The noise cancelling part of the equation comes from an active system whereby a microphone on the outside of the headphones picks up all the ambient noise around you then pipes an equal and opposite signal back into the headphones, theoretically cancelling out the ambient noise and ending up with silence. In actuality, though, these systems never work flawlessly and there will always be a degree of background noise that leaks through. That said, these do seem to be among the least effective we've used in quite a while.
We think this is mainly due to the way the headphones sit on your head. You see, the fit is incredibly light, which does make them more comfortable than most on-ear designs but also means they don't seal very well over your ears, which lets a lot of ambient noise sneak through. Obviously the active system cancels some of this out but we can't help but think a slightly tighter fit (or over-ear design) would've helped in this regard.
That said, one strange upside of this is you still get a good sense of what's going on around you, just with everything being a bit quieter. This means you can still have conversations and hear the fast car approaching you from behind - two examples that truly noise-blocking canalphones can't deal with - but the constant hum of an aeroplane engine is noticeably reduced.
Unfortunately, there are no upsides to the most annoying consequence of these phones' light fit; they fall off incredibly easily. Just tipping your head forward (like you would to get something out of a travel bag, say) causes them to fall off. This makes them incredibly impractical for anything except sitting very still.
Battery life is also a problem. Just one AAA battery is required to power the set and we got a good couple of days usage out of it (Denon says the battery lasts around 40 hours), which is fine. However, if you forget to turn the set off, there's no automatic standby mode so your phones will just merrily run themselves down while stowed away in your bag. Thankfully they are at least usable even without batteries, which is more than can be said for some noise cancelling cans, but you obviously won't get any noise reduction and sound quality also takes a nose dive with bass response particularly suffering.
And this brings us nicely to the ultimate point of these phones; their sound quality. When fully powered up, they sound magnificent. For a start, bass is full and well-defined yet not overpowering, but it's the clarity at the top end that really sets these apart. Every little detail is reproduced effortlessly and there's a smoothness to the whole sound that just feels wonderfully natural. The only possible negative with regards to sound quality is that they can sound a little cold and stark when compared with something like a pair of Grados, for instance, but you quickly get used to this different tonality.
The Denon AH-NC732's are among the best travel headphones we've used in terms of the quality of sound you get. However, we can't help but feel the on-ear design is just fundamentally flawed. After all, you still end up with quite a large case so why not make them a little bigger use larger over-ear cups, making the headphones more secure and comfortable.