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Author 
I need help with a db question. 
tompty 
If i have a 70hz sine wave and a 5khz sine wave both running into my converters at 18dbfs will the final outcome be 18dbfs?
I have always wondered whether the overall db goes up with two nonclashing frequencies are summed or whether its just when frequencies overlap.
thanks! 

tompty 

megaohm 
tompty wrote:  If i have a 70hz sine wave and a 5khz sine wave both running into my converters at 18dbfs will the final outcome be 18dbfs?

It depends on when you 'look' at it.
I think your general grasp of what is going on is correct.
But two waves at different frequencies will add together at certain times depending on how their frequencies are related. 

megaohm 

joshnapkins 
it doesn't matter if they're not the same frequency. think of adding two sine waves of different frequency, at some points their maxima will line up perfectly and you'll get constructive interference at twice the amplitude.
and, as always, when doing any sort of math with dB make sure to convert from log scale to linear scale, do the addition/subtraction/mult/etc. and then convert back to the dB scale. i.e....
10log10(x) = 18 dB, so x = 10^(18/10) = .016
now we add the max amplitude of each signal 0.016+0.016=0.032
now convert back to dB, 10log10(0.032) = 15dBfs 

Ranxerox 
Yes, frequency has nowt to do with it. As a general rule of thumb adding any two signals of the same amplitude together will result in an increase of ~3dB at the output. 

tompty 
brilliant!
thank you all for the helping me out with this. 

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