ANR: Automatic Noise Reduction or sometimes called ENC: Electronic Noise Cancelling. Either is a good description of what is happening. Typical paramotor helmet with ear muffs or headset are PNR: Passive Noise Reduction, this is the foam filled ear muffs that protect your ears from the engine & prop noise.
Combine this PNR with ANR into a Paramotor Helmet and you get a double wammy effect on reducing engine & prop noise.
Standard PNR ear muffs reduce noise by 18-30dB (decibels) depending on the brand and intended use. Most paramotor headsets I have tried are rated around the 26-28dB mark.
Adding the ANR reduces low frequency noise by a further 18-24dB, added to the 28dB protection from the PNR we are in for a quiet ride. The ANR really only works for the low frequency noise i.e.: the engine drone, prop thump. On test the effect really is surprising, with the flick of a switch and in a moment a big reduction in noise, the feeling of being in a sound proof room comes over you.
In the following graph you can see the effect of PNR + ANR has on a range of frequencies.
Combine that to a radio headset push to talk communications with a noise cancelling mic and iPod input, flying PPG becomes even more luxurious.
Link to the Paramotor Helmet with ANR headset we tested from http://www.paramotorhelmet.co.uk
Not all ANR systems are the same, all I can say is this one works very well. Our test paramotor was a Vittorazi Moster.