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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?

Thank you in advance

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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.

Start with a measurement that has a known reference, like:

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.

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  • Thank you very much for you answer, this argument is totally out of my range. I'll try to study it more.
    – Maybe_Able_Programmer
    Oct 8, 2020 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, 2020 at 13:50
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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

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Please understand that this answer is written with a bit of tounge in cheek, partially containing a joke.

Decibel is relative measurement, there are no absolute values. So, assume we define dBA as the relative amount of apples and we define that 1 apple equals 0 dBA. Half an apple would be -6 dBA and two apples would be 6 dBA. Let us also define 0 dBB as one banana. If we now add 0 dBA + 0 dBB what do we get? It would require us to define a new measure, let us use Fruit Salad as the base and hence the measurement would be dBFS.

So if we add 0 dBA + 0 dBB how much do we get in dBFS. As we all now, a Fruit Salad without other ingredients (measured as dBX) than apples and bananas cannot be complete, so I expect the answer to be around -20 dBFS but as they always say "It depends".

On a more serious approach. If you add two sound sources together, both at say 0dBV, the result may be very different depending on the sound sources:

  • Two identical sound sources will give +6dbV
  • Two sources 180 degress out of phase will give no output signal at all or -infinity dBV.
  • A common (not perfect, only common) assumption is to use about +4.5 dBV.
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