02/04/07

3.1 What are some typical applications for active noise control?

The most successful demonstrations of active control have been for controlling noise in enclosed spaces such as ducts, vehicle cabins, exhaust pipes, and headphones. Note, however, that successful demonstrations are many, but successful commercial products are few.

One exception, active noise control headphones, has achieved widespread commercial success. Active headphones use destructive interference to cancel low-frequency noise while still allowing the wearer to hear mid- and high-frequency sounds such as conversation and warning sirens. The system comprises a pair of earmuffs containing speakers and one or more small circuit boards. Some include a built-in battery pack, and many allow exterior signal inputs such as music or voice communications. Used extensively by pilots, active headphones are considered indispensable in helicopters and noisy propeller-driven aircraft. Prices have dropped in recent years. (See Section 3.2 for information about an active control conversion kit available for US$100.) Passenger headsets, which lack the microphone boom found on pilots headsets, are even cheaper. Some sell for less than US$100, and are readily found in catalogs and specialty gift shops such as "Brookstone".

Another application that has seen some commercial success is active mufflers for industrial engine exhaust stacks. Active control mufflers have seen years of service on commercial compressors, generators, and so forth. As unit prices for active automobile mufflers have fallen in recent years, several automobile manufacturers are now considering active mufflers for future production cars. However, if you ask your local new car dealer about the active muffler option on their latest model, you will likely receive a blank stare: no production automobiles feature active mufflers as of this writing.

Large industrial fans have also benefited from active control. Speakers placed around the fan intake or outlet not only reduce low-frequency noise downstream and/or upstream, but they also improve efficiency to such an extent that they pay for themselves within a year or two.

The idea of canceling low-frequency noise inside vehicle cabins has received much attention. Most major aircraft manufacturers are developing such systems, especially for noisy propeller-driven aircraft. Speakers in the wall panels can reduce noise generated as the propeller tips pass by the aircraft fuselage. For instance, a system by Noise Cancellation Technologies (NCT) now comes as standard equipment on the new Saab 2000 and 340B+ aircraft. The key advantage is a dramatic weight savings compared to passive treatments alone.

Automobile manufacturers are considering active control for reducing low-frequency noise inside car interiors. The car stereo speakers superpose cancellation signals over the normal music signal to cancel muffler noise and other sounds. For example, Lotus produces such a system for sale to other automobile manufacturers. Unit cost is a major consideration for automobile use. While such systems are not at all common, at least one vehicle (currently offered only in Japan) includes such a system as a factory option.

The following list of applications for active control of noise and vibration was compiled by Colin Hansen and is used by permission; see IS&VD 1(2). The list includes topics which are currently being investigated by research groups throughout the world.

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3.2 Are all 'active headphones' the same?

No. Two types are often called "active," but only one actually uses noise cancellation. For the sake of discussion, let's call the two types "active headphones" and "amplified earmuffs".

Active headphones rely primarily on noise cancellation for low-frequency quieting. In some, the earmuffs themselves provide relatively little passive noise reduction. In others, the earmuffs provide as much passive reduction as possible, using noise cancellation to get even better performance at low frequencies. In any case, the unit includes a microphone inside each earcup to monitor the "error"-the part of the signal that has not been cancelled by the speakers. A pilot's headset also includes a microphone boom to transmit the pilots voice, and an input jack to transmit communication signals into the earcups. The noise cancellation works best on tones or periodic noise like that from an aircraft propeller. Some models, such as the NoiseBuster Extreme! from Noise Cancellation Technologies (www.nct-active.com), retail for less than US$100.

Amplified earmuffs are quite different, as they do not use noise cancellation at all. A heavy passive earmuff attenuates as much noise as possible. Microphones on the outside of the unit pick up sounds that would ordinarily be heard by the ears. These microphone signals are then filtered before being played by speakers inside the earcups. The most common filtering is to mute loud, impulsive sounds such as gunshots; amplified earmuffs are therefore becoming quite popular at weapons firing ranges. (Example: the Peltor Tactical 7-S)

Amplified earmuffs have also been suggested for use by sufferers of tinnitus ("ringing of the ears"), a condition that can be aggravated by loud noises. But amplified earmuffs should not be confused with true active noise control headphones.

Numerous microphone-based products, such as cell phones and computer microphones, use electronic cancellation methods to reduce background noise.

3.3 What are the benefits of active control?

The many practical benefits of active control technology are not all obvious at first glance. The main payoff, of course, is low-frequency quieting that would be too expensive, inconvenient, impractical, or heavy by passive methods alone. For example, the lead-impregnated sheets used to reduce aircraft cabin propeller noise impose a severe weight penalty, but active control might perform as well with a much smaller weight penalty.

Other possible benefits reflect the wide range of problems on which active control can be applied. For instance, with conventional car mufflers the engine spends extra energy to push exhaust gasses through the restrictive muffler passages. On the other hand, an active control muffler can perform as well with less severe flow restrictions, thus improving performance and/or efficiency. Additional benefits include:

Of these, the potential for reduced maintenance and increased material fatigue life have received new emphasis in recent years. In the long-term, however, benefits may extend far beyond those mentioned above. The compact size and modularity of active systems can provide additional flexibility in product design, even to the point of a complete product redesign.

3.4 What was that short story by Arthur C. Clarke?

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.

In the tradition of Clarke's other works, the story itself is entertaining and well-told. Strictly speaking, however, the basic premise requires some poetic license regarding the physics of sound cancellation. Well-informed readers must rely on their "willing suspension of disbelief" to overlook the inconsistencies. [Of course, I say that with the benefit of over fifty years' hindsight. CR]

3.5 How can I do a simple, inexpensive active control demo?

Because active control employs some interesting physics, readers often ask how to construct a simple, low-cost demonstration as a student project or for instructional purposes. Here are five possibilities:

The easiest way to do a limited demonstration of sound cancellation is to visit the following web site, maintained by the Vibration and Acoustics Laboratory at Virginia Tech in Blacksburg, Virginia: www.val.me.vt.edu

From this site you can download a simple Windows-compatible program that conducts a demonstration of sound cancellation (which, in a narrow sense, is a form of active noise control.) All you need is a PC, a sound card, and two speakers. The program plays a "disturbance" sound from one speaker and a "control" sound from the other, and demonstrate that one speaker can cancel sound from the other. No fuss, no mess.

Of course, you can demonstrate cancellation without the software if you have a stereo amplifier, two speakers, and a way to generate a send a pure-tone signal to the amplifier (such as a signal generator). First, play a pure tone through both speakers. Move the speakers close together and far apart; you'll notice no real change in the sound level. Then, cross-wire one of the speakers (i.e., swap the positive and negative wires). Move the speakers close together and you'll hear the sound level fall dramatically. Experiment with different frequencies to find what works best for your particular setup.

Again, these setups only demonstrate that one sound wave can cancel another, and some would argue that this is not truly active noise control.

The opposite end of the spectrum: It is possible to construct a simple analog feedback controller using op-amps, capacitors, speakers, and other parts available from any electronics supplier. While simple in concept, constructing such a demonstration requires a pretty solid foundation in acoustics, electronics, and control theory. A basic outline is given below, but the details are well beyond the scope of this FAQ. Readers interested in further discussion are encouraged to contact Dr. Dexter Smith (discoveryengineering@compuserve.com) or visit the following web site: ourworld.compuserve.com/homepages/discoveryengineering/fanc1.htm

A simple analog system for feedback active control consists of a microphone sensor, a loudspeaker actuator, and an equalizer to correct for the delay from the speaker to the microphone and for the transfer function of the speaker itself. The microphone is usually placed close to the speaker, since the system transfer function (from power amplifier to output of mic preamp) is increasingly difficult to equalize as the mic moves away from the speaker. (The phase change goes from gradual to rapid as frequency increases). A disturbance input at the sensor (low frequency acoustic noise) can be attenuated by the proper choice of equalization. The zone of silence around the sensor is approximately 1/10th of the wavelength of the noise to be attenuated. The system can be equalized by taking data into a sound card on a PC, determining the transfer function, and equalizing it with a biquad op-amp circuit using, for example, 4 op-amps.

A technical brief published recently in the Journal of the Acoustical Society of America describes how to make a simple active control experiment using a tuning fork, a function generator, and some simple, inexpensive electronics components. The reference is:

• Ostergaard, P.B., "A simple harmonic oscillator teaching apparatus with active velocity feedback," Journal of the Acoustical Society of America, Vol. 99, No. 2, February 1996.

This approach is much more powerful and flexible than any of those mentioned above, but also much more expensive. Several are available; try looking here, for example: www.technofirst.com

This alternative is much less expensive, but not as flexible: the "ANR Adapter" from Headsets, Inc. The ANR Adapter is an add-on kit that transforms an ordinary passive pilot's headset into an active noise control headset. The kit costs only US$100; you supply the headset. The makers claim roughly 22 dB attenuation from 20 Hz to 700 Hz. If you simply want a demonstration in which you flip a power switch to hear active noise control at work, this kit may be for you. (See Section 4.2 for contact information. For a review of the product, see the following magazine article: Picou, Gary, "Low-Rent ANC," The Aviation Consumer, vol.25, No.7, MAY 01 1995, p.10-12.)

Copyright (c) 1994-2007 by Christopher E. Ruckman. All Rights Reserved.