Active noise control (ANC), also referred to as noise cancellation (NC) or active noise reduction (ANR), constitutes a methodology for mitigating undesirable sound through the introduction of a precisely engineered secondary sound intended to neutralize the original. Initial conceptualization of this technology occurred in the late 1930s, with subsequent developmental efforts commencing in the 1950s. These advancements ultimately led to the commercial availability of airline headsets incorporating this technology by the late 1980s. Presently, this technology finds application in diverse contexts, including road vehicles, mobile telephones, earbuds, and headphones.
Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The concept was first developed in the late 1930s; later developmental work that began in the 1950s eventually resulted in commercial airline headsets with the technology becoming available in the late 1980s. The technology is also used in road vehicles, mobile telephones, earbuds, and headphones.
Principles of Operation
Sound manifests as a pressure wave, characterized by alternating phases of compression and rarefaction. A noise-cancellation speaker generates a sound wave possessing identical amplitude but an inverted phase, commonly termed antiphase, in relation to the primary sound. These waves then superimpose, a phenomenon known as interference, resulting in their mutual cancellation, an effect designated as destructive interference.
Contemporary active noise control systems typically employ either analog circuitry or digital signal processing techniques. Adaptive algorithms are engineered to analyze the waveform of ambient noise, whether aural or nonaural. Subsequently, contingent upon the specific algorithm utilized, a signal is generated that either phase-shifts or inverts the polarity of the initial signal. This inverted signal, being in antiphase, undergoes amplification, and a transducer subsequently produces a sound wave directly proportional in amplitude to the original waveform, thereby inducing destructive interference. Consequently, the perceived volume of the noise is substantially diminished.
A noise-cancellation speaker can be positioned proximally to the sound source intended for attenuation. Under such circumstances, the speaker must possess an equivalent audio power level to that of the undesirable sound source to achieve effective noise cancellation. Conversely, the transducer responsible for emitting the cancellation signal may be situated at the specific location where sound attenuation is desired, such as near a user's ear. While this configuration necessitates a significantly lower power level for cancellation, its efficacy is generally limited to a singular user. Achieving noise cancellation across multiple locations presents greater challenges, primarily because the three-dimensional wavefronts of the unwanted sound and the cancellation signal may interact to produce alternating zones of constructive and destructive interference, thereby diminishing noise in certain areas while potentially amplifying it in others. Within confined enclosures, such as a vehicle's passenger compartment, comprehensive noise reduction can be accomplished through the deployment of multiple speakers and feedback microphones, coupled with the measurement of the enclosure's modal responses.
Applications
Applications of this technology can be categorized as either 1-dimensional or 3-dimensional, contingent upon the characteristics of the zone requiring protection. Periodic sounds, even those exhibiting complexity, are more readily canceled than stochastic sounds, owing to the inherent waveform repetition.
The attenuation of noise within a 1-dimension zone is comparatively simpler, typically necessitating only one or two microphones and speakers for effective operation. Numerous successful commercial implementations exist, including noise-canceling headphones, active mufflers, anti-snoring apparatuses, vocal or center channel extraction systems for karaoke machines, and noise management within air conditioning ducts. Within active noise-canceling headphones, an integrated microphone gauges ambient noise, and the residual noise component that would otherwise reach the ear is computed utilizing the headphone's acoustic transfer function. Subsequently, an opposing signal is generated within the headphones to counteract this residual component. At the eardrum, the external sound and the compensatory signal from the headphones interact to achieve attenuation. This interaction results in a substantial reduction of the sound pressure level. Furthermore, desired audio content, such as speech or music, can also be reproduced via the headphones. The designation 1-dimension denotes a straightforward piston-like relationship, either between the noise source and the active speaker (in the context of mechanical noise reduction) or between the active speaker and the listener (as observed in headphones).
The protection of a three-dimensional zone necessitates numerous microphones and speakers, thereby increasing implementation costs. Noise reduction is more readily achieved with a single, stationary listener; however, the challenge of noise reduction becomes significantly complicated if multiple listeners are present, or if the sole listener alters their head position or moves within the space. High-frequency waves are difficult to attenuate in three dimensions due to their relatively short audio wavelength in air. For instance, the wavelength in air of sinusoidal noise at approximately 800 Hz is twice the average distance between a person's left and right ears; such noise originating directly from the front can be easily reduced by an active system, but if it emanates from the side, it may result in cancellation at one ear and reinforcement at the other, leading to an amplification rather than attenuation of the sound. High-frequency sounds exceeding 1000 Hz tend to unpredictably cancel and reinforce from various directions. Consequently, the most effective noise reduction in three-dimensional space primarily involves low-frequency sounds. Commercial applications of 3-D noise reduction include the protection of aircraft cabins and car interiors; however, in these contexts, protection is mainly restricted to the cancellation of repetitive (or periodic) noise, such as that induced by engines, propellers, or rotors. This efficacy stems from the predictable, cyclic nature of such noise sources, which simplifies analysis and cancellation processes.
Contemporary mobile phones incorporate a multi-microphone design to attenuate ambient noise from the speech signal. Sound is captured from the microphone(s) positioned furthest from the mouth (representing the noise signal(s)) and from the microphone closest to the mouth (representing the desired signal). These signals are then processed to cancel the noise from the desired signal, resulting in enhanced voice clarity.
In specific scenarios, noise can be controlled by implementing active vibration control techniques. This approach is particularly effective when structural vibrations generate undesirable acoustic emissions through coupling with the surrounding fluid medium, such as air or water.
Active versus Passive Noise Control
Noise control represents a methodology for mitigating acoustic emissions, employing either active or passive means, frequently implemented for reasons of personal comfort, environmental protection, or regulatory adherence. Active noise control is a method of sound attenuation that necessitates an external power source. Conversely, Passive noise control is achieved through the application of noise-isolating materials, such as insulation, sound-absorbing panels, or mufflers, without requiring an external power supply.
Active noise cancellation is optimally effective for low-frequency applications. For higher frequencies, the spatial constraints for free-field and quiet-zone methodologies become impractical. In acoustic cavity and duct-based systems, the nodal density escalates significantly with rising frequency, rendering active noise control methods rapidly intractable. Conversely, passive noise control strategies demonstrate greater efficacy at higher frequencies, frequently offering sufficient attenuation without recourse to active systems.
History
The inaugural patent for a noise control system—U.S. patent 2,043,416—was awarded to inventor Paul Lueg in 1936. The patent detailed a method for attenuating sinusoidal tones within ducts through phase advancement and for canceling arbitrary sounds in the vicinity of a loudspeaker by polarity inversion. During the 1950s, Lawrence J. Fogel secured patents for systems designed to mitigate noise within helicopter and airplane cockpits. In 1957, Willard Meeker created a functional prototype of active noise control integrated into a circumaural earmuff. This particular headset exhibited an active attenuation bandwidth ranging from approximately 50 to 500 Hz, achieving a peak attenuation of about 20 dB. By the late 1980s, the initial commercially accessible active noise reduction headsets were introduced. These devices could be powered either by NiCad batteries or directly from the aircraft's electrical system.
Active sound design
- Active sound design
- Adaptive noise cancelling
- Coherence (physics)
- Noise-canceling microphone
Notes
References
BYU physicists demonstrate fan noise reduction in computing and office equipment.
- BYU physicists quiet fans in computers, office equipment
- Anti-Noise: Quieting the Environment with Active Noise Cancellation Technology, published in IEEE Potentials, April 1992.
- Christopher E. Ruckman's Active Noise Control (ANC) FAQ (Originally established in 1994 and maintained until approximately 2010; currently inactive).
- Waves of Silence: Digisonix, Active Noise Control, and the Digital Revolution.