Full face are often noisier than 3/4 - you may think your full face is quiet enough - your audiologist would disagree.

Full face helmets provided average noise levels of 88.3 dB(A) at 50 km/h up to 103.6 dB(A) at 120 km/h. While the open face helmets provided average noise levels of 87.2 dB(A) at 50 km/h up to 98.5 dB(A) at 120 km/h.
this is in agreement with my own experience.

The levels are damaging over time. DB is logarithmic so 103 is a lot louder than 98...

It's your hearing.

all you wanted to know about it is here

Noise Induced Hearing Loss in Occupational Motorcyclist

http://www.cieh.org/JEHR/hearing_los...rcyclists.html

A number of reviewers have noted the same thing in their reviews that the open face generally tend to be quieter.

Both my open face Bells ....Classic and the Mag 8 are quieter than the full face Shoei TZ-R I snagged for colder weather and heavy bugs.

That 5 DB advantage will stay in place even with ear plugs or headphones. Over long touring trips that quieter sound adds up to more relaxed and less tired.

Now I know it's not my imagination. Just one more reason I prefer open face.