super72bb,
The reason why the ANR systems are only effective at the lower frequencies is locked in the principle that they employ. They sense the ambient noise, and its phase, and apply an exact inverse of that noise, the exact frequency and the opposite phase, to absorb the noise. Note with an ANR headset on, if you clap your hands, there is a distinct ringing for several seconds. That is the counter noise dying out as it fails to be reinforced.
As the frequency of the noise rises, the wave shape gets much more complex, and the calculations for the counter noise get nearly impossible (for today's computers).
None of the ANR's claim credit for helping reduce hearing loss, nor should they.