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Let's assume I have a mono track in my DAW software. The track contains an audio file which was normalized to 0 db. If I play the track the volume gauge on the stereo out channel will go up only to -3db.
Where does this difference come from? And how do you handle mono tracks during mixing? If nothing special is done the mono tracks will always sound quieter that the stereo tracks in the same mix.
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The difference is due to something known as "panning laws". Imagine you have recorded a mono track, and it is currently panned centrally. You decide you want it panned all the way to the left. Did you want it to also drop to half volume at the same time? Probably not. Yet that is what would happen if the DAW simply turned off the signal going to the right speaker.
So your DAW has a choice if it wants mono tracks to stay at the same volume as they are panned from side to side. It can either boost them by 3dB when they are fully left or right (and run the risk of clipping), or it can cut them by 3dB when they are central (which is what your DAW is doing).
Many DAWs also let you configure the rate at which the 3dB will be added as you pan to one side, so you might be offered "sine" or "square root" options.
Mark is completely correct. I'll add a little to it. Acoustical summing at dead center in a near field should add 3dB to the sound energy over hard Right or Left, when patched directly (not using a pan pot).
The general purpose of the 'panning law' is to compensate for this acoustical summing when panning through center. When done right, as you pan your sound, it will keep the same apparent volume across the soundfield.
This phenomenon does not always behave perfectly, nor is it what mixers always want, so some analog and digital panners use different panning laws; say -5db down at the center, or different degrees of taper across the pan.
Under the circumstances you describe, your last sentence, "If nothing special is done..." is usually going to be untrue. Commonly, different panning laws only become an issue when the mixer needs to make a 'standardized' format conversion without remixing a new printmaster...as when turning an LCR stem into a stereo pair, for instance.
