[Luckeh!!!!]

Basically this

To add decibels, let the dB level = X(1,2,3 etc...) for each source.
First divide the dB value by 10: X(n)/10
Then raise 10 to the power of (X/10): 10^(X(n)/10)
Then add the results: E(10^(X(n)/10))
Then calculate the log of the sum and multiply it by 10.

I've done all that on a spreadsheet so I just need to input the values, or if you know a programming language you can write a prog to calc it for you.

Google might yield downloadable ones.