For the best audio experience, reducing noise from external sources can make a big difference. There are two main ways in which headphones can achieve this:
1. By using a design that completely encloses the ears.
2. By using active noise reduction techniques, AKA noise cancellation.
Enclosing the Ears
Headphones that completely cover the ears are known as circumaural headphones. This simple, common design is the first step in reducing outside noise. Generally speaking, bigger and bulkier designs eliminate more noise. The disadvantage is that they tend to be heavier and less convenient than more compact designs.
Active Noise Cancellation
Active noise cancellation (AKA acoustic noise cancellation) is a very clever technique that actually "listens" to surrounding noise and eliminates it. If you are familiar with how sound waves interact this will make sense, but to put it simply it works like this:
Microphones mounted in the headphones pick up external sounds.
These sounds are reproduced and played back via the headphone speakers "out of phase" (inverted).
The original noise and the new out-of-phase version cancel each other out, leaving silence.
The desired sound from the headphone source is unaffected.
In practice, noise-cancelling headphones do not eliminate all background noise. Some manufacturers claim up to 90% noise reduction but this is probably optimistic. In most situations you will notice a very good reduction but don't expect it to be perfect.
Also, different types of noise are processed with differing results. Constant noises such as vehicle noise, air conditioning and crowds may be reduced significantly. Other noises may not be reduced much at all.
Noise-cancelling headphones need power so one or more batteries are usually required. Some headphones will not work at all if there is no power; others allow you to bypass the noise-cancellation function if the battery goes flat.
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