I think you're wrong, I suspect "severe annoyance" is simply a certain decibel level. Anyone know?
Must desist from those late-night attempts at wit.
As I previously said:
The 55 Lden contour corresponds to the World Health Organisation's definition of "severe annoyance".
Lden is indeed a decibel value, or more accurately the weighted sum of 3 values for (d)ay, (e)vening and (n)ight.
so you are perfectly correct.