Active Noise Control

Active Noise Control quiets offensive noise by using cancellation techniques, or anti-sound. Sound is a pressure wave in air that travels from the source to a listener. If you introduce a second sound wave that is equal and opposite to the original sound, the two waves combine (or cancel) to create no pressure, or silence. The second sound wave is often introduced either near the offending noise source, or near a listener's ears. Applying a cancelling sound near the offending source results in sound attenuation in all regions away from the sound source. Applying a cancelling sound near a listner creates a local zone of silence.

To keep the secondary sound an accurate inverted replica of the offending sound, microphones and computers are used to constantly sense the offending source, and calculate the anti-sound command to the secondary speaker. Microphones are also used to monitor the success of the cancellation and provide feedback to the computer on how to change the anti-sound command.

Active Noise Control uses principles of acoustics, signal processing, audio system design and psychoacoustics. The closeness of the secondary source to an offending source or listener's ear is measured in acoustic wavelengths. Wavelength gets smaller in length as the frequency (or pitch) of the sound goes higher. At middle C (261 Hz) the single secondary source needs to be approximately one-half meter way from the source or ear to provide good cancellation. More complex arrangements of speakers and microphones can be used to create a "wall of sound" that can produce zones of silence without being close to the source or listener.

In applications such as mufflers, HVAC and cooling towers, the offending sound travels in a tube and radiates out the air exits. Because the sound is travelling in the tube before it radiates, the secondary source location becomes more flexible than in other applications. This allows active noise control speakers to placed along the tube instead of at the offending source or near the air exhaust points.


Active noise control works - but where is it going?

Larry Riddle

Many people have heard of active noise control, but with the exception of headphones and some automobiles, few people have experienced its benefits. Why has this happened?

The short answer is cost. Unlike over-hyped technologies, active noise control works. The obstacle has been to make active noise control a cost-effective solution to noise problems. This obstacle has been far more difficult than many people expected. When a technology is more expensive than its benefits are generally worth, only a small market exists for the technology.

Now, new technologies are usually expensive to start out. Early users of a technology pay a higher price for being first, with a competitive advantage as the benefit. Technology developers gain valuable experience working with early customers. This experience and know-how yields product improvements that provide more economical solutions for future customers. In this way, the technology spirals into wider and wider use, as the cost for implementation falls.

So, what happened to active noise control? Why don't we see active noise control technology in our daily lives? The answer lies in the fact that most consumers are reluctant to pay more than 100 to 2000 dollars for noise reduction solutions. This cost range has been difficult to achieve with effective active solutions. The notable exceptions are noise canceling headphones, automobiles and some dishwashers.

The result is that other applications of active noise control have been in industrial situations, where the benefit for active noise control is driven by hearing safety concerns. In some cases, improving speech communication has been important enough to use active noise control. Even in industrial markets, active noise control technology spirals have been rare. The result is that the pervasive use of active noise control still waits for the "killer-app" that can propel it out of the one-of-a-kind, custom project cycle that best describes how much of active noise control technology is applied today.

So, where has active noise control been successfully applied? A partial list:

These products are successful because they have taken active noise control technology and reduced it to practice. Another common element is that with the exception of cabin noise reduction, the solution is 'simple'. By simple, I mean that the solution does not require an extensive number of microphones, sensors, cables, etc. In many cases the controller that produces anti-sound commands can be built with analog circuits, leading to lower costs and portability.

The exception to this rule is aircraft cabin noise reduction. In this case, numerous speakers (or headsets) and microphones are used. In the noise reduction system built into new Boeing aircraft, it appears that the noise reducing apparatus has been integrated into the entertainment system. This has the benefit of reducing the cost of the noise reducing system, since it is being added to an existing acoustic system. The existing entertainment system uses headphones, signal processors, and cabling that can be leveraged by the active noise control (ANC) design.

So where should we go in ANC technology development? I think that continued development of simple solutions is best done through education. Present commercial efforts are mired in patent issues and return on investment questions. Through education, engineers can be trained in the use of active noise control. When these engineers are confronted with design problems, they will be equipped to find niche solutions using active noise control.

Researchers in this field should focus on two approaches. The first is the development of integrated active control solutions for complex noise problems. Automobile audio systems, with integrated personal communications systems and active noise control technology could improve driving enjoyment and safety. A second approach is the search for the 'killer application'. As an example, are there new ways of communicating that rely in a fundamental way on ANC technology? We will have to wait and see.

SSC ANC Applications

Personal Audio Zones

Signal Systems Corporation was the first company to develop personal audio zones (PAZ) for automobile cabins that offered good listening fidelity for music. These personal audio zones utilized noise cancellation technology to deliver different audio programs to different passengers and the driver. We demonstrated the ability to have four passengers listen to their desired audio entertainment without use of headphones at the Consumer Electronics Show in 2006 and 2007. The system was particulary effective because with automobile audio, impercetable delays could be inserted in the speaker outputs, which allows the cancellation system to better anticipate the undesired audio, and hence achieve more cancellation.

A new and fortuitous pscho-acoustic effect was also discovered in this project. The region of silence perceived by the listener was larger than the zone of cancellation for the higher frequency content of the undesired audio being cancelled. Listeners apparently mentally suppressed undesired audio streams beyond the physical cancellation effect, when presented with a un-cancelled desired audio program at the same time.

Active Noise Reduction Technology

Noise levels in crew compartments below the flight decks on Navy ships during flight operations have traditionally been extremely noisy due to jet engine noise as well as launch machinery transients. Levels exceed Navy desired levels by over 10 dB. Current state of the art materials do not work well at low frequencies. SSC proposes to develop an active noise solution that consists of a modular, fully integrated smart material that can be applied to the interior compartments of ships. The smart acoustic panel will reduce radiating noise by employing advanced smart materials such as polyvinylidene fluoride (PVDF) and lead zirconate titanate (PZT) actuation materials, embedded MEMs based acoustic and vibration sensors, and polyurethane foam for passive vibration absorption. A method to use smart acoustic materials to provide local noise control of manned spaces will also be investigated if global control proves to be unfeasible.

SSC developed a MEDEVAC Active Noise Cancellation Acoustic Pillow (MANCAP) featuring active noise cancellation (ANC) and passive noise reduction measures to create a quiet zone around injured personnel's ears during evacuations in and around noisy military vehicles. The MANCAP concept allows access to the patient's face for respirators and medical treatment while installed on any standard NATO litter. The MANCAP will reduce the local noise level of the patient to less than 80 dBA in a military helicopter by leveraging our ANC algorithms, real time software, hardware and headrest technology previously developed under commercial and DoD sponsorship, including Army, Navy and Special Forces efforts.

Battlefield Acoustics

We have applied our ANC technology to robotic vehicles. This software eliminates unwanted signals from microphones mounted on an unmanned vehicle. We have tested our system on numerous systems including the Demo III Experimental Unmanned Vehicle (XUV) built by General Dynamics Robotic Systems Technology. Our noise cancellation software achieved over 40 dB narrowband cancellation while eliminating over 30 tones from the robots' background noise level.

Some of these results are presented in the Air Coupled Sensor Workshop Presentation Slides (Adobe Acrobat PDF Format 437 KB)

Active Noise Control for Smart Materials

We have been developing active noise cancellation systems for acoustic composite tiles for several years. We have developed a real-time 48 channel controller using programmable DSPs. This system was integrated into a smart skin that was demonstrated underwater. Our control system designs are based on hierarchical approaches.

We used on-line evolutionary techniques for fully adaptive control systems to change sensor and actuator configurations as well as filter coefficients. These systems define the state of the art in affordable and effective controllers for intelligent materials.

A related publication is: L. Riddle and J. Murray, 'Smart Structure Active Sonar Echo Cancellation Using Frequency Scheduled Control,presented at the 1998 SPIE Conference on Smart Materials

A full length paper describing the trade-offs involved in making a cost effective ANC system for complex problems is described in:

L.Riddle, J. Murray and S. Lease, 'Active Noise Control Architecture for the Smart Skin Demonstration,' ACTIVE-99, International Symposium on Active Noise Control, Dec 2-4, 1999.

ANC FAQ

Christopher E. Ruckman, wrote a useful ANC FAQ in 1994-1999. It provides a solid introduction to the technology.

An edited list of active noise control web sites is maintained at the Open Directory Project.

A technical overview of active noise control headphones can be found at ANC - Trends - Concepts and Technical Challenges

Companies

Universities