The Half Power Point of -3dB
The half power point essentially establishes that a -3dB difference is half the power of any frequency played through a speaker. A mono signal played through 1 speaker is going to be 100% volume, but when played through 2 speakers you have double the speakers so essentially only need half the power from both to perceive no change in level.
Important note about stereo:
Location Sound is captured with mono microphones and even if something is recording to a stereo track (from a mono source) it does not make it true stereo. If you record 1 source in Center and then playback, you will equally play the source through both speakers giving the illusion of center stereo. This is known as phantom stereo. And there you have it. Panning for gold standard audio.
Phantom stereo is when you have a mono source that is played through a stereo speak setup (2.0 / 2.1) This is not true stereo because it is playing exactly the same signal through both speakers giving the illusion of the source playing in the center of a stereo field.