More Basics: What is sound? Frequency? Wavelength?

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, 

  a sound wave can travel in three dimensions rather than two, and 

  the wave speed is much faster (340 meters per second in air). 

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 some respects, sound and vibration are quite similar; it may be useful to think of sound as a 

vibration traveling through air. Many of the same concepts apply for both sound and vibration, 

but there are certain significant differences. For example, when sound travels through air, all 

frequencies of sound travel at the same speed (340 meters per second). By contrast, for some 

types of vibration traveling through a structure such as a wall or floor, low frequencies travel 

faster than high frequencies. 

In this context, noise is simply unwanted sound. Philosophers wonder: "If a tree falls in the forest 

and nobody is there to hear it, does it make any noise?" When they phrase the question in 

precisely that way, the answer is NO for this reason: "sound" is not really "noise" unless 

someone hears it AND finds it offensive. 

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. Let’s jump directly to 

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. 

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 treatments usually depend on the acoustic wavelength, 

making them thicker and more massive for lower frequencies. The light weight 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 more difficult 

to use. 

Is active control new?

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. In recent years, researchers have 

published technical articles at the rate of several hundred per year. There are now dozens of 

companies that specialize in active control products, and the topic is widely studied in 

universities and government research laboratories. 

Are there different kinds of active control?

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. (ASAC is distinguished from ANC only in how it is applied, since in either case you have 

a controller using actuators to control the response of a plant.) 

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! 

 / Column A    \    / Column B (optional) \    / Column C     \

 | ----------- |    | ------------------- |    | ------------ |

 | active      |    | vibration           |    | cancellation |

<  adaptive     >  <  noise                >  <  control       >

 | semi-active |    | sound               |    | damping      |

 | electronic  |    | structural-acoustic |    | suppression  |

 \             /    \ vibro-acoustic      /    \ isolation    /

Is active noise control like noise masking?

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 noises 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 -- at least, in some locations if not all. 

How can adding sound make a system quieter?

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 usually 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. The principle is called "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. 

Now, 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." As you get farther away, the sound energy is spread out over an 

increasingly large area. 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. There are many situations in which walls, ducts, 

buildings, roadways, or other surfaces can act as waveguides for sound. 

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. 

In some cases, an active control system can actually absorb acoustic energy from a system. Of 

course, the amount of energy absorbed by the system is usually tiny compared to mechanical 

losses or other losses in the system, but absorption is one possible mechanism for active systems. 

When does active control work best?

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!) 

What is adaptive active control?

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. 

Applications of active noise control

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 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 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. 

---------- begin quote from C. Hansen---------- 

  Control of aircraft interior noise by use of lightweight vibration sources on the fuselage 

and acoustic sources inside the fuselage. 

  Reduction of helicopter cabin noise by active vibration isolation of the rotor and gearbox 

from the cabin. 

  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. 

  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. 

  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. 

  Lightweight machinery enclosures with active control for low frequency noise reduction. 

  Control of tonal noise radiated by turbo-machinery (including aircraft engines). 

  Reduction of low frequency noise propagating in air conditioning systems by use of 

acoustic sources radiating into the duct airway. 

  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. 

  Reduction of noise inside automobiles using acoustic sources inside the cabin and 

lightweight vibration actuators on the body panels. 

  Active headsets and earmuffs. 

---------- end quote from C. Hansen, IS&VD 1(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 (http://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 popular Peltor Tactical 7-S retails for around 

US$130. Peltor Inc., 41 Commercial Way, E. Providence, RI 02914, phone 401.438.4800, fax 

401.434.1708) 

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. 

A new product has recently come to market: the Andrea Anti-Noise(R) PC Headsets/Handsets 

with Active Noise Cancellation Microphone Technology. This product includes an earpiece with 

a boom-mounted microphone, and active noise control is used to filter out background noise 

from voice signals recorded by the microphone. The manufacturer claims the product can 

"substantially increase the speed and accuracy of voice-computing applications by electronically 

canceling background noise and echo speaker feedback over traditional microphones." Iinterested 

readers should contact Andrea directly and mention this FAQ. (Andrea Electronics Corporation, 

11-40 45th Road, Long Island City, NY 11101, USA, phone 1.800.442.7787). 

Additional information about active (and passive) headphones may be found in the rec.aviation 

FAQ (news:rec.aviation.answers). 

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: 

  increased material durability and fatigue life 

  lower operating costs due to reduced facility down-time for installation and maintenance 

  reduced operator fatigue and improved ergonomics 

Of these, the potential for reduced maintenance and increased material fatigue life have received 

new emphasis in the last few 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. 

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 forty years' hindsight! 

CR] 

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: 

Option 1: Noise cancellation demo

The easiest way to demonstrate sound cancellation is to visit the following web site, maintained 

by the Vibration and Acoustics Laboratory at Virginia Tech in Blacksburg, Virginia: 

http://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. 

Option 2: Build an analog feedback controller

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: 

http://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. 

Option 3: Build Ostergaard’s feedback vibration controller

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. 

Option 4: Buy an off-the-shelf active control module