Noise Cancelling Headphones
Active-electronics 'noise-cancelling' headphones have become quite popular over the last five years thanks to very slick advertising campaigns from big-name manufacturers. Unfortunately, this headphone design does not provide the best sound quality and overall acoustic isolation compared to a sealed or an ear-canal headphone (ECH). The next paragraph gives a brief explanation of how noise-cancelling technology works and the ambient noise isolation it provides compared to a 'sealed' or 'ear canal headphone'. We also strongly recommend that you visit our "Sealed Headphones" article first to see if one of these types will work better for your needs, as these designs tend to provide a lot more bang for the buck than a noise-canceling unit may offer.
In "active" noise-cancelling headsets, a small microphone is mounted in each earpiece and picks up the sound arriving at the ear. This ambient noise signal is sent to a small electronics unit attached to the headphone cable and is inverted and summed with the music coming in from your player and then sent back to drive the headphones. As the inverted ambient noise signal drives the headphones, it creates sound that is exactly inversely opposite and therefore cancels the ambient sound arriving at the ear. This technology can achieve about a 10dB drop in a limited spectral region of enviromental noise, usually below 1,000kHz. In other words, you get a limited attenuation of external sounds, mainly in the lower parts of the frequency range. Thusly, 'active' technology functions well at blocking out the constant low-end rumble heard inside an airplane cabin but does much less well at attenuating inconsistent, higher-pitched noises.
On the other hand, a standard full-sized 'sealed/closed' earcup headphone will provide a similar -10dB of isolation BUT over a significantly wider audio frequency range. Finally, high quality in-ear earphones, also known as ear canal headphones or ECHs, offer a bare minimum of minus -20dB of ambient noise attenuation, sometimes as much as minus -37dB --- in other words, nearly-total peaceful silence with just you & your music! With 'standard' headphones or ear canal earphones, another plus is retaining full control over the quality of music reproduction via the wide variety of headphones/earphones available on the market and, ultimately, even the ability to add a dedicated (portable) headphone amplifier for truly maximized audio performance!
Because of all this complex electronic hardware these units typically contain, 'Active Noise-Canceling' is a headphone category containing few really superb-sounding products. However, Sennheiser -- yet again -- have diligently done their homework and developed a basic line of several active noise-cancelling headphones that perform decently at this thorny application. The PXC 150, PXC 250, and PXC 300 all work quite well at blocking ambient noises, sound acceptable, and are very lightweight, ergonomic, & durably-built with a 2-year product warranty.
Sennheiser has also released two new flagship noise-cancelling high-end headphones, the amazing PXC 450 and the less expensive PXC 350. These are both larger full-size, nicely comfy, over-the-ear designs that offer surprisingly good sound quality for headphones of this type.
Wikipedia Link to learn more about noise-cancelling headphones.