# Decibel sum problem on 0 dB + 0dB

I don't know a lot of what I'm going to ask you. What I know is that to sum different sound sources, logarithm should be used (like the attached img).

THE QUESTION IS: why `0 dB + 0 dB = 0 dB` is not true? I undertand this is not a standard sum, but why two sound sources with 0 dB emissions, generate sounds? It's probably best to say that initially, the question you are trying to ask is meaningless.

In order to understand why, you should research exactly what a "decibel" is and how it is used.

To summarise the key points, 'dB' is not an absolute reference to a measurement of anything, it is a way of describing relative measurements based on a known reference. In order for this to be meaningful, you have to know what your reference point is.

With Decibels and audio, there can be many reference values. When you are using the term "0dB" this basically means that the value is the same as another 'absolute' value.

0dBU, or 0dBV or 0dBSPL

a measurement of 0dBU increased by 3dB would equal +3dBU.

A measurement of 0dBU changed by 0dB would equal 0dBU

A measurement of 0dBV changed by -5dB would equal -5dBU

This is a bit more meaningful and hopefully makes more sense.

• Thank you very much for you answer, this argument is totally out of my range. I'll try to study it more. Oct 8 '20 at 8:23
• In no way was the intention to be dismissive of the question, but decibels are a slip-up point for many that are new to audio, but once you get the hang of them, a lot of things will fall into place. Oddly, without realising it, your original question is - in some ways - true 0dB + 0dB = 0dB. Essentially no change added to no change equals no change. So your original assumption that it was wrong, is somewhat erroneous.
– Mark
Oct 8 '20 at 13:50

When talking about decibels, they are not a specific value but a ratio between two numbers.

If you have 0dBV it does not equal 0 volts. It is equal to 1 volt. 1+1=2.

2 volts = 6dBV