adding up the noise

[FALKEN]

a quick brain fart:

say you have a noise floor of -76db per channel.

say you just have 4 channels plus 2 mixdown channels.

-76db = 0.000122765 V

0.000122765 V x 6 = 0.00073659 V

0.00073659 V = -62 db

this does not take into account noise in other phases.

is this math correct?

is -62 db good?

 

[Wil816]

You can't add noise that way. That's good news though, because your final noise will be less than you thought. Take the square of the noise voltage from each source, add those squares together, and then take the square root. In your case of -76db = 122.8µV, taking six of those is SQRT(6*122.8µV²), which is 301µV or -68.2dB, or about 6dB better than what you thought.

If you only have two sources, you can picture the principle with a right triangle. Each noise source's voltage is one of the sides that ends at the right angle, and you find the length of the hypotenuse. So 3mV and 4mV of noise add up to 5mV, not 7, because 3²=9, 4²=16, and 9+16=25, and the square root of 25 is 5. Two 1mV noise sources add up to 1.414mV (the square root of 2), not 2mV.

 

[FALKEN]

Thanks!!!!!

 

[ofajen]

Another way of explaining the same thing: the general rule is that each doubling of the number of tracks mixed together adds about 3 dB to the incoherent noise floor. So, four tracks mixed together would raise the noise floor about 6 dB or so. So, the noise floor of the mix should be in the -70 dB range. The two tape tracks you're mixing to are separately played back, so they don't add to the noise floor because of combining tracks, as above. Of course, if the mix recorder tracks had a higher noise floor than -70 dB, you would hear that noise more than the noise of your mixed tracks.

Cheers,

Otto