pba
Thanks. All too new for me. As I said before, in-ear devices weren't wanted in the 90s, so the technical solution was ANR (or a silent main gearbox).
Apart from the reduction in noise dose, the other crucial test was speech intelligibility. One of the reasons (+ TEMPEST) why ANR was fully integrated into the comms system/aircraft, meaning the helmet became a comms LRU as well as an AEA. Farnborough have a dedicated lab, and local students were watered, fed and handed a tenner to come in for a session. They wore a helmet, it was fed with real aircraft noise from a survey, and they wrote down the words they heard. So, you could 'change aircraft' midstream. The main work in this area was done during Digital ANR development, which was completed in 2000, and led to adaptable software. I think it's one of those areas that will be forever in development.
Your mention of warning tones makes me twitch. Not integrated, no fly - that was the rule. Breaching that led to the loss of ZG710 in 2003. A fundamental part of the programme was checking all warning tones, and adjusting them if necessary. Conversely, one odd problem was the crews could hear the GPS ToD update tone every second, which was reduced.
It's success can be easily appreciated by one fact. Without ANR, AEW2 allowable flying hours were 59 per year. With Mk1 analog (simple, battery powered), 315. Post-padding upgrade, full integration (aircraft power, etc) and lessons learned from trials, 1350 in the front, 700 in the back. A good example of how big a difference seat position makes. Your friendly project team should have the equivalent data for current kit and aircraft.