours truly.
The key to all this is the phasing that happens when you send an identical signal to a pair of speakers or stacks that are separated in space. Due to different arrival times depending on the listening position, some frequencies add and get louder, and others cancel out and get quieter. (They rarely disappear completely.) So let's say that your stuck in a mix position where your head is in a null where some of the mid frequencies are cancelling out. Your guitars will sound funny and quiet. So you turn them up till they sound right but then you're really hammering anybody who happens to be sitting in a spot where those same frequencies are adding up. Separating them makes the signal to each stack more unique and less likely to interfere with what the other stack is doing.
You could be heavy handed and pan one guitar hard to the left and the other hard to the right, but then you're depriving the people on the far outsides of the venue of the opposite panned guitar. If you've just got one mic per axe then start panning slowly until you get the sound to shift out of center. It's a good idea to walk around the venue with one, then the other, then both guitarists playing. (I'm using a lot of guitar examples here but this applies to everything.) Once you get more comfortable with the technique you won't have to do this as much.
To take it a step farther there's a lot you can do with multiple mics, or splitting the signal from a single mic. If you EQ a pair of inputs differently you can get a different signal coming from each side and still get the benefit of stereo separation while keeping something in the center of the image, like a vocal or lead guitar solo. To go back to guitars, you could pick two identical mics (or split the signal) and compress one so that when the player really hammers on it one side gets louder and the other stays the same, pulling the image to one side. That kind of dynamic processing can happen without your input once the settings are right, and it can really create some interest for the audience. Whether they realize what's happening or not, to have instruments shift locations in the image depending on how they're played is more interesting to listen to than ones that sit still.
I could go on and on and on about this, but rather than try to list off every possible technique I'll just encourage you to go out and experiment. If you're not free to move around a venue during a soundcheck or event, get some buddies you trust to help you out and throw some hand signals back to you. Also, Dave Rat wrote some pretty good blog posts about dynamic panning techniques that are worth reading. He even went so far as to use two parallel sound systems to help eliminate phasing and create a clearer image and mix.
Hit the comment's my Brethren of the Knob and Fader. You're the Scotty to my Kirk, tell me how it is.