By: Mike. G0KOT
As presented to the Vange Amateur Radio Society 23rd Apr 2009
Arthur C. Clarke's short story entitled "Silence Please" appeared in his 1954 collection "Tales from the White Hart" (reprinted in 1970 by Harcourt, Brace & World Inc., New York). In it, Harry Purvis recounts the tale of the ill-fated "Fenton Silencer," an anti-noise device that goes disastrously awry.
Noise-cancelling headphones reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control (ANC). 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.
Noise cancellation makes it possible to enjoy music without raising the volume excessively. It can also help a passenger sleep in a noisy vehicle such as an airliner.
Retail noise cancelling headphones typically use ANC to cancel the lower-frequency portions of the noise; they depend on more traditional methods (like the materials from which the earcups are made) to prevent higher-frequency noise from reaching the ear. This approach is preferred because:
- it reduces the demand for complicated electronic circuitry.
- at higher frequencies, active cancellation is less effective. 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, which is not technically feasible.
- due to the shorter wavelength of higher frequency sounds, passive isolation becomes more effective.
The first patent for a noise control system was granted to inventor Paul Lueg in 1934 U.S. Patent 2.043.416, describing how to cancel sinusoidal tones in ducts by phase-advancing the wave and canceling arbitrary sounds in the region around a loudspeaker by inverting the polarity. By the 1950s, systems were created to cancel the noise in helicopter and airplane cockpits including those patented by Lawrence J. Foqei in the 1950s and 1960s such as U.S. Patent 2,866.848 . U.S. Patent 2.920.138. U.S. Patent2,966.549 and Canadian patent631.136. In 1986. Dick Rutan and Jeana Yeager used prototype headsets built by Bose in their around-the-world flight.11^
Dr. Lawrence J. Fogel (March 2, 1928 - February 18, 2007),
Dr. Fogel's first decade of engineering (1948-1960) began with an interest in antenna design, ensuring reliability of weapons systems, and inventing interfaces for human operators. He analyzed and designed flight instrumentation for high performance and vertical takeoff aircraft as well as helicopters. He also engineered design concepts for various individual instruments, including all altitude indicators, airspeed indicators, altimeters, as well as related displays. His keen insight led to five patents issued from 1958-1961 pertaining to two separate inventions regarding active noise cancellation and a revolutionary cockpit display known as the "Kinelog."
Active noise cancellation has been studied for many years, using many different control
Feedforward  as well as feedback [S] and hybrid techniques have proven their values.
Synthesis techniques applicable to ANC include LQG , Hz and H, .