The concept of active control (ANC) is not new. Paul Lueg in 1936 received the first patent for ANC. In the 1950s more research was done with little success. However, the advent of integrated circuits has greatly advanced ANC research. Digital Signal Processors have greatly reduced the cost of circuitry used in active control. ANC necessitates an accurate analysis of the incoming sound waves. The algorithms that are used to analyze the wave were developed in the late 1980s. ANC research became more prevalent in the 1970s and has continued forward with the added DSP technology. Heating Ventilation and Air Conditioner (HVAC) were the first devices to benefit from ANC technology that was developed at the CNRS4 Laboratory and Mechanics in Marseilles, France. Since then, the French Navy has adopted the technology for submarine air ducts. Renault auto makers have a long history of ANC research for applications with engine noise, and recently most other auto makers have started research (http://www.adit.fr/Produits/TF/Anciens/TF25a.html). Commercially, ANC has met with success in the avionics industry reducing noise inside the cockpit. ANC is also being currently studied in universities (Pennsylvania State University and Virginia Tech) and government laboratories (NASA Langeley Research Center and NASA Glenn Aeroacoustic Propulsion Laboratory). The current focus is on reducing the noise of aircraft engine noise through ANC inside of aircraft cabins/cockpits.

Sound is a wave that travels through a medium, such as air. The vibration of an object generates sound. Noise is simply defined to be unwanted sound. Human response to noises can vary greatly. Factors that can influence individual response include intensity, frequency, and time pattern of the noise; the amount of background noise present prior to the intruding noise; and the nature of work or human activity that is exposed to the noise. The adverse effects of noise include interference with concentration, communication, and sleep. At the highest levels, noise can induce hearing damage. In most areas, automobile and truck traffic is the major sources of environmental noise. Traffic activity generally produces an average sound level that remains fairly constant with time. Air and rail traffic and commercial and industrial activities are also major sources of noise in some areas. Noise sources associated with utility operations include both stationary sources (such as turbines, compressors, generators, cooling towers, automatic safety relief valves, and fans) and mobile sources (such as maintenance trucks) (http://www.sdgedivest.com/chapters/04-10nse.htm). There are two methods of controlling/reducing noise: active and passive control. Passive control is the traditional method of creating a physical barrier barring the penetration of the waves. Passive methods involve a physical material (insulation, water, and rubber) that can absorb the energy of the sound waves, thus dampening them. Passive techniques work best on medium to high frequency noise. Lower frequency noises have more energy and thus more material is needed to absorb the energy rendering passive noise control methods bulky. Active control essentially tries to eliminate sound or vibration components by adding the exact opposite sound or vibration waves. How this is done can be easily seen using two simple examples:

Figure 1: Two waves with equal amplitude and identical phase add together constructively, resulting in a doubling of overall amplitude.

Figure 2: Two waves with equal amplitude and opposite phase add together destructively, resulting in a canceling of overall amplitude. The phase describes the relative position of the wave in its rising and falling cycle. If two waves are in phase, they rise and fall together, whilst if they are exactly out of phase, one is rises as the other falls, and so they cancel out. (This effect can be heard in the interactive demo at http://www.isvr.soton.ac.uk/active). A basic noise cancellation setup consists of a physical space in which a microphone/s senses the sound signal. It is then processed (i.e. in a Digital Signal Processor) and the inverse signal is broadcasted through actuators (speakers). The broadcasted waves then interfere with the noise to cancel it out.

 

The most commercially successful active noise control systems are headsets used in aviation. Active Noise Control offers a major advancement in headset technology in that old passive methods insulate the ear directly. However, the voice transmission microphone is outside of the earmuffs (the part around the ear), and therefore picks up the noise and transmits it to inside of the earmuff.

Another widespread use in active noise control is HVAC noise control. By positioning a similar setup: microphones to sense the noise signal, processors to create a signal for anti-noise, and speakers, in the ducts fan noise is cancelled. HVAC devices are relatively simple when it comes to sound wave geometry. The wavelengths are longer (low frequency) and the space they occupy can be easily modeled as two dimensions. This simplicity aids in ease of broadcasting the anti-signal.

ANC Set up in HVAC device Success of ANC in HVAC

from:http://www.ibp.fhg.de/rata/aktiv1-e.htm

The current trend in research is how effective the different signal processing algorithms are. Recent work has concentrated on active vibration control and adaptive control algorithms for both feed-forward and feedback systems. This adaptive control algorithm has been applied to three-dimensional space. Multiple channel ANC based on both the filtered x- least mean square (FXLMS) and the H algorithm looking at both feed-forward and feedback experiments was conducted by M.R. Bai and Z. Lin from the Department of Mechanical Engineering, Nation Chiao-Tung University, Sin-Chu, Taiwan. They found that ideally, for the most attenuation of noise, sensors are placed at pressure maxima/nodes. All algorithms yield noise reductions for periodic noises [this is why ANC is successful in headsets and HVAC, engines and fans' noises are periodic], and feed-forward structure is experimentally a feasible approach for broadband [contains a wide range of frequencies] cancellation. This would result in a broader application of ANC. They also found that the Multiple Input Multiple Output feed-forward H technique is effective in dampening transient noises. The application here is a more effective means of dampening out non-periodic noises. Potentially ANC could be applied to cancel any unwanted noise creating a complete quiet zone--perhaps an eerily quiet zone. This would involve a large number of sensors to monitor the incoming noise, a large number of processors, as well as a large number of loudspeakers to broadcast the "anti-noise." ANC holds the most direct potential in low frequency periodic noises. ANC also holds potential in the aircraft industry as replacing passive control. ANC is significantly less heavy than passive control; and in the aircraft industry weight is a dominating factor. However, another trend in research is to combine ANC and passive noise control to create a hybrid effect. At Virginia Tech's Vibrations and Acoustics Laboratory (VAL) C. Guigou and C.R. Fuller have developed a foam PVDF smart skin. It was designed for aircraft interior noise control. The skin is to reduce the sound by the action of passive absorption of the foam (for high frequencies) and the active input of a PVDF element

 

driven by an oscillating electrical input (which is effective at lower frequencies). The device consists of cylindrically curved sections of PVDF piezoelectric film embedded in partially reticulated polyurethane acoustic foam (http://www.val.me.vt.edu). The results of this show large noise attenuation. However sound control is not only limited to active and passive methods that combat noise. Controlling noise at the source is also effective. For example, to reduce ambient noise the idea of a focused speaker broadcasting a direct beam of sound to the listener would better suited. Professor Barry Vercoe and F. Joseph Pompei of the MIT Media Lab have developed Audio Spotlight. Standard loudspeakers transmit sound which necessarily spreads very quickly, and control of sound projection and position is only about as flexible as where you can hang a loudspeaker. The Audio Spotlight is a device that will project sound much like a spotlight projects light; shining it at a listener allows only them to hear it, while shining it at a surface causes the sound to appear to originate from there, creating something of a 'virtual loudspeaker'. Beamsteering by phased arrays allows the sound to move, enabling the user to dynamically place sound exactly, and only, where it is desired.

http://sound.media.mit.edu/projects.html

However, to reduce outside noises such as jet traffic or noisy street noises ANC technology hasn't been developed nearly enough. The H algorithm using MIMO holds the most potential. Currently, there are no commercial/private applications of 3-D ANC scenarios in houses/offices outside of HVAC systems. ANC works best on periodic noises (i.e. engines). However, when dealing with a permanently standing structure, such as a house, there is the Doppler Effect. Since the sound source is translating the sound signal translates with the source as it is broadcasted. This causes for a band of frequencies, and thus the noise's pitch changes relative to a stationary point. This is what happens if you stand by the side of the tracks and a train approaches; it approaches as a dull rumble growing louder and louder until it screeches (higher frequency) by you and then vanishes back into a dull rumble. The Doppler Effect complicates the sound signal's geometry and the complexity of the system. So as of now, this currently remains an academic research project.

SOURCES:

http://www.val.me.vt.edu/
http://www.isvr.soton.ac.uk/active/
http://sound.media.mit.edu/
http://www.ibp.fhg.de/rata/aktiv1-e.htm
http://www.adit.fr/Produits/TF/Anciens/TF25a.html http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/u11l1c.html http://www.sdgedivest.com/chapters/04-10nse.htm
http://www.ibp.fhg.de/rata/aktiv1-e.htm

Bai, M.R. and Lin, Z. Active Noise Cancellation for a Three Dimensional Enclosure by Using Multiple Channel Adaptive Control and H Control. Journal of Vibration and Acoustics: Vol 120, October 1998, p958-64.

Headset Suppliers:

LightSPEED Technologies, Inc.
15812 SW Upper Boones Ferry Road Lake Oswego, Oregon 97035, USA Tollfree (US): 800.332.2421 International: 503.684.5538 FAX: 503.684.3197
http://www.lightspeed-tek.com/ls-contact.html

Bose:

http://www.bose.com/products/aviation/
There are links off the web page to contact them

HVAC Suppliers:
Noise Control Technology
Connecticut NCT Group, Inc.
One Dock Street, Suite 300
Stamford, CT 06902
PH: 203-961-0500 FX: 203-348-4106
http://www.nct-active.com/contact.htm

Digital Signal Processors-DSPs:
Texas Instrument
http://dungeon.ti.com/sc/docs/psheets/abstract/apps/spra042.htm

Foam:
Virginia Tech
C.R. Fuller-Active/Passive Foam
c.r.fuller@larc.nasa.gov