Well?...on my 10 scale (with 10 being dead quiet) I'd rate my old HJC's and the Scorpion as solid 8's...the arai quantum as a "7" and the X11 as a 5-6.(hydrabusa @ Mar. 16 2007,14:41) Jinkster, I've had a few like yourself...since Bell, Biffee, arai and shoei shark...etc.
I didn't like one so far...except RF1000, it is most quiet one that I can find, and how is X-11? I heard its nice but, due to the extra vent it makes alot of wind noise...in this case, I have one Shark RSR2...needed a extra vents for summer, but it feel like wearing a half helmet, every single noise will pick...I don't know where it is coming fromso let have lil more details about the X-11...
Upshot?..the X11 is noticably just a tad louder than others i've owned but not overwhelmingly so..matter of fact?...I never took notice of it being louder and would've never known that it is slightly louder until last biketoberfest...and i emptied a full tank in one sitting on I-95 south...something i don't normally do...and the last 1/2 hour or so of that run is when i noticed that the X11 is a tad louder than others i've owned..however...it's nothing i'd notice under my normal "ride for 45minutes-1hr between breaks/rest-stops" sorta riding and it's virtues far over-whelm it's slight decible deficiet as i experience far less wind related neck fatigue and while this lid might not be "The Lightest"?...but it damn sure feels light to me..real light..as it seems the fantastic aero's keep it feeling light at warp speed..if i knew i was going mongo miles, superslab ST'ing for days on end?...I might consider wearing some plugs but...other than that?...I never notice it's loudness under typical riding venues.
Sidenote: I thought my scorpion flowed some bigtime air...but it's nothing compared to the ventilation in my X11.
Hope that helps and L8R, Bill.