i have this graph generated by the noise reduction software. Can help clarify why it stated there's a 2 dB correction applied?
please look at the "Calculations" view in the Sound Insulation Reporter software. In that view, please click on the "Average L2" calculation.
You'll see a graph similar to this one:
The L2 level average is getting close to the background noise at certain frequency bands. In this example there's a +6dB area on top of the background noise.
When the L2 average overlaps with that 6dB area above the background noise, the applied standard requires to process some correction values on the L2 measurments.
The width of that area as well as the amount of correction varies from standard to standard, and sometimes even within different versions of a standard.
Here's a description of that calculation in the ASTM E336 Standard you are using for version 2017 and 2020
ASTM E336 Version 2017
ASTM E336 Version 2020
You may download the full standard from here https://www.astm.org/Standards/E336.htm
I hope this helps.
Thank you very much Gregor, that's very helpful and clear. Really appreciate this.