This whole sound mat stuff is interesting.
Back in 1993 when 4Wheeler Magazine tested the then brand new FZJ80 it was the quietest vehicle they had ever tested. I do not remember the specific number but the cruising speed number was something like 70 db.
I took some decibel readings for a 'before and after' out of curiosity. I'll check the 'after' once I'm done. But I bet it didn't reduce much by a measured amount.
The biggest improvement I immediately noticed was the quality of noise at highway speeds - the background road/wind noise was reduced. I'm guessing it's the higher frequencies being blocked/filtered better. Either way just more pleasant experience especially on longer trips and less background noise - conversations are a little easier and the sound system sounds better - easier the hear the detail from the speakers. Might all be placebo effect but glad I went through the effort.