Noise cancelling headphones do exactly what the name implies... cancel noise through a process called "active noise cancellation." Through the marvel of modern technology, a single pair of headphones with built in tiny microphones and a few electronics can monitor the environment and reduce unwanted background noise. For those more technically minded, noise cancellation works by a microphone near the ear picking up unwanted ambient noise, and an electronic component that reproduces the opposite polarity of the unwanted sound through the headphones. The effect of the sound plus the opposite polarity of the sound cancel each other out, so what you hear is nothing.

For those not totally hip on the principles of sound, the effectiveness of active noise cancellation diminishes as the frequency of the noise increases. Thus, most active noise control systems work best for lower frequency, continuous sounds like engine noise on a plane, trains moving on tracks, and the humming of a large fan. What it won't work well on is higher frequency or constantly changing sounds like speech and little kids banging on a pan with a metal spoon. Fortunately, passive noise control techniques like pads and ear cups due a pretty effective job of combating those higher frequency audio signals. Expect to see both kinds of techniques implemented on most headphones, and when put together can do a pretty good job of muzzling most kinds of outside noise.

Noise cancelling headphones aren't perfect, and do introduce a few irritations on their own. First, many kinds of noise cancelling headphones will introduce their own kind of noise, typically in the form of a hiss that's audible during soft music passages or when no audio signal is present. With decent volume, it's typically not noticeable.

While this could just be me, I often feel a bit of pressure from active noise systems that can make your ears feel tired after long listening sessions. The effects of this will vary from person to person and with the model of headphones being used. I own two kinds of Noise cancelling headphones, and the effect is stronger with one than it is with the other.

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