You can download it in Flash SWF format or as a C source / Win32 binary (binary compiled with the DJGPP online-compiler).
The summing of decibel values is a bit of a confusing subject, i believe because decibels are units with no dimension (ratio like %) and also that they are logarithmic.
You can read this whole article for more information:
The important bit here is to learn to distinguish different types of measurement (decibels). What you are seeing on the meters in your audio program is dbFS originally designed for ADCs (analog-to-digital converters) and assimilated by modern audio tools. dbFS is the reverse of dbSPL.
The above article explains an easy way to sum dbSPL values and also provides a table to convert dbSPL to dbFS (16bit only):
If you have 80dbSPL + 80dbSPL the result would be approx 83dbSPL (80+3), because the difference between the two is 0 units - from the "rule of thumb".
a dbSPL calculator works with the following equation:
result = 10*(log(10^(input1/10)+10^(input2/10))/log(10)
where "input1" and "input2" are the dbSPL (!!) values you want to sum. 80db+80db=83db
to sum two sinewaves in your audio program (with dbFS measures):
result = 20*(log(10^(input1/20)+10^(input2/20))/log(10)
But why 20*? Here is a explanation quote from wikipedia:
So result for the sum of -9dbFS and -9dbFS, would be -2.97940008672037 for equal input/output impedance.