When a signal is panned centrally, exactly the same signal will be output on both the left and right audio channels. If, on the other hand, the signal is panned fully to one side or the other, it's only output on one [g=190]channel[/g]. If you listen to the results over loudspeakers, the doubled acoustic energy in the room when replaying a centrally-panned sound will produce a louder signal. And as we\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\'re talking about acoustic power, the signal level will appear to increase by about 3dB, compared to the level when only one [g=190]channel[/g] is outputting the signal.
So if you pan from hard left, through centre, to hard right, it will sound as if the level rises slightly as it passes through the centre. To overcome this, mixer designers engineer the panning law to introduce a 3dB level drop at the centre, relative to the edges. However, if mono compatibility is important, a 6dB centre attenuation is necessary because the left and right channels are mixed together to make a derived mono. With the same signal in both channels (from a centrally panned signal), the voltage addition results in a 6dB level increase compared to the level of a signal in a single [g=190]channel[/g], so many broadcast desks employ a 6dB centre attenuation in their pan pots.
Manufacturers of mixers for general purpose often hedge their bets, and arrange a centre attentuation of 4.5dB, being halfway between the two camps! This is an excellent compromise in my opinion and seems to work well in all circumstances. However, bear in mind that because of the panning law, altering an instrument's stereo panning will also change its perceived level in the mix by a small but often significant amount. For this reason, I advocate panning sources to the required positions before crafting the final [g=184]balance[/g]. Some people recommend getting the [g=184]balance[/g] in mono first, but the [g=184]balance[/g] will change by anything up to 6dB if you then pan sources around, which isn't very helpful. And if you think all this is complicated, spare a thought for surround sound panning laws!