From Newsgroup: rec.music.classical

Noise-cancelling headphones are a special type of headphones which suppress 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. 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.[1]
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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 (and vice versa). The opposite sound waves collide and are eliminated or "cancelled" (destructive interference). 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.[2]
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, 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.[3]
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.
By the 1950s, Dr. Lawrence Jerome Fogel created systems and submitted patents regarding active noise cancellation 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 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.
In 1989, Bose Corporation 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.[4]
Several airlines provide noise-cancelling headphones in their business 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[citation needed]. 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.[4]
Noise-cancellation headphones have been used as sleeping aids as well.[5] 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 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.[6]
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.[7]
There is a general danger that listening to loud music in headphones can distract the listener and lead to injury and accidents.[8][9] Noise-cancelling headphones add extra risk. Several countries and states have made it illegal to wear headphones while driving or cycling.[10]
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.[11][12][13]
A December 2016 study from the Hong Kong Journal of Occupational Therapy found that noise-cancellation headphones helped children with autism spectrum disorder cope with behaviors related to hyper-reactivity and auditory stimuli.[14]
The 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 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.[15]
For frequent flyers or commuters on public transit, reducing the noise around you is the difference between enduring a trip and enjoying it. Noise-cancelling headphones can make your music easier to hear and your world a little more peaceful.
The Bose Noise Cancelling Headphones 700 is the best set of over-ear noise-cancelling headphones that Bose currently offers. This pair is worth the investment for the frequent traveler who puts a premium on performance and comfort, and we recommend the 700 over newer, pricier Bose offerings for as long as it is still available.
These headphones can provide the superior noise cancellation that Bose has been known for, but the level of the active noise cancellation (ANC) is adjustable on a scale from 0 to 10, enabling you to find the amount that works best for you.
The XM5 is loaded with helpful features, including Alexa and Google voice activation, dual-device connectivity, noise-reducing microphones, and even the ability to accept or reject calls with a nod or shake of your head.
If you want a surprisingly great pair of wireless noise-cancelling headphones for around $100, we recommend the Soundcore Space One. In our tests, the active noise cancellation was nearly as effective as that of the Bose 700, and the level is adjustable. The sound, while not as refined as that of the Bose 700, is nonetheless enjoyable to listen to and can be customized to your preference in the Soundcore app. The chassis is lightweight and comfortable to wear, with full controls.
Senior staff writer Brent Butterworth also tested all of the finalists and performed the noise-cancelling measurements. Brent has been reviewing audio gear professionally since 1990. He previously worked as an editor or writer for Sound & Vision, HomeTheaterReview.com, Home Theater Magazine, and numerous other publications.
Testing the noise cancelling is more complicated. We do this both by ear and by using test equipment. To test the noise cancelling by ear, I play airplane noise at a loud level through a JBL L16 wireless speaker and try each set of headphones, moving my head and adjusting settings to see which mode cancels noise most effectively. I stand in front of a fan to check the effect of wind noise on the microphones and take the top performers on walks outside near busy streets.
Brent then performs lab tests, measuring the degree to which the headphones block different frequencies of sound. To do this, he places each set of over-ear headphones on his GRAS 43AG ear-and-cheek simulator connected through an M-Audio USB interface to a Windows laptop, plays pink noise through the same speaker system described above, and uses TrueRTA audio-spectrum analyzer software to see how much sound leaks through the headphones.
To test noise-cancelling earbuds, Brent places the right-channel earpiece of each set of in-ear headphones into a GRAS KB5000 anthropometric pinna mounted on the GRAS 43AG ear-and-cheek simulator fitted with a GRAS RA0402 high-resolution ear simulator. He connects this fixture through an M-Audio USB interface to a Windows laptop, plays pink noise through the same speaker system, and uses the TrueRTA audio-spectrum analyzer software to see how much sound leaks through the earbud.
To provide a simple way of looking at these measurements, Brent calculates the average amount of noise (in decibels) that the headphones cancel in the 100 Hz to 1,200 Hz frequency band, which is where more airplane-cabin noise occurs, based on his analysis of the four recordings he made in the cabins of different airliners. The higher the number, the greater the average noise reduction. Here is a rundown of our top performers, compared with a couple of pairs of headphones that lack active noise cancellation:
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