Quieter (from the 2nd line of the thread title) is via ear plugs. No helmet is quiet enough on its own to save your hearing. You don't sacrifice hearing important things around you because everything else is quieter, too.
I know I'm late to the thread but I have to disagree with this. I have used a Shoei RF1100 for years and it definitely required earplugs. Not knowing any better, I was still satisfied with it. Then recently, I purchased a Shoei Qwest and please believe me when I tell you there is simply no comparison between the two. The Qwest is remarkably quiet, and I don't require the use of earplugs at all, even at highway speeds with the windshield all the way down (and that's a stock w/s btw).
I wish I knew enough about dB to give a scientific description that would explain the difference without hyperbole, but I don't. I also know that head shapes determine how much air/wind enters at the bottom and that has an effect too. But for me, my Qwest is VASTLY, by orders of several magnitude, quieter than the RF1100. If you have a way to test one, I urge you, in the strongest possible terms, to take one for a spin.