Noise-cancelling headphones are headphones that reduce unwanted ambient sounds using active noise control. This is distinct from passive headphones which, if they reduce ambient sounds at all, use techniques such as soundproofing.
Noise cancellation makes it possible to listen to music 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 headphones increase the signal-to-noise ratio significantly more than passive noise attenuating headphones or no headphones, making hearing important information such as safety announcements easier. Noise-cancelling headphones can improve listening enough to completely offset the effect of a distracting concurrent activity.
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Theory
To cancel the lower-frequency portions of the noise, noise-cancelling headphones use active noise control. They incorporate a microphone that measures ambient sound, generate a waveform that is the exact negative of the ambient sound, and mix it with any audio signal the listener desires.
Most noise-cancelling headsets in the consumer market generate the noise-cancelling waveform in real-time with analogue technology. In contrast, other active noise and vibration control products use soft real-time digital processing.
To prevent higher-frequency noise from reaching the ear, most noise-cancelling headphones depend on soundproofing. Higher-frequency sound has a shorter wavelength, and cancelling this sound would require locating devices to detect and counteract it closer to the listener's eardrum than is currently technically feasible, or would require digital algorithms that would complicate the headphone's electronics.
Noise-cancelling headphones specify the amount of noise they can cancel in terms of decibels. This number may be useful to compare products, but does not tell the whole story, as it does not specify noise reduction at various frequencies.
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In aviation
By the 1950s, systems were created to cancel the noise in helicopter and aircraft cockpits. Noise-cancelling aviation headsets are now commonly available.
A number of airlines provide noise-cancelling headphones in their business and first class cabins. Noise cancelling is particularly effective against aircraft engine noise. In these cases, the headphones are about the same size as normal headphones. The electronics, located in the plane handrest, take the sound from the microphone behind the headphone, invert it, and add it back into the audio signal, which reduces background noise.
As a sleeping aid
There is a increased use of noise cancellation headphone over a period of last few years as sleeping headphones. These are particularly helpful for people suffering from insomnia or other different types of sleeping disorder. The use of these active and passive noise cancellation headphones helps to achieve better noise isolation from ambient sounds. For sleeping disorder patients car honking, the sound of snoring or any other small disturbance creates problems. Noise cancelling sleep headphones and ear plugs are designed to cater this segment of patients.
Drawbacks
Noise-cancelling headphones have the following drawbacks:
- They typically cost more than regular headphones.
- Active noise control requires power, usually supplied by a USB port or a battery that must occasionally be replaced or recharged. Without power, some models do not even function as regular headphones.
- Any battery and additional electronics may increase the size and weight of the headphones compared to regular headphones.
- The noise-cancelling circuitry may reduce audio quality and add high-frequency hiss, though reducing the noise may result in higher perceived audio quality.
Source of the article : Wikipedia
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