Audio Manually evaluating an AudioSource's rolloff

I am trying to get the rolloff curve of an AudioSource, in order to evaluate its rolloff at an arbitrary distance.

I found

 AudioSource.GetCustomCurve()

, but it only returns the right curve if the source's rolloff mode is set to "Custom".
I could potentially calculate a linear rolloff by myself, but I have no idea how the Logarithmic rolloff works.

Is there a built-in way to get the rolloff curve/evaluate the rolloff regardless of the mode ?
Else, how do I evaluate a logarithmic rolloff ?

 

Any help would be highly appreciated.
At this point assuming there is no magical built-in way, I'd simply need to know the math behind the Logarithmic Rolloff to find my way out.

 

We use custom animation curves that are stored as parameters. You could do the rolloff based on these animation curves and then they are tweakable easily too!