▶ Noise-canceling earphones come in either active or passive types. Technically speaking, any type of earphone can provide some passive noise reduction. That's because the materials of the earphones themselves block out some sound waves, especially those at higher frequencies. The best passive noise-canceling earphones, however, are circum-aural types that are specially constructed to maximize noise-filtering properties. That means they are packed with layers of high-density foam or other sound-absorbing material, which makes them heavier than normal earphones. The tradeoff of all that extra weight is a reduction in noise of about 15 to 25 decibels (dB). But considering jet engines create 75 to 80 dB of noise inside the aircraft cabin, passive models have some serious limitations. That's where active noise-canceling earphones come in.
▶ Active noise-canceling earphones can do everything that passive ones can do -- their very structure creates a barrier that blocks high-frequency sound waves. They also add an extra level of noise reduction by actively erasing lower-frequency sound waves. How do noise-canceling earphones accomplish this? They actually create their own sound waves that mimic the incoming noise in every respect except one: the earphone's sound waves are 180 degrees out of phase with the intruding waves.
▶ Noise cancellation makes it possible to listen to audio content without raising the volume excessively. It can also help a passenger sleep in a noisy vehicle such as an airliner. In the aviation environment, noise-cancelling earphones increase the signal-to-noise ratio significantly more than passive noise attenuating earphones or no earphones, making hearing important information such as safety announcements easier. Noise-cancelling earphones can improve listening enough to completelyoffset the effect of a distracting concurrent activity.
There are a number of great applications for active noise cancellation devices. Noise cancellation almost requires the sound to be cancelled at a source, such as from a loud speaker.
That is why the effect works well with headsets, since you can contain the original sound and the canceling sound in an area near your ear. One obvious application is that people
working near aircraft or in noisy factories can now wear these electronic noise cancellation headsets to protect their hearing!