http://www.eurekalert.org/pub_releases/2011-07/aiop-mhh072911.php

Studies have revealed the biggest source of noise for
motorcyclists is actually generated by air whooshing over the riders' helmets.
Even at legal speeds, the sound can exceed safe levels. Now, scientists have
identified a key source of the rushing din. Researchers from the University of
Bath and Bath Spa University placed motorcycles helmets atop mannequin heads,
mounted them in a wind tunnel, and turned on the fans. By placing microphones at
different locations around the helmet and at the mannequin's ear, the
researchers found that an area underneath the helmet and near the chin bar is a
significant source of the noise that reaches riders' sensitive eardrums. The
team also investigated how helmet angle and wind speed affected the loudness.
Future tests will move beyond the wind tunnel to real-life riders on the open
road. The findings, described in the Journal of the Acoustical Society of
America, may one day be used to design quieter helmets, saving riders' ears for
the enjoyment of hard biker rock, the researchers say.

Thanks to Dean Williams