In a comment I observed , "NB: if you want ~6 db gain (composite) . . . . " I have seen the useage of "NB" before. Does "NB" mean no-brainer ?

According to dictionary.cambridge.org/dictionary/english/nb it means "written before a piece of important information to make readers notice it". That it certainly does - and makes us wonder why, and where it came from.

And that's a no brainer. 😁

Or perhaps NB = Notice, Brainiac. :)