OMG!!! Hondas getting louder? You may have to paint them yellow so they don't embarrass the other red ones in the campground!!! Tongue in cheek of course!!
I'm sure with many hours on them, they probably are somewhat louder, and your technique for measuring is also probably within the norm! It's not a requirement to be scientifically correct when measuring!!! Sometimes your ears are the best for comparisons.
I know my Champion (horrors!!) is a bit louder than my sons H---a, but when both are running at max or off eco, either will not be mistaken for anything other than a generator!!