Active noise control
Active noise control (ANC) (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for reducing unwanted sound.
Sound is a pressure wave, which consists of a compression phase and a rarefaction phase. 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 phase cancellation. Depending on the circumstances and the method used, the resulting soundwave may be so faint as to be inaudible to human ears.
A noise-cancellation speaker may be co-located with the sound source to be attenuated. In this case it must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference. In small enclosed spaces (e.g. the passenger compartment of a car) such global cancellation can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.
Modern active noise control is achieved through the use of a computer, which analyzes the waveform of the background aural or nonaural noise, then generates a signal reversed waveform to cancel it out by interference. This waveform has identical or directly proportional amplitude to the waveform of the original noise, but its signal is inverted. This creates the destructive interference that reduces the amplitude of the perceived noise.
The active methods (this) differ from passive noise control methods (soundproofing) in that a powered system is involved, rather than unpowered methods such as insulation, sound-absorbing ceiling tiles or muffler.
The advantages of active noise control methods compared to passive ones are that they are generally:
- More effective at low frequencies.
- Less bulky.
- Able to block noise selectively.
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 cancelling 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. Fogel in the 1950s and 1960s such as U.S. Patent 2,866,848, U.S. Patent 2,920,138, U.S. Patent 2,966,549 and Canadian patent 631,136. In 1986, Dick Rutan and Jeana Yeager used prototype headsets built by Bose in their around-the-world flight.
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.
Protection of a "1-dimension zone" is easier and requires only one or two microphones and speakers to be effective. Several commercial applications have been successful: noise-cancelling headphones, active mufflers, and the control of noise in air conditioning ducts. The term "1-dimension" refers to a simple pistonic relationship between the noise and the active speaker (mechanical noise reduction) or between the active speaker and the listener (headphones).
Protection of a 3-dimension zone requires many microphones and speakers, making it less cost-effective. Each of the speakers tends to interfere with nearby speakers, reducing the system's overall performance. Noise reduction is more easily achieved with a single listener remaining stationary in a three-dimensional space but if there are multiple listeners or if the single listener moves throughout the space then the noise reduction challenge is made much more difficult. High frequency waves are difficult to reduce in three dimensions due to their relatively short audio wavelength in air. The wavelength in air of sinusoidal noise at approximately 500 Hz is double the distance of the average person's left ear to the right ear; such a noise coming directly from the front will be easily reduced by an active system but coming from the side will tend to cancel at one ear while being reinforced at the other, making the noise louder, not softer. High frequency sounds above 1000 Hz tend to cancel and reinforce unpredictably from many directions. In sum, the most effective noise reduction in three dimensions involves low frequency sounds. Commercial applications of 3-D noise reduction include the protection of aircraft cabins and car interiors, but in these situations, protection is mainly limited to the cancellation of repetitive (or periodic) noise such as engine-, propeller- or rotor-induced noise.
Antinoise is used to reduce noise at the working environment with ear plugs. Bigger noise cancellation systems are used for ship engines or tunnels. An engine's cyclic nature makes FFT analysis and the noise canceling easier to apply.
The application of active noise reduction produced by engines has various benefits:
- The operation of the engines is more convenient for personnel.
- Noise reduction eliminates vibrations that cause material wearout and increased fuel consumption.
- Quieting of submarines.
- BYU physicists quiet fans in computers, office equipment
- Anti-Noise, Quieting the Environment with Active Noise Cancellation Technology, IEEE Potentials, April 1992
- Christopher E. Ruckman's ANC FAQ
- Noise-Canceling Headphones construction
- Principles and Experiments of an ANC for a computer box
- Down with Noise, Practical control systems for combatting audible noise show up in aerospace, general aviation, and military roles, Steve Elliott, IEEE Spectrum Magazine July 1999
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