Researchers from the University of Bath and Bath Spa University conducted an extensive study on motorcycle helmets and wind noise. http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PMARCW000012000001050001000001&idtype=cvips&doi=10.1121/1.3602104&prog=normal
The roar of a motorcycle engine can be really loud, however the biggest source of noise for a motorcyclists is actually the whooshing over the riders’ helmets. The University put helmets atop mannequins heads and placed them in a wind tunnel. They then placed microphones at different locations around the helmet and at the mannequin’s ear, the researchers found that the area underneath the helmet and near the chin bar was a considerably noisier then other parts of the helmet. The researchers looked closer at the helmet angle, wind speed and how it affects the wind noise.
So far the researches has shown temporary threshold shifts. However, I do find it a little alarming that in some cases they found that some of the helmets actually made the noise louder in frequencies below 500 Hz.