Karmakaze Designs

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A number of headphone manufacturers offer noise reduction or noise cancelling headphones that say they block out ambient noise so that you can listen to your music without distraction or just relax in near silence in a noisy environment. These noise reduction headphones actually do work and they employ several methods to block out unwanted noise.

The first and most simple way that noise reduction headphones work is to for an acoustically insulated barrier between your ears and the outside sounds. Headphones cover your ears and if they fit snugly and have acoustically damping foam or padding materials inside then they can physically block some of the noise that would otherwise reach your ears. Many commercial ear protection devices use only this technique, but to be very effective the ear cups need to be fairly big and heavy. This type of ear protection is often used in noisy work environments and is effective across a wide range of the audio spectrum. For lighter recreational noise reduction headphones this method is also used, but generally to lesser effect than the big, heavy ear protection devices.

To make up for their lighter size, recreational noise reduction headphones may also use an active noise cancelling system. Since sound waves are cyclical pressure waves, creating an equal and opposite pressure wave eliminates the wave altogether. By using a microphone to sense the sounds outside the ear cup of the headphones, a counter wave can be created through the speakers inside the ear cup that is phased opposite that of the outside noise, effectively cancelling out most of it. This active noise cancellation isn’t 100% effective, but can substantially reduce the amount of noise that makes it to the ear. This technique is especially effective for lower frequency which have a longer wavelength. The constant drone of an airplane engine or a lawnmower motor, for example, can be dramatically reduced by noise reduction headphones with active noise cancelling systems.

Noise cancellation works well with headphones in part because of the fact that the size and shape of the area inside the headphone is controlled by the manufacturer. Active noise cancellation systems cannot work with nearly as great an effect in rooms or other uncontrolled spaces. Echoes, reverberations and peaks and valleys as sound waves intersect other sound waves means that the waves are not precisely the same in any two places within the room. As a result, the noise to be cancelled varies dramatically within the room and moving just a foot or two to one side results in a different sonic environment.

The best that can be done for noise reduction in a room-sized environment is to install acoustic insulation to try to absorb or damp the sound waves as they hit the walls of the room. These kinds of acoustic room treatments can lower the level of noise in a room, but not nearly as well as noise cancelling headphones do.