Noise reducing headphones are those which are made to provide a more pleasant listening experience by blocking ambient noise. If successful, they cancel out a large portion of the noise around the listener, providing peace and quiet in an otherwise noisy environment. Many airline patrons use noise reducing headphones, especially when sitting near the food preparation area or a crying baby. In spite of their relatively high cost, noise reducing headphones can become a necessity for the frequent air traveler, rather than a luxury.
There are two ways in which noise reducing headphones can work to block ambient noise. The first, passive noise reduction, is accomplished by blocking out sounds with insulating material, such as a high-density foam. When these materials form part of the headphones, they reduce the perceived ambient noise by a factor of ten or more.
Oftentimes, however, passive noise reduction is not enough. Inside a passenger jet, for example, that amount of noise reduction still lets enough engine noise through to cause discomfort to someone who wants to quietly relax. This is where active noise reduction comes in. The best noise reducing headphones depend on active noise reduction to a great degree, to accomplish their purpose. The degree of effectiveness of active noise reduction is usually what distinguishes a top quality headphone from a mediocre one.
Active noise reduction uses the science of acoustics to "repel" ambient noise from the wearer's ears. This technology goes beyond passive noise reduction to actually produce sound waves that resemble those of the ambient noise in many ways. They are almost identical, except that they are a mirror image, so to speak, of the ambient noise. When these two sets of sound waves interact, they cancel each other out in a process called destructive interference. The end result is total silence. This gives the wearer the ability to focus on what he is listening to, or simply to block out sounds around him, allowing him to rest.
Noise reduction technology does have some limitations. While it is excellent at blocking out sounds that are constant and unchanging, like a jet engine, it has a harder time blocking out more sudden or high-frequency sounds, such as a ringing telephone or other random sounds. Also, because active sound reduction creates a sound wave of its own, this may in some cases compromise the quality of the music or other audio being heard. These trade-offs, though, are usually well worth it for some much-needed peace and quiet.