I did a fairly careful test over the weekend of what Active Noise Cancelation does.
I compared it using a professional GenRad SPL Meter and didn't see a whole lot of difference. Maybe 3dB (which is significant).
But then I realized there is a problem with using the SPL meter. It's listening for the peak of any frequency. Even ones you might not even hear.
So then I recorded the ambient noise with a fairly good Olympus Hand Held Digital Recorder.
I made sure to go the same speed, using ACC.
Going over the same section of the road.
I removed the Fuse on the Amp to Disable it.
The image below is a Spectrum Analysis. It basically shows the level at each frequency. This is done over a 6 second period of it Enabled and not enabled.
Note that the scales are slightly different.
A 3dB difference is significant.
By memory, which can be completely wrong, it felt like higher frequency, harsher noises were reduced. The low frequency stuff did not feel like it changed. Like someone suddenly added more insulation. Harsh sounds were more muffled. Or like a window is open a crack and you just shut it.
I sure wish I could turn Active Noise Cancelation off and on while driving. So I could A/B it better to my ear and rule out any other possible variables between runs.
Climate was OFF. Mic was hand held near my ear. It really should be on a tripod.
Also keep in mind if there was more ambient noise it may be more aggressive about reducing it.