Panning law, or panning rule, is a recording and mixing principle that states that any signal of equal amplitude and phase that is played in both channels of a stereo system will increase in loudness up to 6.02 dBSPL, provided there is perfect response in the loudspeaker system and perfect acoustics in the room.
[1] Often, the acoustic summing of a room and system are inferior to the ideal, so the specific relative level will increase from −3 dB to 0 dB as the mono signal is panned from center to hard left or right.
For example, Yamaha digital consoles employ a typical (compromise) 3 dB panning rule where the signal is at full level when pan position is centered and becomes progressively louder (up to + 3 dB) as it is panned to the right or left.
The 3 dB panning rule is a commonly applied compromise to comply with the mediocre acoustic summing capabilities of most control rooms.
However, the console manufacturer SSL used to employ a 4.5 dB panning rule, because it was believed that their expensive consoles would normally be used in tuned rooms that had acoustic summing capabilities closer to the ideal.