Pan Law, in practical application, states that an audio source, of equal amplitude and phase, that is played in both channels of a stereo system and panned from center to left or right will sound natural to increase 3db.
Read the Wikipedia link below for more details. (Actual change is 6.02db for perfect response and perfect acoustics, but practical application for most speaker setups is to think 3db).
Read more at: http://en.wikipedia.org/wiki/Pan_law