"P Stamler" wrote in message


A good rule of thumb is that if noise source #2 is about 10 dB less
than noise source #1 (9.89dB and some change, if you want to be more
precise) it will raise the combined noise by 1dB, all other factors
being equal.

My handy-dandy spread sheet says that the contribution from a source 10 dB
smaller than the larger source is about 0.5 dB - actually more like 0.4 dB.

To be precise, if you mix a noise signal at -20 dB with a noise source
at -30 dB, the sum is -19.5861 dB.

This result is consistent at other levels, for example if you mix a noise
signal at -80 dB with a noise source at -90 dB, the sum is -79.5861 dB.

My estimate of a 0.5 dB increase is based on the fact that uncorrelated
noise is properly added geometrically, which involves taking the square root
of the sum of the squares.

To get a 1 dB increase you have to mix noises that are about 6 dB apart.