Calculations with db are wrong

[BernhardJosef]

Describe the bug
The result of adding 2 levels is wrong
The result of subtracting 2 levels is wrong

To Reproduce
Level level1 = new Level(1.6, Level.ParseUnit("dB"));
Level level2 = new Level(0.7, Level.ParseUnit("dB"));

Level level3 = level1 - level2;
Level level4 = level1 + level2;

Expected behavior
db additions or subtractions should be executed without any transformations

Additional context
see https://sengpielaudio.com/calculator-db.htm

[angularsen]

Hi, thanks for reporting.

If you are able to, could you please create a pull request with a unit test that highlights the two errors and what the results should be? Huge bonus points if you also provide a fix for the implementation.

It will help move this along much faster, I have limited time to look into it.

[angularsen]

Relevant changes coming in #1587
Let's re-evaluate after those are merged.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[github-actions]

This issue was automatically closed due to inactivity.

[angularsen]

I think the core issue here is that Level + Level and Level - Level currently encode one specific interpretation of dB arithmetic, but users can reasonably expect another.

Today, logarithmic quantities implement +/- as linear-space aggregation:

Level total = level1 + level2;

This means roughly:

10 * log10(10^(level1 / 10) + 10^(level2 / 10))

So this does not produce direct value arithmetic:

1.6 dB + 0.7 dB != 2.3 dB

That behavior is useful when adding quantities represented in dB, but it is surprising when the user wants to add/subtract dB values as gains/ratios.

For v6, I think we should make this explicit and avoid +/- for logarithmic quantities. My current preference is to provide named APIs for the two meanings:

// Convert to linear space, operate, convert back.
Level total = level1.LinearAdd(level2);
Level total = levels.LinearSum();
Level mean = levels.LinearMean();

// Operate directly on the logarithmic values.
Level gain = level1.LogValueAdd(level2);
Level delta = level1.LogValueSubtract(level2);
Level gain = Level.FromDecibels(level1.Decibels + level2.Decibels);

The exact names can be discussed, but I think the important part is this split:

• Linear* means convert to linear space, do the operation, convert back.
• LogValue* means operate directly on the logarithmic unit values.
• +/- should probably be removed for logarithmic quantities, because both interpretations are common enough that the operators are ambiguous.

How does this match your expectation?

[BernhardJosef]

I think, you implemented sound level measurement in dB and this is ok. My use of dB is a ratio of 2 quantities and this is also a very common application. Several ratios(attenuation, amplification, ...) expressed in logarithmic can be added or subtracted instead of multipication.

So I think you should add dB as unit to Ratio. Adding a ratio to a quantity multiplies the quantity with the ratio. Multiplication of a quantity with a ratio should have the same result.
dB is one of several ambiguous units abbreviations.

Your proposed work around is not applicable for me. I use UnitsNet within C#-scripting and the expressions are written by the users of our Software.
I would prefere a logarithmic ratio instead of changing dB for sound level measurement.

I can support you with the test of ratio unit dB