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So here is the catch.... the more sources with the same loudness lvl we add progressively we will receive less loudness change from the same source.
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Your ears (and most perception) is also proportional or logarithmic. If you have a singer and you add another singer it becomes noticeably louder (about 3dB louder). But if you have 10 singers and you add one more you're probably not going to notice the change. You'd have to go from 10 to 20 singers to make it (about) 3dB louder.
There are different dB calculations for amplitude (voltage digital level) and for power (Watts).
Double the amplitude (or double the voltage) is +6dB and four times the power. Double the power is +3dB.
And doubling the number of singers is approximately double the power. On
average the amplitude will be +3dB, but some peaks may be +6dB higher, and other peaks will be lower where the "waves" are out-of-phase.
For amplitude dB = 20Log(A/Aref) Aref can be the 0dB reference level, or an arbitrary "starting point" if you just want to know the dB
change.
For power dB = 10Log(P/Pref).
Even though the references are different, a dB change in your digital level corresponds to the same dB change in your acoustic level. For example if 0dB in your DAW is 90 dB SPL out of your speakers. Reducing the digital level to -3dB will give you 87dB out of your speakers. (Most of us don't have calibrated systems so we don't know the SPL.)