Noise Cancelling Headphones

What are they?
Noise-canceling headphones reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control. Essentially, this involves using a microphone, placed near the ear & electronics circuitry which generates an "anti-noise" sound wave with the opposite polarity of the sound wave arriving at the microphone. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphone. Keeping noise low at the ear makes it possible to enjoy music without raising the volume unnecessarily. It can also help a passenger sleep in a noisy vehicle such as an airliner.

How Effective they are?
Retail nose cancelling headphones typically only cancel the lower-frequency portions of the noise; they depend upon traditional noise suppression techniques (such as their ear cups) to prevent higher-frequency noise from reaching the interior of the headphone.
Before going for a pair, keep the following in mind:

• Because of the shorter wavelength of the high-frequency sound. Active cancellation is much less effective at higher frequencies.
• They consumer power, usually supplied by a battery that must occasionally be replaced or recharged.
• They may not cancel all sound effectively. Some noise-cancelling headphones can be overloaded by low-frequency pressure waves, distorting the desired signal.
• They work well for sounds that are continuous, such as the sound of a refrigerator, but are rather ineffective against speech or other rapidly changing audio signals.
• They may introduce additional noise, usually in the form of high-frequency hiss.
• They are generally more bulky than traditional headphones.