I have an aero LG Course helmet. It is significantly quieter than my old Giro helmet. However, at 23mph the wind rushing over your ears is what's causing most of the noise. Along the lines of Cat Ears I use the Slipstreamz Spoiler and they work pretty well at reducing the intensity of the wind rushing past your ears. They also look quite a bit less dorky if that matters to you. You have to make sure they're stick out at ~90 degrees or so to divert the wind enough to make a difference.