Hmm... can't see their point here? Way I see it the system operates by replacing one continual background noise with an appropriate "anti-noise" - if that wasn't the principle then you wouldn't be able to hear anyone speak to you.
However, I do have some concerns over the "advantages" claimed. Used to operate turboprop hardware and a colleague who had ANR headset could never detect props out of synch and I would suspect there could be some form of defect with the aeroplane which COULD go unnoticed. Further - get a lot of guys telling me that these headsets are better for your hearing and they try to justify this by showing me how much lower the volume setting is on their ACP than on mine..... Fact is that the ANR sets have some amplification of their own to add to that of the ACP. Could prove this myself by bringing in my ghetto blaster and coupling an input to it from the intercom (probably get the sack for it) - a 100 Watt subwoofer next to your lughole would probably require the ACP setting to mins or below.
By the way I did try one of these magic sets few weeks ago and noticed little difference. It seemed to simply replace a mid range background noise with that of a lower frequency...... What's the real benefit I wonder?