Bose's noise-cancelling range, currently made up of the QuietComfort 3 and 15, is the best-known of its kind. It has been around for over 10 years now, but in more recent years has had to deal with the rise of affordable noise-isolating earphones like the Sennheiser CX 300. Both types of headphone attempt to block out noise in different ways, but noise cancelling is far more technologically-advanced (or at least involved) than isolation.
On the back of each QC3 earpiece is a small metal grille. Behind this is a microphone that monitors ambient noise. The noise cancelling brains within the headphones then pipe sound waves equal to the inverse of this ambient noise through the speakers. The peaks and troughs of the two sets of sound waves cancel each other, leaving you with blissful silence.
Of course, it's not quite as perfect as all that. Noise cancelling creates a sensation of pressure within your ears and a slight - very, very slight in the case of the Bose QuietComfort QC3 - white noise hiss occasionally intrudes.
It's superbly effective against the low-end hums generated by passing cars, jet engines and the rumble of the wind through a hundred thousand distant leaves. Flicking the on/off switch on the side of the QC3 demonstrates quite how surprisingly noisy most of the world is, and how we've become accustomed to so many of nature's own dirges.
Let's not descend into a rant about how we should all appreciate the dawn chorus though. They excel in cutting-out this low-end noise, but have much less of an effect on higher frequencies. On the train, the base engine rumble will disappear but you'll still be able to hear the nasal announcements pipe-in over the tannoy, the conversations of nearby passengers and - worst of all - any screaming babies letting rip in the vicinity. The volume of these sounds will be decreased a bit, but that's in part due to the only semi-open design of the ear cups.
The on-ear design also has some ramifications for noise cancellation. Ideally, the Bose QuietComfort 3 want the only noise to reach your ears to be that which is let in through the back port, but the on-ear pads offer less complete (or at least less reliable) isolation than the over-the-ears style of the QuietComfort 15. This is something Bose itself has admitted to, but says that the difference is mitigated by the noise cancelling techniques. In our testing, which included trips on trains, across busy roads and jaunts on the London Underground, the headphones provided excellent cancellation within the limits of the technology. Even with no music playing, they turn tube stations into near-empty museums, where all you can hear is the light clip-clop of heels on marble floors.