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Essentially, this involves using a microphone, placed near the ear, and electronic circuitry which generates an "antinoise" sound wave with the opposite polarity of the sound wave arriving at the microphone. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphone. Keeping noise low at the ear makes it possible to enjoy music without raising the volume unnecessarily. It can also help a passenger sleep in a noisy vehicle such as an airliner. Retail noise cancelling headphones typically only cancel the lower-frequency portions of the noise; they depend upon traditional noise suppression techniques (such as their earcups) to prevent higher-frequency noise from reaching the interior of the headphone. This approach has three benefits: The demands on the electronic circuitry are vastly simplified.; Because of the shorter wavelength of the high-frequency sound, active cancellation is much less effective at higher frequencies. To truly cancel high frequency components (coming at the ear from all directions), the sensor and emitter for the cancelling waveform would have to be adjacent to the user's eardrum, an impractical situation.; For the same reason (shorter wavelength), effective passive isolation becomes easier with rising frequency.;
HistoryBy the 1950s, systems were created to cancel the noise in helicopter and airplane cockpits including those patented by Lawrence J. Fogel in the 1950s and 1960s such as U.S. Patent 2,866,848 (filed in 1954), U.S. Patent 2,920,138, U.S. Patent 2,966,549 and Canadian patent 631,136. Dr. Amar Bose (founder of the Bose Corporation) began work on his noise-cancelling headphones in 1978 on board an airplane. During the international flight, he was provided with a set of headphones from the airline and almost immediately realized how dissatisfied he was with their quality and the loud engine noise. After nearly a decade of research, Bose would release the first noise-cancelling headphones. Since then, many other corporations (such as Panasonic and Sony) have contributed their own noise-cancelling headphones. Sennheiser Corporation was also a pioneer in noise-cancelling headphones, for an airline in that case. In 1986, Bose applied their noise-cancellation technology to develop headphones to protect the hearing of pilots participating in the first non-stop around-the-world flight.
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