In this country we measure the noise at the source,-bike under full load.
When the track is inspected every year,a measuring point is selected,typically a long straight.The track has to have a marshall who measure the db level at every practice,if needed.At local races youll sometimes have the official people turn up,and at every National event they will always be there,testing the noise level of all the bikes.The max levels:
Micro 50(polini,PW 50,etc) 111db,Micro 80(PW 80,SUZ ds 80,etc) 113 db
Mini 65-85 121db,
125 128db
250 128db
500 124db
4-strokes 129db
Many bikes struggle even with standard silencers,but with fresh,correct packed wool theres rarely problems.
The sound of a 4-stroke is not necessarily measured louder in db´s,but a neighbour to the track will almost always hear the 4-strokes louder.Its just like listening to loud music at a distance,you only hear the bass notes,and the bass drum.