The Lightspeed website actually has a rather good explanation as to how ANR works.
http://www.anrheadsets.com/anr-tutorial.html

I've found that engine "roar and rumble" is what is cancelled rather than squeaks, pops, etc.

David.

I would also recommend the Lightspeed headsets. I've got a 20XL and the noise reduction is great. I've worn a pair for over 4 hours before and they stay really comfortable.

A really good thing about this unit is that if you forget to turn the power off (it is inline with the headset cord) it switches itself off after 20 minutes.

Regards,

STurns

From what I can remember from A Level Physics the ANR would normally only cut out the general "roar and runble" and not the individual pops and bangs as the ANR works on a feedback basis so by time the ANR has "responded" to a roar/bang etc you will have already heard it.

The same principal works on some of the "better" motor cars.

PFO

Go, yes I am quite silly, not only that, I still whack bush like a soft lad.