NOISE REDUCTION

VIBRATION CONTROL

Includes Noise Reduction, Vibration Testing, Acoustics Measurement, Shock Isolation

This Page of FAQ was written to serve four purposes:

Provide concise, accurate answers to common questions about active noise control.

Dispel popular misconceptions about what active noise control can and cannot do.

Refer readers to web links, technical references, and other sources of information.

Stimulate public interest in acoustics.

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. 

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. 

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. 

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    /

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. 

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. 

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 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) ---------- 

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). 

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. 

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] 

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 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. 

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. 

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 only 

if you have a budget on the order of US$2000 or so: the EZ-ANC from Causal Systems. This 

comprehensive kit includes hardware, software, and a complete theoretical/user's manual. (See 

Section 4.2 for contact information, or check out their web page: 

http://www.io.org/~causal/cs/csdir01.htm). Other companies also offer controllers that you can 

purchase and apply to your own problems; see, for example, http://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 3.2 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.) 

Reducing noise and vibration in aircraft

Today’s aircraft industry is placing increasing emphasis on creating a comfortable environment for passengers and crew. There are many potential treatments for reducing noise and vibration in aircraft. One of the keys for effectively implementing these is to understand the benefits and limitations of each type of treatment.

Noise and vibration treatments can be separated into two categories, passive and active. Passive treatments include Rubber-To-Metal (RTM) mounts, Fluidlastic® mounts, Fluidlastic® Torque Restraint (FTR) mounting systems, Tuned Vibration Absorbers (TVA), and a range of cabin wall/interior treatments. Active treatments, which require controller electronics, consist of three main types:

• Active Isolation Control (AIC) systems introduce actuators into mounts to prevent vibration from being transmitted into the structure to reduce cabin noise and vibration (1). The AIC system commands actuators to minimize the vibration and noise signals from accelerometers or microphones.

• Active Noise Control (ANC) systems use loudspeakers and microphones in the cabin to reduce interior noise as measured by the microphones (2).

• Active Structural Control (ASC) systems encompass a wide range of solutions, including placing actuators near sources or on structures that radiate noise (3). Typically, an ASC system will use microphones as sensors, but accelerometers can also be used.

As will be discussed, the best comprehensive solution to aircraft interior noise problems will be “hybrid” in nature, optimally combining active and passive technology.

Active Noise Control (ANC) and Interior Treatments

ANC systems utilize loudspeakers inside the cabin to create a secondary noise field which cancels the primary field due to the engines or propellers (6). For an ANC system to create global reductions, one of two criteria must be met. Firstly, the acoustic response must be lightly damped and possess low modal density in the frequency range where the noise must be reduced. When this occurs, a few actuators can be used to reduce noise at all points throughout the cabin. Secondly, speakers can be placed within a ¼ wavelength of discrete sources. Unfortunately, neither of these criteria can normally be met in aircraft. Depending on the size of the cabin, the transition from sparse to dense modal response typically occurs at a frequency less than 50 Hz. Since most aircraft sources such as turbofan engines or propellers produce noise at frequencies above 50 Hz, global noise reduction will not be possible using the first criteria. Further, since the sources are distributed rather than discrete, the second criteria can rarely be used.

If global noise reduction cannot be achieved, then local control can be utilized. Local control involves creating zones of quiet around the error microphones. The size of the zone of quiet is related to frequency being controlled. In general, the radius of the sphere of quiet, will be roughly one-tenth the wavelength of the sound. At 200 Hz, the radius of the sphere of quiet is 6 inches (0.15 meters). It is possible to enlarge the zone of quiet by a number of techniques including using multiple microphones. However, if the frequency is 2,000 Hz, the sphere of quiet would be too small to be practical. While ANC has its limitations, it can be very effective for controlling low-frequency noise in turboprop aircraft. A production ANC system for the Beech King Air provides up to 12 dB spatially averaged reduction in the propeller-induced noise, producing dramatic subjective improvements in passenger and crew comfort (2).

In addition to ANC, a variety of passive interior treatments are used in today’s aircraft. Usually integrated into the cabin wall, these treatments include acoustic insulating materials behind the trim, acoustic absorption material on the inside of the trim, and constrained layer damping on the inside of the aircraft skin. Also, elastomeric mounts can be used to reduce noise transmitted through trim attachment points, luggage racks, and seating. Due to size and weight constraints, these passive treatments generally work well only at frequencies above 1000 Hz. In the case of propeller-induced noise, passive acoustic treatments (e.g., constrained layer damping and acoustic damping) provide virtually no noise attenuation.

Active Structural Control (ASC) and Passive Tuned Vibration Absorbers (TVA)

TVA (3) and ASC (7) systems control structural vibration and consequently noise in the cabin. Tuned vibration absorbers work by increasing the localized impedance of the structure. This is accomplished by using a tuned resonant mass and spring (elastomeric stiffness). While this resonant behavior is used to greatly reduce the weight of the TVA, it generally limits the performance of the TVA to a narrow range of engine speeds and necessitates a second set of TVAs if noise reduction is desired at other frequencies (such as the harmonics of the tone). Further, TVAs can be used to increase the overall damping in the structure, particularly when the structure is lightly damped. ASC systems represent a very wide range of solution possibilities, including placing actuators on the aircraft cabin wall or in dominant vibration transmission paths (3). The curve plotted in Figure 3 can also be used as a simple tool to predict the performance of TVAs and ASC when the acoustics are strongly coupled to structural vibration. In this case, r is the ratio of the structural impedance divided by the passive impedance of the TVA or actively created impedance of the ASC actuator. Note that as a rough rule of thumb, the impedance of the TVA or ASC system must be approximately that of the  impedance of the structure (r=1.0) to achieve a 6 dB reduction. Since the actuators in an ASC system have a force producing element, ASC can impart significantly more impedance over a wide frequency range with less weight relative to TVAs. In comparison to ANC, ASC and TVAs can be conveniently packaged behind the existing trim.

In cases where the acoustic response is strongly coupled to vibration, TVAs are being used in production aircraft to provide significant noise and vibration reduction (7). Unfortunately for some aircraft, the acoustic response may not always be strongly coupled to the structural vibration, limiting the benefit of TVAs and eliminating the usefulness of the curve in Figure 3 as a predictive tool. This situation occurs when the dominant interior acoustic modes are spatially different (at the fuselage walls) from the dominant structural modes (3). For example, consider a case where the disturbance frequency matches an acoustic resonance. The modes dominating the structural vibration can be differently shaped than those dominating the acoustics. However, it is the structural motion that spatially couples to the resonant acoustic mode that drives the noise, and thus it is this motion that must be controlled. When this is the case, simply controlling the vibration (as would be accomplished with TVAs) will not significantly reduce interior noise. An ASC system with microphones can overcome this limitation by adaptively reducing structural motion that radiates noise into the cabin. This will reduce cabin noise, but control spillover into non-radiating motion may increase vibration. Intelligent placement strategies and other schemes may limit this phenomena to provide reduced noise and vibration for the passengers and crew.

In addition to the behavior discussed above, the placement of actuators/TVAs with respect to the noise source is another factor that largely affects the performance of a TVA or ASC system. In situations where the source of cabin noise is well defined and localized, good global noise reductions are possible with few actuators or TVAs placed near the source (within ¼ wavelength) to “block” energy from propagating into the cabin. This strategy can be used instead of AIC on commercial jets. This results in global reductions in noise levels with a few actuators, even at high frequencies. When the ASC actuators or passive TVAs cannot be placed near the source, truly global reductions are more difficult to achieve, especially when the structure and acoustics are not directly coupled. In any situation, an ASC system can provide localized noise reductions. This has been demonstrated in turboprop aircraft to be very effective, providing good noise reductions (an average of 10 dB) near the passenger head locations. Table 1 summarizes application issues for TVAs and ASC.

Hybrid Systems/Total Aircraft Solutions

An effective and comprehensive noise and vibration strategy for a given aircraft can only be arrived at logically if the benefits and limitations of each technology are assessed, and if the cost, weight, and performance goals are defined. While treatment technology has advanced rapidly, the aerospace community must become more sophisticated in how it evaluates noise and vibration. As an example, the aircraft industry generally uses dBA. Unfortunately, dBA is just one of many important measures. For example, dBA can be very misleading in determining the benefits of any treatment in loud, low frequency environments, because dBA tends to discount low-frequency noise (8). As proof of this statement, passengers prefer the acoustic environment of commercial jets to commuter turboprops, even in cases where both aircraft have the same dBA levels. This is because the perceived noise in turboprop aircraft is heavily dominated by low frequency noise (frequencies that are lower than jets and are further attenuated by the A-weighting scale). Moreover, the effects of vibration are often as important to passenger comfort as noise. Today, sophisticated aircraft acoustic teams use an array of  measurements including dBA, dBC, tonal emergence, speech interference level, and g’s of vibration.

Once performance goals are set, the relative benefits of each treatment must be considered. Figure 4 shows general guidelines for applying various active and passive solutions on the basis of frequency. In general, passive systems work best at high frequencies and active systems work best at low frequencies. In cases where active systems can be applied near the source, performance at frequencies in excess of 400 Hz is possible and may provide solutions that are more weight effective than passive treatments. Passive RTM and Fluidlastic® solutions can provide vibration and noise benefits over an extremely wide frequency range.

When considering an active system, other factors, as shown in Table 2, must also be considered. ANC systems have an advantage that minimal or no prior knowledge is needed about the sources and paths, since cancellation is provided directly to the passengers and crew. Generally, AIC and ASC system require more of a prior knowledge in the form of test data or assumptions based on experience. While AIC systems typically use fewer actuators than ASC systems, the AIC actuators need to provide more force, limiting the choices of applicable devices. Also, AIC may require a redesign of the mounting system to accommodate the actuators. This makes AIC more difficult to demonstrate, but the weight and performance advantages of AIC are often worth the effort.

The implications of Figure 4, Table 2 and cost, lead to the notion of a total aircraft solution. As an example of this concept, Figure 5 shows the state-of-the-art turboprop aircraft solution. The figure shows and AIC system integrated with an RTM to provide a hybrid solution at the front of the engine mounting system. Assuming that the aft part of the engine does not transmit significant vibration, RTM mounts may possibly be the best alternative. TVAs may be located on other flanking paths including the fire wall and bleed airlines. In the cabin, passive interior treatments will be needed to control boundary layer noise above 1000 Hz. RTM mounts suspend auxiliary power units and attach luggage racks and interior trim. For low frequency induced noise created by propellers, ASC on the frames between the propellers will often provide the best solution. Finally, an ANC system in the cockpit reduces crew fatigue. This figure demonstrates just one of the many potential combinations of technologies to address the total noise and vibration problems of aircraft.

Conclusions

The technical demands of creating comfortable aircraft will require a range of solutions and products. The advent of active systems provides aircraft designers with cost effective and weight efficient solutions for many noise problems that are low frequency in nature. Either as an integrated system, or as separate but complimentary systems, passive and active technologies will be combined to create the best overall hybrid solutions to meet the demands of the aerospace industry.

• Global reduction of noise and vibration.

• Good low frequency performance.

• Good high frequency performance.

• Easy to install.

• Low weight.

• Local noise control.

• Solution independent of sources or paths.

• Easiest to demonstrate.

• Does not affect structure.

• Opportunity to retrofit.

• Can control both noise and vibration.

• Easy to demonstrate.

• Minor impact on structure/trim.

• Good low frequency performance.

• Good high frequency performance with path identification.

• Easy to retrofit.

Some firms need vibration control or monitoring with a professional engineer. Some can can perform noise and vibration measurement at your facility. Some serve your our customers.   Some also have many years of combined experience in Acoustics, and Shock Consulting. Technology development is something that a consultant firm should be specialists at. They specialize in solving problems, or any issues relating to Noise, Shock, and Acoustical Consulting Services. They often have patented technology using professional engineersto  improve noise and vibration characteristics of products or components, helping manufacturers solve noise and vibration problems and reduce production costs, while saving end users maintenance and energy costs.


Vibration in physics, is commonly an oscillatory motion. It is a movement first in one direction and then back again in the opposite direction. For example, by a swinging pendulum, by the prongs of a tuning fork that has been struck, or by the string of a musical instrument that has been plucked. Random vibrations are exhibited by the molecules in matter. Any simple vibration is described by three factors: its amplitude, or size; its frequency, or rate of oscillation; and the phase, or timing of the oscillations relative to some fixed time (see harmonic motion). Sound is produced by the vibrations of a body and is transmitted through material media in pressure waves made up of alternate condensations (forcing of the molecules of the medium together) and rarefactions (pulling of the molecules of the medium away from one another). In sound the vibration is longitudinal, for the movement is to and fro along the direction in which the sound is traveling. When a sound wave of one frequency strikes a body that will vibrate naturally at the same frequency, the vibration of the body is called sympathetic vibration. A reinforcement of sound resulting from sympathetic vibration is called resonance. When the vibrations of a sound-producing body cause another body to vibrate in the same frequency, not normally its own, the vibration is known as forced vibration. Heat is commonly defined as the energy of molecules, part of which consists of the energy of their vibrational motion.


Vibration isolation technology is a specialty implemented with expertise in mechanical engineering. New science has ground breaking techniques for Vibration and noise control. The best consultants have educations from top universities. Mechanical engineering is a core compentency for Structural Analysis, and Structural noise. Industrial problems can result in an Acoustics problem at a manufacturing facility. Industrial noise can be solved by actively utilizing Vibration monitoring techniques.


Structural excessive sound can be aleviated with proper Shock control. Noise problems require consulting services to help you figure out the problem at hand. Environmental noise reduction technology helps bring Acoustic technology to the facility. Acoustic technology combined with an intensive studtying of Structural dynamics can also benefit manufacturers. Dynamic analysis may be the solution as well. Look for a complete Acoustics and Noise reduction Vibration consultant consulting company.


Seek out a mechanical engineering and consultants company. Use vibration testing or monitoring services with a pro engineer. Engage consulting for measurement, analysis and testing services.


Consultants can bring drastic noise reduction technology to a facility with testing and analysis. Locate a one stop shop vibration consultant company. Want a pro engineer? Don't want to deal with using your internal resources? Need a mechanical engineering company? Interview a pro engineer to solve your acoustical problem with the help of this page. Testing services are incredible in assisting in vibration control. Find the best engineering shop. Some engineering services will meet all of your needs. A good team of engineers can do anything, ranging from design, analysis and vibration monitoring. They can even do new product development and current product retrofit. The top engineers are the masters of problem assessment and provide you with well engineered solutions. Vibration testing services can help a company's product succeed through vibration control. Sometimes, they can even just do vibration consulting.

Use vibration isolation technology to reduce vibration problems. You can examine Vibration Analysis, Vibration Testing with any good Vibration consulting firm. Often, these firms also specialize in Noise Reduction Acoustics Engineering.

Much new technology provides quality, innovative noise control  materials and engineering solution directly to industrial, commercial and governmental institutions worldwide. Included are quality noise control materials  that are highly competitive in both cost and quality. Most engineering staff is highly knowledgeable with much experience in providing effective noise and vibration control solution to a wide range of industrial and commercial products. The sale staff should be extremely professional and courteous. Only consider a company that is committed to delivering quality, cost effective products and services to their customers.

Look on the web for high performance product lines especially designed to absorb, isolate and remove unwanted noise and vibration.

EXAMPLE TECHNOLOGIES

Ester Embossed                            

Ester Embossed Acoustical Foam is designed to provide maximum absorption in minimum thickness. The flexible polyurethane foam is manufactured to optimize uniform cell structure, airflow resistance as well as good heat, humidity, flame and chemical resistance. The attractive textured pattern increased surface area, density and airflow resistance for maximum sound absorption. Ester Embossed material can be applied to various household appliances, business equipments, medical devices and  transportation vehicles.

Ester With Urethane Film                

Ester With Tedlar                             

Ester With Metalized Mylar            

Ester Acoustical Foam with different types of Film Facings is designed to provide a durable, abrasion and puncture resistance as well as an impervious facing to most petroleum, moisture and dirt. Material is suitable for various applications such as air ducts, clean rooms, vehicle interiors, printers as well as other industrial applications. It is also ideal for engine rooms, medical and food processors.

Ester With Perforated Vinyl           

Ester Foam with Perforated Vinyl is designed to provide maximum absorption and resilience with 14% open area. Its attractive leather like appearance makes it ideal for vehicle interiors, marine headliners and implant application such as walls, partitions.

Key Benefits

Damping Sheet        

DS-VO damping sheet is a flexible, extensional damping material with a viscoelastic surface and pressure sensitive adhesive backing. It  designs to provide a high degree of damping over broad temperature and frequency ranges. Typical applications include business machines, industrial and  medical equipments, speakers and home entertainment system equipments, trucks, boats etc...

Key Benefits

Damping foil       

Damping foil composite is design to reduce resonance vibration in thin sheet metal. It is constructed from light weight aluminum bonded to a viscoelastic and pressure sensitive  backing . Typical applications to include Aircraft fuselage skins, business equipment enclosures, auto doors, CD ROM drives. It can also be used as isolation gasket for stepper motor.

Key Benefits

VB Composite        Our Vinyl Barrier, VB-VO, often used between 2 layers of foam, provides high transmission loss and is certified with UL for UL94-VO flammability rating. VB-VO provide excellence sound barrier for pipe, duct wrap, machinery lagging, carpet underlay, in plant applications etc...

 DS Composite          

Our DS Composite material provides a combination of structural damping of our DS-VO and highly absorption ester foam.

 Vinyl Barrier

Key Benefits

VB-V0 Vinyl Barrier is a tough, limp mass, loaded vinyl designed to provide maximum sound attenuation as well as chemical and flame resistance. It is non-toxic, contain no lead or asbestos and will not rust, shrink or cause metal corrosion. VB-VO Vinyl can be applied to various household appliances, business equipments, medical devices and  transportation vehicles.

Key Benefits

Vibration Control by Confinement (VCC)

The VCC (Vibration Control by Confinement) approach to noise and vibration control and suppression problems is a groundbreaking technique that fundamentally changes how industry addresses the negative effects of excess noise and vibration energies. The VCC technology may be used to enhance quality of life, improve performance of consumer and industrial products, extend useful life of machinery and structures, and reduce the cost associated with the implementation of active control and smart technologies.

Contrary to the current conventional passive and active vibration control approaches that are applied in time and/or frequency domains, our VCC method utilizes those domains plus the space domain in which spatial distribution of vibration energy is controlled. In other words, we control the flow of power by directing vibration energy away from the sensitive areas of the system. It has been shown that if one can control and/or trap the vibration energy away from areas that are mission/performance/safety sensitive, then the excess energy can be more effectively removed. In some instances, the energy may remain in the trapped area if it does not affect the performance of the system. In other cases, the trapped energy can be removed more efficiently than conventional passive and/or active methods.

A company called QRDC has shown profound performance improvements by conventional vibrations control techniques when combined with VCC. For instance, they have demonstrated a 30% to 50% improvement in the performance of Constrained Layer Damping (CLD) when combined with VCC. They have also shown 80% to 98% improvement when conventional passive or active control techniques are enhanced with VCC. If VCC is applied as a stand-alone technique, it has unparalleled results.

The picture to the right demonstrates the VCC approach. The figure displays the basic process of what we refer to as the "Energy Steering and Shoveling Device." This process takes place when one implements the smart energy-based vibration control technique. The key to the technique is that the excess energy is taken from the critical areas (sensitive interfaces) or critical modes and placed in the non-critical areas (non-sensitive interfaces) or non-critical modes. In doing so, some of the energy is also dissipated in various forms. Another way of looking at our energy managing concept is that one can provide certain conditions in the structure so that it redirects its vibration energy from one place to another and from one mode to another mode. The main contribution is the development of a smart structure which performs this steering and shoveling of energy by design, in a systematic manner, and most importantly, by controllable means. When energy management is induced, vibration energy is confined to a limited region within a structure. Consequently, the vibration energy injected into a structure by an external disturbance cannot randomly propagate away from its initial contact point with the structure but is directed to be confined, diverted, or trapped to selected regions.

One of the questions addressed by the VCC technology is whether it is possible to attenuate vibrations of the selected critical regions of a vibrating system to acceptable levels faster than its non-critical regions. Here, a critical region or a critical mode is defined with respect to a set of mission, safety, and performance criterion given in an application. Based on the VCC-based smart structure concepts with smart energy managing features, one can achieve significant noise and vibration reduction with a smaller amount of input energy than that consumed by commonly practiced controllers, thus enabling current and future actuators to perform more efficiently and effectively. Furthermore, our approach will result in a significantly less complex control system with a reduced cost when compared with current active vibration control systems.

GLOSSARY

Absorption Coefficient: The absorption coefficient of a material or sound absorbing device is the ratio of the sound absorbed to the sound incident on the material or device.

Acoustical Material: A material used to alter a sound field. The material may be used to absorb, damp or block acoustical energy.

Airborne Noise: A condition when sound waves are being carried by the atmosphere.

Ambient Noise: All the sounds from many sources associated with a given environment.

Anechoic Room: A test chamber which has a lining of absorbent acoustical material to eliminate all sound reflections. It is most often used to determine the sound radiation characteristics of equipment.

Damping: The process of dissipating mechanical vibratory energy into heat. In noise control, a damping material is usually applied to a vibrating surface to reduce the noise radiating from that surface.

Dissipative Silencer: A device inserted into an air duct or opening to reduce noise transmitted through the duct or opening. Noise reduction is accomplished through the use of internal sound absorbing materials.

Flanking Transmission: Noise that reaches an observer by paths around or over an acoustical barrier.

Frequency Spectrum: A graph or plot of the sound pressure level in each band from a set of octave or 1/3 octave bands.

Insertion Loss: The reduction of sound power level attained by inserting a silencer or muffler in an acoustic transmission system (see ASTM E-477).

Loudness: Loudness is the subjective human definition of the intensity of a sound. Human reaction to sound is highly dependent on the sound pressure and frequency.

Mass Law: A rule for estimating the transmission loss of a barrier in its mass controlled region. The rule states that transmission loss increases/decreases 6 dB for each doubling/halving of either frequency or barrier surface density.

Noise: Any undesired sound.

Noise Reduction (NR): The reduction in sound pressure level caused by making some alteration to a sound field.

Noise Reduction Coefficient (NRC): A single number rating which is the average of the sound absorption coefficients in the octave bands centered at 250, 500, 1000 and 2000 Hz expressed to the nearest integral multiple of 0.05 (see ASTM C-423).

  Octave Band (O.B.): A range of frequencies where the highest frequency of the band is double the lowest frequency of the band. The band is usually specified by the center frequency, i.e., 31.5, 63, 125, 250, 500 Hz, etc.

Radiation: The process whereby structure-borne vibration is converted into airborne sound.

Reverberation: Reverberation is the echoing of previously generated sound caused by reflection of acoustic waves from the surface of enclosed spaces.

Reverberation Room: A test chamber so designed that the reverberant sound field within the room has an intensity that is approximately the same in all directions and at every point. It is commonly used to measure sound absorption, ASTM C-423 and transmission loss, ASTM E-90.

Sabin: The unit of measure of sound absorption. The number of square feet of sound absorbing material multiplied by the material absorption coefficient.

Sound: Pressure waves that are traveling in the air or other elastic materials.

Sound Absorption: The acoustical process whereby sound energy is dissipated as heat rather than reflected back to the environment.

Sound Level Meter: An instrument used to measure sound pressure level. Sound level meters are commonly either Type 1, precision instruments, or Type 2, general purpose instruments. Both types can have weighting and filter networks to provide dB readings by octave band in the A, B, or C scales.

Sound Power Level (Lw): A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts).

Sound Pressure Level (Lp): A measure of the air pressure change caused by a sound wave, expressed on a decibel scale referenced to 20µPa.

Sound Transmission Class (STC): A single number rating derived from measured values of transmission loss in accordance with ASTM 413. The rating provides an estimate of the performance of a barrier in certain common noise attenuation applications.

Structure-borne Noise: Mechanical vibration in a structure which can ultimately become audible sound. Until such time as radiation occurs, these vibrations are inaudible and of little concern.

Transmission Loss (TL): The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expressed in decibels.

You may need elastomeric shock mounts and vibration isolation products. Many engineers utilize dynamic testing capabilities and their years of experience to solve all types of shock and vibration control problems. Many have also developed a six degree-of-freedom analysis program to completely evaluate mounting systems. A special Varicalc program analyzes systems' natural frequencies as well as isolators.

Anti-Vibration Mountings

Used to control shock and vibration in a variety of industries and applications.

Following are descriptions of various styles of rubber isolation mounts / isolators. I do not have specific information about shock and vibration mounts shown including dimensions, load ratings, and stiffness data.

Pivotal LevelersPivotal Levelers:

Pivotal Levelers are self-aligning, self-contained rubber mounts that handle equipment up to 8,000 pounds, providing stability, isolation and shock protection while allowing complete portability.

Patented Pivotal Levelers keep equipment level and isolate noise and vibration for about one-third to one-half the cost of ordinary machine mounts.

You can quickly and easily attach Pivotal Levelers to everything from computers and compressors to presses and pumps. These self-aligning, self-contained units handle equipment up to 8,000 lbs., providing stability, isolation and shock protection while allowing complete portability. They permit ¼" or more of leveling, depending on the attachment hardware and method you use.

Pivotal Levelers are made of bonded neoprene/steel for long service life and resistance to ozone and oils.

Economical Pivotal Levelers keep equipment level on uneven floors while reducing noise, vibration and shock. These typical methods of use demonstrate the great versatility of Pivotal Levelers.

Mounting FeetRubber Mounting Feet:

Mounting feet let you isolate all types of machinery and equipment from vibration and floor motion and have a load capacity from 6 to 1,000 pounds. They are made of neoprene bonded to a steel insert. All models are available with zinc-plated steel glides.

Multi-use Mounting Feet are used on business machines, light
manufacturing machinery, precision equipment in fields such as
optics, and almost anywhere else vibration presents a problem.

They have a load capacity from 6 to 1000 pounds. The point load
(see chart) is accomplished by loading only on the steel insert. The
spread load method uses a washer or places the equipment directly
on the entire 1-3/8" diameter top surface.

Note: Attachment hardware (bolts, nuts and flange nuts) can be supplied at additional cost.

Machine Leveling MountsMachine Leveling Mounts:

Machine Leveling Mounts are designed to reduce machine noise and meet OSHA standards for anchorage with internal adjustment capability up to ½ inch above loaded height. Oil resistant rubber is utilized.

Whenever you need to protect heavy production equipment from shock and vibration, a rugged series of Leveling Mounts will do the job to your full satisfaction. Their no-walk, no-creep performance lets you place your equipment wherever you wish without bolting it to the floor and to move it easily without re-anchoring or re-shimming. Even when immersed in oil, water or other hazards, the high strength steel housing and neoprene base resist failure for years of daily punishment.

Firms offer an average of six standard load ranges to closely match your weight and dimensional requirements. Punch presses, milling machines, injection molding equipment, lathes, compressors, mixers…any equipment that generates motion internally or is affected by external vibration can be economically mounted for top performance plus complete portability. Leveling is simple and quick with internal adjustment capability up to ½ inch above loaded height, just by a few turns of the leveling bolt. Then the locknut is tightened on the machine foot for a permanent, precise mount.

Benefits: Benefits: Easy Leveling, Reduce Installation and Relocation Costs, No Special Foundations, Isolate Shock and Vibration, Reduce Noise Levels, Meets OSHA anchoring Standards, Reduce Maintenance Costs, Improve Production Efficiency

Applications: Applications: Injection Molding Machines, Die Casting Machines, Punch Presses, Lathes, Grinders, Jig Borers, Screw Machines, Milling, Machines, Brakes, Four Slides, And many others.

I do not have load ratings and dimensional information on all sizes of machine leveling mounts.

Fail-Safe Compression MountsFail-Safe Compression Mounts:

Ideal for isolation of diesel engines and generators with a wide load range of 50 to 420 pounds. These rubber mounts also protect against harmful shock loads. These fail-safe* isolators are ideal for isolation of diesel engines and generators used in construction equipment, recreational vehicles and off-road equipment. The low natural frequency allows them to be used for computer and electronic equipment when there is a need for a "ruggedized" installation. They are also excellent isolators for compressors, motors, pumps and other machinery when skid mounted.

The mounts offer a wide load range of 50 to 420 lbs., has a high stiffness ratio of 6:1, axial-to-radial.

The standard elastomer is neoprene, which is resistant to ozone, fuel and oils. It performs well at a temperature range of –20ºF to +180ºF. Optional materials such as nitrile, butyl, silicone and others are available to meet your environmental conditions or military specifications.

Firms offer about two series of fail-safe compression mounts for various load ratings, and mounting dimensions.

Heavy Duty Compression MountsHeavy Duty Compression Mounts:

Designed for applications under heavy industrial machinery and provide low natural frequency of 8-15Hz for efficient isolation of machine speeds as low as 750 rpm.

These rugged, high performance mounts are normally used for vertically applied loads to prevent the transmission of noise and vibration caused by the rotation of unbalanced equipment such as centrifuges, blowers, pumps, vibrators and air handling systems. They effectively isolate disturbing frequencies as low as 900 rpm (15 Hz), providing up to 90% isolation at 1,500 rpm (25 Hz). Their elastomer-in-compression design effectively interrupts noise transmission paths to prevent sounding board effects. Cold-rolled steel and a neoprene elastomer resist ozone and oils in an operational temperature range of –20º to +180ºF.

Standard Compression Mounts:

These Mounts offer natural frequencies as low as 6 Hz at maximum lead. Materials are neoprene and steel.

Mounts are available in about eight different sizes to cover a large load range. I do not have specific information about the products available. The "double-deflection" are softer mounts that yield a lower natural frequency (approx. 4 1/2 Hz at rated load).

Standard and Double Deflection Compression Mounts are available for natural frequencies as low as 4.5Hz to 6Hz at maximum load.

Dome Mounts:

Feature interlocking metals that result in a fail-safe mount ideal for isolating medium - to large - size engines, fans, blowers, pumps and air handling equipment. These isolators provide excellent protection from harmful shock and vibration inputs.

The interlocking metals of the Dome Mount series result in a fail-safe mount. This feature and low stiffness make them ideal for isolating medium- to large- size engines as well as fans, blowers, pumps and air handling equipment. They have an approximate natural frequency of 9Hz at maximum load.

The elastomer is neoprene and the metals are zinc-plated steel.

Low Frequency Mounts:

Consisting of a metal spring molded in rubber, these mounts interrupt the sound path, prevent noise amplification caused by sounding board effects, and stop vibrations from being transmitted to the floor or work-surface. Low Frequency Mounts’ unique design bonds a steel spring inside a matrix of oil/ozone-resistant neoprene. The springs absorb low frequency vibrations, slowing and passing them on to the resilient neoprene. This material—made even more stable by the springs—interrupts the sound path, prevents noise amplification caused by sounding board effects, and stops vibrations from being transmitted to the floor or work surface.

Low Frequency Mounts are made for the toughest vibration applications. They tame the effects of paint mixers, air conditioning units, air compressors and more, indoors and out. There are models for loads from 50 to 4,700 lbs. which exhibit 3.5 Hz natural frequency at maximum load.

Cupmounts:

Cupmounts help your sensitive equipment defend itself against high-impact shock and can be used to mount your equipment in compression, tension and shear applications. Three Way Protection: Help your sensitive equipment defend itself against high-impact shocks by installing Tech Products Cupmounts. These rugged and versatile mounts also control vibration and interrupt structure-borne noise. Under normal loading conditions, they exhibit natural frequencies of approximately 25 Hz and isolate disturbing frequencies above 35 Hz.

Fail-safe Construction: Available in four basic sizes, these compact, low-profile isolators have interlocking components of steel (other metals available) and standard neoprene or high damped silicone elastomers. They can be used to mount your equipment in compression, tension and shear applications. No matter how the mount is oriented or the load is directed, the elastomer is in compression.

Land, Sea and Air Uses: Land, Sea and Air Uses: Great resistance to severe shock makes cupmounts ideal for protecting sensitive equipment on rough-terrain vehicles or railroad cars. Factories of all types use them for everything from numerically controlled machinery or electronic control panels to blowers. And they stand guard against shock on shipboard equipment, shipping containers, and both aircraft and missile electronics. Oil resistant standard cupmounts operate over a temperature range of –20ºF to +180ºF. For more severe environments, choose optional silicone elastomers to provide increased corrosion resistance and operation over an even wider temperature range.

All models are available with through center holes or metric thread sizes, in addition to the trapped hole dimension shown. Standard models have metal parts of cold rolled steel (zinc plated) and a neoprene elastomer; other metals may be requested, and high damped silicone is an optional elastomer for special applications.

Stable-Flex Mounts:

Specifically engineered to isolate light weight low speed equipment such as small engines, generators, compressors, pumps, and various mobile applications. The rugged design allows for excellent stability under extreme shock loads. Stable Flex Mounts have been specifically engineered to isolate light weight, low speed equipment. The complex geometry of the elastomer element in the mount provides a low axial stiffness and excellent lateral stability.

These fail-safe isolators yield an axial natural frequency of approximately 8 Hz at the rated loads, for effective isolation of low speeds. They are constructed of neoprene bonded to zinc-plated low carbon steel for superior ozone and oil resistance. Other specialty elastomers are available, such as high damped silicone.

Common Applications include: Small Engines, Generators, Compressors, Pumps, Other Industrial Equipment, and Various Mobile Applications.

Universal Mounts:

Resist ozone, oils and fuels while providing fail-safe, all-attitude isolation for equipment up to 4,550 pounds in mobile applications. Low-cost, easy-to-install Universal Mounts provide fail-safe, all-attitude isolation for vehicle cabs, engines, transmissions and other equipment up to 4550 lbs. in mobile applications.

Consisting of two parts—an elastomeric ring and an elastomeric bushing bonded to a center metal spacer—Universal Mounts are held in place with a through bolt.

With an operating temperature range of –20º to +180ºF, the standard neoprene elastomer resists ozone, oils and fuels while providing adequate rebound protection. Other elastomers are also available.

Armor Plated Universal Mounts:

Armor plated universal mounts have a steel wear surface bonded to the rubber which eliminates the need for machining a radius in the support structure. The Armor Plated Universal Mount can be used in the same applications as the standard Universal Mounts. But since it has bonded-in steel wear surfaces, it can be used in more extreme environments. These steel wear surfaces eliminate the need for machining a radius in the support structure.

Other Mounts:

Made of oil, fuel and solvent-resistant neoprene with a high load capacity, stability, and the ability to be installed at any mounting angle. Only a few series provide excellent isolation of both shock and vibration. all-attitude mounts are a money-saving way to protect equipment from vibration and shock.

High load capacity, stability, and the ability to be installed at any mounting angle make them ideal for a wide variety of applications, including vehicle cabs; truck, bus and marine engines; generators; air conditioning units; motors and electronic equipment.

These mounts can be installed at any convenient mounting angle thanks to their "shouldered" core design that make optimum use of the neoprene elastomer’s shear and compressive properties. They offer a fail-safe installation when the proper snubbing washers are used. At maximum load, the natural frequency of these mounts is about 8.5 Hz, providing effective isolation from disturbing frequencies of 12 Hz and above.

Oil, fuel and solvent-resistant neoprene—with a temperature range of –20º to +180ºF— provides isolation in all planes, regardless of the direction of the exciting forces.

Center Bushing Mounts:

The answer to fail-safe, multidirectional isolation in heavy-duty static trial load applications. Center Bushing Mounts are fail-safe, multi-direction isolators for a variety of heavy duty applications. During Installation, a self-contained rebound is formed when the mounts resilient element spreads under compression. An internal sleeve serves as a positive spacer to control pre-loading.

These low-deflection, one-piece safety mounts are rated by static load in the axial direction. They can handle dynamic loads up to three times their static load ratings. Versatile Center Bushing Mounts will also take dynamic radial loads, but are not recommended for static radial loads.

Neoprene elastomer operates over a temperature range of -20 to 180 degrees (F).

Seven different sizes are offered to cover a wide variety of loads. Select specific load ranges and review dimensions, load ratings, and stiffness data before purchasing any.

Ring & Bushing MounstRing & Bushing Mounts:

All-rubber mounts are incorporated directly into the structural components of equipment and offer fail-safe operation when installed in pairs. Neoprene Ring and Bushing Mounts are incorporated directly into the structural components of equipment such as office machines, motors and pumps, as well as air conditioning, electronic and scientific equipment. They offer fail-safe operation when installed in pairs.

Opposing holes in the elastomer provide excellent low-frequency isolation. The location of the holes cushions shock and isolates vibration parallel to the mount axis. The bushing’s holes isolate vibration perpendicular to its axis. Select specific load ranges and review dimensions, load ratings, and stiffness data before purchasing any.

All-Attitude Mounts:

Featuring an aluminum center plate suited for built-in electronics and assure effective isolation in all directions for both shock and vibration. Elastomeric mounts usually have an aluminum center plate suited for built-in electronics. There is a holder style, where a base mounted configuration is needed. Three models carry maximum load ratings of 4, 5, 7 and 10 lbs., based on 0.036" double-amplitude input. An elastomer-in-compression design assures effective isolation in all directions. Fail-safe construction retains the equipment under a 30G, 11 millisecond shock input at rated loads, even if the elastomer is destroyed. The highly-damped silicone elastomer provides optimum vibration isolation over a temperature range of –65º to +300º and low transmissibility at a resonance of about 3.5 maximum. Radial to axial stiffness ratio is approximately 0.6. Many models meet the environmental requirements of MIL-E-5400, with mounting hole patterns conforming to MIL-Size 0. They are unaffected by ozone, fungus or high humidity. Other all-attitude isolators protect stationary loads of 15-50 lbs. and vehicular loads up to 30 lbs. from shock, vibration and noise. An elastomer-in-compression design provides effective, fail-safe protection in any mounting position, even if the elastomer should be destroyed. The 522 Series plate style is for low profile use. Neoprene elastomer is standard but high damped silicone (HDS) is also available to meet application needs. HDS models—which meet MIL-E-5400 shock and vibration requirements—have a temperature range of -65º to +300ºF and a maximum transmissibility of about 3.5. Neoprene models have a temperature range of –20º to +180ºF with excellent resistance to oil and ozone. Radial to axial stiffness ratio on all models is about 0.6.

Bubble Mounts:

Low-frequency isolation for electronic or medical equipment, avionics, computers, small pumps and compressors and more from shock and vibration.

Especially useful in mobile or medical equipment, these aluminum and neoprene isolators operate in temperatures between –20º and +180ºF. They resist oil, ozone and most solvents. These mounts are also available made from silicone for both high and low temperature applicaitons.

Platemounts:

Self-snubbing under extreme shock loads and offer fail-safe operation of electronic and electro-mechanical equipment, appliances, office machines and transportation equipment. Fail-safe operation of electronic and electro-mechanical equipment, appliances, office machines and transportation equipment are typical applications for Tech Products Plate Mounts.

Designed for light to medium loads, they isolate mounted equipment from external vibration and/or isolate vibration produced by the mounted equipment itself.

Plate mounts feature steel and natural rubber which operate at temperatures of –20º to +180ºF. Neoprene elastomer is also available.

Stud/Plate MountsRubber Stud/Plate Mounts:

Rubber Stud/Plate Mounts, known by many names, including bumpers, snubbers, feet, sandwich mounts, attachments, shockmounts, shearmounts, cylindrical mounts, isolators, levelers and insulators, offer solutions to thousands of noise and vibration problems. Again, stud/Plate mounts are known throughout the industry by several names including: bumpers, snubbers, feet, sandwich mounts, shockmounts, shearmounts, cylindrical mounts, isolators, levelers, and insulators.

The standard elastomer for these mounts is natural rubber.

Below is a table showing typical environments where isolation is necessary and the types of vibration isolators used in these environments. Engineers are usually happy to evaluate any application and make isolator recommendations.

Shock & Vibration Isolators
Natural Rubber, Neoprene and Specialty Elastomers

 Words On This Page: 12180