Noise-cancelling headphones reduce unwanted ambient sounds using

active noise control Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The concept was first develop ...

. This is distinct from passive

headphone Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an a ...

s which, if they reduce ambient sounds at all, use techniques such as

soundproofing Soundproofing is any means of impeding sound propagation. There are several basic approaches to reducing sound: increasing the distance between source and receiver, decoupling, using noise barriers to reflect or absorb the energy of the sound w ...

. 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 Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...

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 Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

portions of the

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

, noise-cancelling headphones use active noise control or ANC. A

microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...

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 Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrop ...

of the noise wave is high, the cancelling wave is low (and vice versa). The opposite sound waves collide and are eliminated or "cancelled" (

destructive interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

). 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. According to an experiment conducted to test how lightweight earphones reduced noise as compared to commercial headphones and earphones, lightweight headphones achieved better noise reduction than normal headphones. The experiment also supported that in-ear headphones worked better at reducing noise than outer-ear headphones. 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 wanted sounds (such as voices) 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. 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 In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

, 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

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a ...

s. This number may be useful for comparing products but does not tell the whole story, as it does not specify

noise reduction Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an u ...

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 Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, lighter- ...

. This system was designed to reduce noise for the pilots in the cockpit area and help make their communication easier and protect hearing. Fogel is considered to be the inventor of active noise cancellation, and he designed one of the first noise-cancelling 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. In 1989,

Bose Corporation Bose Corporation () is an American manufacturing company that predominantly sells audio equipment. The company was established by Amar Bose in 1964 and is based in Framingham, Massachusetts. It is best known for its home audio systems and spea ...

introduced its "Series I Aviation Headset" which became the first commercially available ANR headset. It included a noise-cancelling function and was powered either by NiCad batteries (with a claimed battery life of 8 hours) or by power from the aircraft. Series I aviation headsets are distinguished by the clear windows on the earcups or by noting that the on/off switch and volume control are located on the separate control module. Several airlines provide noise-cancelling headphones in their

business Business is the practice of making one's living or making money by producing or buying and selling products (such as goods and services). It is also "any activity or enterprise entered into for profit." Having a business name does not separ ...

and first-class cabins. Bose started supplying American Airlines with noise-cancelling headphones in 1999 and started offering the "Quiet Comfort" line for the general consumer in 2000. 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 hand rest, 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

Noise-cancellation headphones have been used as sleeping aids as well. Both passive isolating and active noise-cancellation headphones or earplugs help to achieve a reduction of ambient sounds, which is particularly helpful for people suffering from

insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy, ...

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.

In hospitals

The use of noise-cancelling headphones for patients in intensive care units has been implemented to reduce the amount of noise exposure that they face while in a hospital environment. Active noise control technology is shown to have a relationship with sleep disturbance, delirium, and morbidity, therefore bringing up concerns about lowering the levels of noise for patients receiving care.

Health and safety

There is a general danger that listening to loud music in headphones can distract the listener and lead to injury and accidents. Noise-cancelling headphones add extra risk. Several countries and states have made it illegal to wear headphones while driving or cycling. It is not uncommon to get a pressure-like feeling when using noise-cancelling headphones initially. This is caused by the lack of low-frequency sounds as being perceived as a pressure differential between the inner and outer ear.

Autism

A December 2016 study from the '' Hong Kong Journal of Occupational Therapy'' found that noise-cancellation headphones helped children with

autism spectrum disorder The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...

cope with behaviours related to hyper-reactivity and

auditory stimuli In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be ...

Drawbacks

Noise-cancelling headphones typically cost more than regular headphones. The active noise control requires power, usually supplied by a USB port or a

battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...

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.

References

{{DEFAULTSORT:Noise-Cancelling Headphones Audio engineering Noise reduction