The FAQ you are now reading discusses active noise control, a novel way of using basic physics to control noise and/or vibration. What is an FAQ, you say? Well, the Internet supports thousands of “newsgroups” — discussion forums covering every imaginable topic. An FAQ (Frequently Asked Questions list) is a summary written to answer specific questions that arise repeatedly in the newsgroups. This particular FAQ was written for the newsgroups news:alt.sci.physics.acoustics and news:comp.dsp, which focus on acoustics and digital signal processing, respectively. This FAQ has four purposes:
The Acoustical Society of America awarded its 1994 Science Writing Award for this FAQ. The Science Writing Award is intended to “recognize and stimulate distinguished writing (or producing) that improves public understanding and appreciation of acoustics.” The award, one of two given each year, has never before been given for a
work published only on the Internet.
An article based on this FAQ appeared in the most recent issue of “Echoes”, the quarterly newsletter of the Acoustical Society of America (Spring 1996). This FAQ was last updated on February 22, 1996.
The Active Noise Control FAQ is updated monthly; see the version date cited above. You have several options to obtain the latest version:
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To cite this FAQ as a reference, I suggest a citation like this:
Ruckman, C.E. (1995) “Frequently Asked Questions: Active Noise Control,” Internet FAQ document. Available via anonymous ftp from ftp://rtfm.mit.edu/pub/usenet/news.answers/active-noise-control-faq, or via Usenet in news:news.answers.
The following people contributed to the discussions upon which this
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Copyright (c) 1994,1995,1996 by Christopher E. Ruckman
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If you are not familiar with how sound works, the following brief refresher course may help. Don’t be put off by occasional technical jargon; most of the FAQ includes analogies and examples to illustrate ideas in plain language. (The author apologizes to acousticians everywhere for presuming to summarize their craft in a mere three paragraphs!)
Sound is a pressure wave traveling in air or water. A sound wave resembles the surface wave made when you throw a stone into a calm pool of water, except that
the sound wave consists of tiny fluctuations in the air pressure rather than fluctuations in water height,
Sound is usually generated by vibration of an object or surface such as a speaker cone, a violin body, or human vocal cords. The vibrating surface “radiates” pressure waves into the adjoining air or water as sound. (Sound can also be generated by turbulent airflow, by bubbles collapsing, or by many other phenomena.)
The frequency (number of wave crests per unit time that pass a fixed location) measures the tone or pitch of a sound. For example, a bass guitar plays lower frequencies than a violin. The wavelength, or distance between wave crests, is related to frequency: lower frequencies have longer wavelengths. In air, all frequencies of sound travel at the same speed. When bending waves travel through a flexible structure, however, low frequencies travel faster than high frequencies.
In this context, noise is simply *unwanted* sound. There is an old trick question: “If a tree falls in the forest and nobody is there to hear it, does it make any noise?” The answer is “no” because sound cannot be *noise* unless somebody hears it and finds it offensive. However, if the question is phrased “Does it make any *sound*,” then you have a deep philosophical question to ponder!
2.1 What is active control of noise/vibration?
The question is usually posed like this: “I heard about a new noise control technology called Active Something-Or-Other … can I use it to make my house quiet when the kid next-door plays ‘Black Sabbath’ on his electric guitar?”
The technology in question is “active noise control,” a.k.a. “active noise cancellation,” a.k.a. “anti-noise,” and it is one of the hot research topics in acoustics these days. Here is the bottom line: yes, active noise control works in the proper circumstances, but no, you cannot use it to soundproof an entire house.
Active control is sound field modification, particularly sound field cancellation, by electro-acoustical means.
In its simplest form, a control system drives a speaker to produce a sound field that is an exact mirror image the offending sound (the “disturbance”). The speaker thus “cancels” the disturbance, and the net result is no sound at all. In practice, of course, active control is somewhat more complicated; see below.
The name differentiates “active control” from traditional “passive” methods for controlling unwanted sound and vibration. Passive noise control treatments include “insulation”, silencers, vibration mounts, damping treatments, absorptive treatments such as ceiling tiles, and conventional mufflers like the ones used on today’s automobiles. Passive techniques work best at middle and high frequencies, and are important to nearly all products in today’s increasingly noise-sensitive world. But passive treatments can be bulky and heavy when used for low frequencies. The size and mass of passive treatment usually depend on the acoustic wavelength, making them thicker and more massive for lower frequencies. The lightweight and small size of active systems can be a critically important benefit; see later sections for other benefits.
In control systems parlance, the four major parts of an active control system are:
- The plant is the physical system to be controlled; typical examples are a headphone and the air inside it, or air traveling through an air-conditioning duct.
- Sensors are the microphones, accelerometers, or other devices that sense the disturbance and monitor how well the control system is performing.
- Actuators are the devices that physically do the work of altering the plant response; usually they are electromechanical devices such as speakers or vibration generators.
- The controller is a signal processor (usually digital) that tells the actuators what to do; the controller bases its commands on sensor signals and, usually, on some knowledge of how the plant responds to the actuators.
Analog controllers may also be used, although they are somewhat less flexible and thus more difficult to use.
The idea of active noise control was actually conceived in the 1930’s (see the Lueg patent mentioned below), and more development was done in the 1950’s. However, it was not until the advent of modern digital computers that active control became truly practical. Active control became a “mainstream” research topic in the 1970’s and 1980’s, and in recent years research papers have been published at the rate of several hundred per year. There are now several rather large companies that specialize in active control products, and the topic is widely studied in universities and government research laboratories.
There are two basic approaches for active noise control: active noise cancellation (ANC) and active structural-acoustic control (ASAC). In ANC, the actuators are acoustic sources (speakers) which produce an out-of-phase signal to “cancel” the disturbance. Most people think of ANC when they think of active noise control; some examples are mentioned below. On the other hand, if the noise is caused by the vibration of a flexible structure, then ASAC may be more appropriate than ANC. In ASAC, the actuators are vibration sources (shakers, piezoceramic patches, etc.) which can modify how the structure vibrates, thereby altering the way it radiates noise. (The distinction between ANC and ASAC is somewhat arbitrary, since both cases correspond to a controller using actuators to reduce the plant response.)
Active vibration control is a related technique that resembles active noise control. In either case, electromechanical actuators control the response of an elastic medium. In active noise control, the elastic medium is air or water through which sound waves are traveling. In active vibration control, the elastic medium is a flexible structure such a satellite truss or a piece of vibrating machinery. The critical difference, however, is that active vibration control seeks to reduce vibration *without* regard to acoustics. Although vibration and noise are closely related, reducing vibration does not necessarily reduce noise.
Actually, you can generate your own catchy phrases with the following handy buzzword generator. Simply choose one word from each column, string them all together without commas, and paste the result or its acronym into your document or conversation!
Active noise control is quite different from noise masking. Acoustic masking is the practice of intentionally adding low-level background sounds to either a) make noise less distracting, or b) reduce the chance of overhearing conversations in adjoining rooms. In active noise control, the system seeks not to mask offending sound, but to eliminate it. Masking increases the overall noise level; active control decreases it, in some locations if not all
It may seem counter-intuitive to say that adding more sound to a system can reduce noise levels, but the method can and does work. Active noise control occurs by one, or sometimes both, of two physical mechanisms: “destructive interference” and “impedance coupling”. Here is how they work:
On one hand, you can say that the control system creates an inverse or “anti-noise” field that “cancels” the disturbance sound field. This works by the principle of destructive interference. A sound wave is a moving series of compressions (high pressure) and rarefactions (low pressure). If the high-pressure part of one wave lines up with the low-pressure of another wave, the two waves interfere destructively and there is no more pressure fluctuation (no more sound). Note that the matching must occur in both space *and* time — a tricky problem indeed.
On the other hand, you can say that the control system changes the way the system “looks” to the disturbance, i.e., changes its input impedance. Consider the following analogy:
Picture a spring-loaded door, one that opens a few centimeters when you push on it but swings shut when you stop pushing. A person on the other side is repeatedly pushing on the door so that it repeatedly opens and closes at a low frequency, say, twice per second. Now suppose that whenever the other person pushes on the door, you push back just as hard. Your muscles are heating up from the exertion of pushing on the door, but end result is that the door moves less. You *could* say that the door opens and that you “anti-open” it to “cancel” the opening. But that wouldn’t be very realistic; at least, you would not actually see the door opening and anti-opening. You would be more accurate to say that you change the “input impedance” seen on the other side of the door: when the other person pushes, the door just doesn’t open.
(The spring-loaded door is supposed to represent the spring effect of compressing air in a sound wave. This is not a perfect analogy, but it helps illustrate impedance coupling.)
In some cases, destructive interference and impedance coupling can be two sides of the same coin; in other cases destructive interference occurs without impedance coupling. The difference is related to whether the acoustic waves decay with distance traveled:
Sound from a speaker hanging in the middle of a stadium decays (is less loud) at a distance because of “spherical spreading.” The sound energy is spread out over an increasingly large area as you get farther away. Go far enough away and, for all intents and purposes, the sound decays completely down to nothing. On the other hand, sound in a “waveguide” such as a duct can travel long distances without significant decay.
If a control system actuator is close to the disturbance source, destructive interference and impedance coupling can both occur. But what about when the actuator is far away from the disturbance, so far away that any wave it creates decays completely down to nothing before reaching the disturbance? There can still be destructive interference near the actuator, even though the actuator cannot possibly affect the impedance seen by the disturbance. Example: the tiny speaker in an active control headphone will not affect the impedance seen by a cannon firing a mile away, but it can create destructive interference within the headphone.
Active noise control works best for sound fields that are spatially simple. The classic example is low-frequency sound waves traveling through a duct, an essentially one-dimensional problem. The spatial character of a sound field depends on wavelength, and therefore on frequency. Active control works best when the wavelength is long compared to the dimensions of its surroundings, i.e., low frequencies. Fortunately, as mentioned above, passive methods tend to work best at high frequencies. Most active noise control systems combine passive and active techniques to cover a range of frequencies. For example, many active mufflers include a low-back-pressure “glass-pack” muffler for mid and high frequencies, with active control used only for the lowest frequencies.
Controlling a spatially complicated sound field is beyond today’s technology. The sound field surrounding your house when the neighbor’s kid plays his electric guitar is hopelessly complex because of the high frequencies involved and the complicated geometry of the house and its surroundings. On the other hand, it is somewhat easier to control noise in an enclosed space such as a vehicle cabin at low frequencies where the wavelength is similar to (or longer than) one or more of the cabin dimensions. Easier still is controlling low-frequency noise in a duct, where *two* dimensions of the enclosed space are small with respect to wavelength. The extreme case would be low-frequency noise in a small box, where the enclosed space appears small in all directions compared to the acoustic wavelength.
Often, reducing noise in specific localized regions has the unwanted side effect of amplifying noise elsewhere. The system reduces noise locally rather than globally. Generally, one obtains global reductions only for simple sound fields where the primary mechanism is impedance coupling. As the sound field becomes more complicated, more actuators are needed to obtain global reductions. As frequency increases, sound fields quickly become so complicated that tens or hundreds of actuators would be required for global control. Directional cancellation, however, is possible even at fairly high frequencies if the actuators and control system can accurately match the phase of the disturbance.
Aside from the spatial complexity of the disturbance field, the most important factor is whether or not the disturbance can be measured *before* it reaches the area where you want to reduce noise. If you can measure the disturbance early enough, for example with an “upstream” detection sensor in a duct, you can use the measurement to compute the actuator signal (feedforward control). If there is no way to measure an upstream disturbance signal, the actuator signal must be computed solely from error sensor measurements (feedback control). Under many circumstances feedback control is inherently less stable than feedforward control, and tends to be less effective at high frequencies. For a concise comparison of feedforward vs. feedback control, see Hansen, IS&VD 1(3).
Bandwidth is also important. Broadband noise, that is, noise that contains a wide range of frequencies, is significantly harder to control than narrowband (tonal or periodic) noise or a tone plus harmonics (integer multiples of the original frequency). For example, the broadband noise of wind flowing over an aircraft fuselage is much more difficult to control than the tonal noise caused by the propellers moving past the fuselage at constant rotational speed.
Finally, lightly damped systems are easier to control than heavily damped ones. (Damping refers to how quickly the sound or vibration dies out; it should not be confused with “dampening”, which describes whether the system is wet!)
Adaptive control is a special type of active control. Usually the controller employs some sort of mathematical model of the plant dynamics, and possibly of the actuators and sensors. Unfortunately, the plant can change over time because of changes in temperature or other operating conditions. If the plant changes too much, controller performance suffers because the plant behaves differently from what the controller expects. An adaptive controller is one that monitors the plant and continually or periodically updates its internal model of the plant dynamics.
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 most demonstrations have not yet made the transition into successful commercial products.
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, and now start around US$650 for active pilots headsets. (See Section 2.11 for information about an active control conversion kit available for US$100.)
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.
Quote from C. Hansen, IS&VD:
1. Control of aircraft interior noise by use of lightweight vibration sources on the fuselage and acoustic sources inside the fuselage.
2. Reduction of helicopter cabin noise by active vibration isolation of the rotor and gearbox from the cabin.
3. Reduction of noise radiated by ships and submarines by active vibration isolation of interior mounted machinery (using active elements in parallel with passive elements) and active reduction of vibratory power transmission along the hull, using vibration actuators on the hull.
4. Reduction of internal combustion engine exhaust noise by use of acoustic control sources at the exhaust outlet or by use of high intensity acoustic sources mounted on the exhaust pipe and radiating into the pipe at some distance from the exhaust outlet.
5. Reduction of low frequency noise radiated by industrial noise sources such as vacuum pumps, forced air blowers, cooling towers and gas turbine exhausts, by use of acoustic control sources.
6. Lightweight machinery enclosures with active control for low frequency noise reduction.
7. Control of tonal noise radiated by turbo-machinery (including aircraft engines).
8. Reduction of low frequency noise propagating in air conditioning systems by use of acoustic sources radiating into the duct airway.
9. Reduction of electrical transformer noise either by using a secondary, perforated lightweight skin surrounding the transformer and driven by vibration sources or by attaching vibration sources directly to the transformer tank. Use of acoustic control sources for this purpose is also being investigated, but a large number of sources are required to obtain global control.
10. Reduction of noise inside automobiles using acoustic sources inside the cabin and lightweight vibration actuators on the body panels.
11. Active headsets and earmuffs