question: How do noise canceling headphones actually work?
comment: They have a microphone on the outside that picks up the external noise. The circuitry then creates a negative version of this and adds it to the music or whatever is playing. Outside noise getting in to the ears is, or should be, cancelled out by adding the negative version of it. Just turning off the microphone circuit stops creating the negative version so stops cancelling the outside noise.
passages: Noise-cancelling headphones
  
  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.
   Section::::Alternative technologies.
   BULLET::::- Electronic noise control: Electronics, sensors, and computers are also employed to cancel noise by using phase cancellation which matches the sound amplitude with a wave of the opposite polarity. This method employs the use of an active sound generating device, such as a loudspeaker to counteract ambient noise in an area. See noise-canceling headphone. Workers in noisy environments may favor this method over ear plugs.
   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.
   Section::::Applications.
  
  Applications can be "1-dimensional" or 3-dimensional, depending on the type of zone to protect. Periodic sounds, even complex ones, are easier to cancel than random sounds due to the repetition in the wave form.
   In a reciprocal LFI network, each player can hear, and be heard by, the same people. For example, in the following chart we can see each player's headphone-mates:
   Noise-canceling microphone
  
  A noise-canceling microphone is a microphone that is designed to filter ambient noise
  
  Section::::Technical detail.
   Each voice has two DCOs, one VCF, one VCA, three ADSR envelopes, and a noise source.
  
  Each DCO can produce triangle, sawtooth, a predefined mix of sawtooth and triangle, and rectangular waves. The pulse width of the rectangular waves ranges from 0 to 100%, and has a dedicated knob. The relative levels of the two DCOs is set with a balance knob.
  
  The pitch and pulse width of each DCO and the relative level of the two can be modulated.
   Within the first days of the FIFA World Cup 2010 Prosoniq came out with a free "VuvuX" AudioUnit plug in to remove the Vuvuzela noise from the audio commentary without affecting speech and background atmosphere. According to their web site the plug in is not using a notch filter and is based on their sonicWORX de-mixing technology which utilizes statistical signal properties.
   Noise cancellation generators for active noise control are a relatively modern innovation. A microphone is used to pick up the sound that is then analyzed by a computer; then, sound waves with opposite polarity (180° phase at all frequencies) are output through a speaker, causing destructive interference and cancelling much of the noise.
  
  Section::::Residential soundproofing.
   The concept is also explored in Arthur C. Clarke's 1950 short story "Silence Please", which features a device capable of cancelling sound waves.
   Bose headphones
  
  Bose has sold aviation headsets since 1989 and consumer headphones since 2000. The current range of headphones/headsets consists of over-ear, in-ear, aviation and military models.
  
  Section::::Over-ear headphones.
  
  Development of the company's noise-cancelling headphones (and first over-ear headphones) began in 1978 when the chairman, Amar Bose, tried a set of airline-supplied headphones during a flight and found that engine noise from the aircraft prevented the music from being enjoyed.
   Section::::Interference reduction.
  
  Balanced audio connections use a number of techniques to reduce noise. 
   Active noise-cancelling headphones use a microphone, amplifier, and speaker to pick up, amplify, and play ambient noise in phase-reversed form; this to some extent cancels out unwanted noise from the environment without affecting the desired sound source, which is not picked up and reversed by the microphone. They require a power source, usually a battery, to drive their circuitry. Active noise cancelling headphones can attenuate ambient noise by 20 dB or more, but the active circuitry is mainly effective on constant sounds and at lower frequencies, rather than sharp sounds and voices. Some noise cancelling headphones are designed mainly to reduce low-frequency engine and travel noise in aircraft, trains, and automobiles, and are less effective in environments with other types of noise.
   Section::::Superconductor planar implementation.
   The "QuietComfort 25" (QC25) over-ear headphones were released in 2015 as the replacement for the . Unlike its predecessors, the QC25s can be used (without the noise-cancelling function) while the battery is flat. Another change is the elimination of the Hi/Low switch which increased the input gain for low-output sources like MP3 players.
  
  In 2014, the QC25s won the "What Hi-Fi?" Sound and Vision annual award for "Best Noise Cancelling Headphones".
  
  Section::::Over-ear headphones.:QuietComfort 35 II.
   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.
  
  Section::::Theory.
   Section::::Operation.
   In 1986, Bose and Sennheiser both presented active noise-cancelling headsets for aircraft pilots, with the Sennheiser design appearing in a Funkschau ("see article on the German Wikipedia") paper and the Bose design appearing in an ASME paper. A prototype Bose product was used to prevent hearing loss in pilots during the first non-stop around-the-world flight in the Rutan Voyager. The company was the first to release active noise cancelling headphones as a consumer product.
  
  Section::::Over-ear headphones.:QuietComfort 25.
   Sound is a pressure wave, which consists of alternating periods of compression and rarefaction. A noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase (also known as antiphase) to the original sound. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out – an effect which is called destructive interference.
   Section::::History.:Alternative manufacturers.
  
  Although the Univox units are the most well-known incarnation of this circuit, Shin-ei licensed this circuit out to dozens of manufacturers. 
  
  Section::::Sound.
  
  This fuzz is an octave fuzz using two germanium diodes to produce the square wave clipping. 
  
  The controls are 'Balance' (volume), 'Expander' (fuzz amount), a two position 'tone' switch, and an on/off footswitch on top. 
   The "QuietComfort 20" (QC20) and QC20i in-ear headphones were released in 2015 and are the company's first in-ear noise cancelling headphones. The QC20 model is for Android, Windows and Blackberry devices, while the QC20i is designed for Apple devices and includes volume controls on the remote.
  
  The noise-cancelling hardware and rechargable battery is contained with a box located near the headphone jack, similar to the . The earbuds are similar to the IE2 headphones. A button on the remote labelled "Aware Mode" reduces noise-cancellation to let in ambient noise.
  
  Section::::Industrial headphones.
  
  Section::::Industrial headphones.:A20 Aviation Headset.
   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 growing number of portable media players are including audio processing chips that allow digital effects like 3D audio effects, dynamic range compression and equalisation of the frequency response. Some devices adjust loudness based on Fletcher–Munson curves. Some media players are used with Noise-cancelling headphones that use Active noise reduction to remove background noise.
  
  Section::::Digital signal processing.:De-noise mode.
   A group of lawyers requested court injunctions against the Toronto Police Service from using newly purchased Long Range Acoustic Devices (LRAD), also known as sound cannons, during protests. Sound cannons have been used in previous summit protests and have the ability to produce sound at ear-piercing volumes, potentially causing hearing impairment. The Ontario Superior Court of Justice later ruled that officers can use sound cannons, with a few restrictions.
  