I read Honda's research info on sound deadening road noise
the 20" Passport rims have a sound deadening strip, but didn't know how well they worked until your comment.
Road noise goes down as the amount of rubber, between the rim and the road increases.
You mention the Db levels were the same, between your OEM 20s (with sound deadening strip) and that of new 18s (without the sound deadening feature). Effectively, the sound deadening strip equates to having more rubber between the rim and the road.
Now I need to find out if any Honda/Acura 18" rims have the sound deadening strip and can be used on my Passport. I haven't checked the rims sizes on newer Hondas/Acura, which might be designed to be more sound conscience.
More on Honda's R&D road noise