measuring noise levels via a contour recording

I have a Contour GPS, mounted to my motorbike helmet. My aim is to compare wind noise with different windscreens fitted to my bike. So I thought I could record a video, and then use software to see how loud the recording is. Then the question is how to compare the noise levels in the different videos. Relative is easy; I can just use one of the noise levels apps on my phone. Or maybe there is some movie playback software that gives an indication. Does anyone have any idea how to get absolute decibel levels? I guess I could take the phone decibel level app with me to get an idea. Or calibrate using some stationary motor (eg tractor or chainsaw). But maybe there's a smarter way? thanks .. Jason