Noise-cancelling headphones, or noise-canceling 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 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 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.
Theory
To cancel the lower-frequency portions of the noise, noise-cancelling headphones use active noise control or ANC. A microphone captures the targeted ambient sounds and a small amplifier generates sound waves that are exactly out of phase with the undesired sounds. When the sound pressure of the noise wave is high, the cancelling wave is low. The opposite sound waves collide and are eliminated or "cancelled". Cancellation focuses on constant droning sounds like road noise and is less effective on short/sharp sounds like voices or breaking glass. It also is ineffective in eliminating higher frequency noises like the sound of spraying. Noise-cancelling headphones often combine sound isolation with ANC to maximize the sound reduction across the frequency spectrum. Noise cancellation can also be used without sound isolation to make voices, etc. easier to hear. Noise cancellation to eliminate ambient noise is never passive because of the circuitry required, so references to passive noise cancellation actually are referring to products featuring sound isolation. 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 sound isolation or 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 for comparing products but does not tell the whole story, as it does not specify noise reduction at various frequencies.
In aviation
By the 1950s, Dr. Lawrence Jerome Fogel created systems and submitted patents about active noise cancellation specifically in the field of aviation. This system was designed to reduce noise for the pilots in the cockpit area and help make their communication easier and protect hearing. Fogel was considered to be the inventor of active noise cancellation and he designed one of the first noise canceling headphones systems. Later on, Willard Meeker designed an active noise control model that was applied to circumaural earmuffs for advanced hearing protection. 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
Over the last few years, the use of noise-cancellation headphones as sleeping aids has increased. Both passive isolating and active noise-cancellation headphones / ear plugs help to achieve a reduction of ambient sounds, which is particularly helpful for people suffering from insomnia or other sleeping disorders, for whom sounds such as cars honking and snoring impact their ability to sleep. For that reason, noise-cancelling sleep headphones and ear plugs are designed to cater to 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, although reducing the noise may result in higher perceived audio quality.
