Iīve only just noticed this thread so perhaps my comments are preaching to the converted but some of the early posters missed the point about ANR entirely.
It does work better at some frequencies than others. The point about the ANR systems I had experience of testing is that it adds attenuation where the passive attenuation of a hard ear cup is worse. It does not matter so much that it does not work at high frequencies because a good ear cup will passively do a lot to reduce these anyway.
IIRC the Sea King had a tonal spike at about 630 Hz and this is where the ANR I used was very effective. I had lots of Sea King and Chinook pilots who were very enthusiastic about ANR. Whether these systems became operational I donīt know.
Anyway, protect your hearing as noise induced hearing loss only adds to whatever hearing loss you may get through ageing!