Originally Posted by
Stitchbitch
ANR doesn't stop high frequencies, these can damage hearing.
The damaging frequencies are determined by a noise survey in each aircraft type, and (especially in the military) each Mark. The basic ANR is then adapted which, in practice, is often component value changes (analog ANR) or via software (digital ANR). The audio cues required in each type are part of this survey, and often a compromise is needed. But, typically, the damaging frequencies (as opposed to annoying) are around 800-900Hz, from transmissions. In other words, you just don't go out and buy an 'ANR'. You need one that matches the aircraft and, especially if it's an integrated system, approved by the aircraft design authority. The obvious example to me is that the Sea King AEW systems (both analog and digital) were superb, but of no use whatsoever in a Lynx. Purely by chance, they worked very well in a Sea Harrier.