Having developed an analog ANR system for Sea King AEW that brought noise dose down to 84dB(A), in 2000 MoD completed the 2nd phase, Digital ANR, that brought it down to 73dB(A); well within the anticipated (at the time) legal limit. That limit was delayed by many years but I believe it is now in force.
Any ANR system has to be tailored for the noise in that particular aircraft; and very often that Mark of aircraft or a particular seat in the aircraft. So, the Analog ANR designed for Sea King happened to work, by sheer luck, in Sea Harrier; but was absolutely useless in Lynx. The damaging noise came from different sources and at different frequencies.
While damaging noise is the primary concern, annoying noise is too. In the Sea King, the radar transmitter whining away in your ear.
With the digital system, the idea was before flying you plug your helmet in to a work station and simply press the button appropriate to your aircraft/mark and, occasionally, seat. It blew an EPROM. (The pilot in AEW received a very different noise from Observers; both damaging). I understand this programme was halted after successful initial trials, but stand ready to be corrected.
Another feature of this programme was to avoid pages of mind boggling decibel notation, and simply express the performance in allowable flying hours per year. ANR allowed Mk2 aircrew to go from 59 (far too low) to 320 (about right). But 600 was required for the ASaC Mk7. Litigation you see.
More recently, I've had cause to speak to Bose about their system. It is far too simplistic. Like most, it is a simple broadband noise reducer, which leaves aircrew short on audio cues. That is fine in the back of an armoured vehicle or for passengers in the aircraft, but not aircrew who rely on speech intelligibility and resistance to fatigue. The speech intelligibilty tests and trials were very complex and comprehensive.
My recommendation. Always ask if the system you are considering has been cleared for your aircraft.
An oddball.... In addition to it being an Aircrew Equipment Assembly, with ANR, especially aircraft powered, the helmet becomes part of the comms sub-system. It sits within the TEMPEST boundary, if you're flying such a beast. What ANR also does is expose design weaknesses in other areas; typically the quality of audio amplifiers in the intercom and the transducers in existing helmets. On the latter, the ones MoD used cost about £10 each; the best were French at about £1k each. MoD resisted.....but you get what you pay for. Interesting topic.