Many will also argue that modulars are noisier than full face helmets. That is true with a few exceptions including Schuberth, who has mastered the design and engineering of their helmets to eliminate as much noise as possible. Since Schuberth introduced the C3 a couple years ago, there are more premium modulars that have been released since then to compete including the Shoei Neotec and HJC RPHA Max that also are trying to provide a quieter riding experience. However, if you compare something as premium as the C3Pro to a basic full face helmet at a <$100 price point then the C3Pro is very much quieter than that one.
Alternatively, if you compare the C3Pro to other premium, high end full face street helmets such as the Arai Defiant or Shoei RF-1200 which have both been designed to greatly minimize wind noise, you may find they all come somewhat close to each other.
The Defiant and RF-1200 have been very popular models among riders who are seeking full face helmets that can greatly reduce wind noise levels. Since Schuberth greatly publicizes their perceived noise level in decibels, I think you can safely assume that the Schuberth models might be a hair quieter than it’s competitors. It’s difficult to say which one is the quietest, but these definitely fall into the ‘more quiet than others’ category.
You must also take into consideration the fairing / windscreen setup of your motorcycle. My bike has nothing, so even the quietest helmets will still be noisy because I have nothing in front of me. Depending on what kind of setup you have on your bike, that will certainly affect overall wind noise as well.
Since this post, I've purchased a Shoei RF-1200 which I find to be almost as quiet as my Schuberth. However, I do think the windscreen on my old SV made a difference in reducing noise as well. My current Triumph has nothing!