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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.
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What Is Noise?
Noise
is unwanted sound which may be hazardous to health, interfere with speech
and verbal communications or is otherwise disturbing, irritating or
annoying.
What Is Sound?
Sound
is defined as any pressure variation in air, water or other fluid medium
which may be detected by the human ear.
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What Are The
Characteristics
Of Sound?
The two most
important characteristics which must be known in order to evaluate the
sound or noise are it’s amplitude and frequency. The amplitude or
height of the sound wave from peak to valley determines the loudness or
intensity. The wave length determines the frequency, pitch or tone of the
sound.
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How
Are These Characteristics Expressed?
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The
frequency of sound is expressed in wavelengths per second or cycles per second
CPS). It is more commonly
referred to as Hertz. Low frequency noise is 250 Hertz (Hz) and below. High
frequency noise is 2000 Hz and above. Mid-frequency noise falls between
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250
and 2000 Hz.
The
amplitude of sound is expressed in decibels (dB). This is a logarithmic
compressed scale dealing in powers of 10 where small increments in dB
correspond to large changes in acoustic energy.
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What
Are Wavelengths?
Sound
wavelengths are the linear measurement of one full cycle of displacement
where the motion of air molecules is first compressed and then rarefield or
expanded. The wavelength is determined by the ratio of the speed of sound
to the frequency.
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Wavelengths =
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Speed of
Sound
Frequency
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What Are Octave Bands?
Standardized octave bands are
groups of frequencies named by the center frequency where the upper limit
is always twice the lower limit of the range. Test data for
performance of acoustical materials is standardized for easy comparison at
the center frequencies. Equipment noise levels and measurement
devices (dB meters) also follow the preferred octave bands.
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What Are Octave Band Center Frequency
Wavelengths At 70°F?
The speed of sound in air at 70°F is 1,130 feet per second. Wavelengths in
feet for each center band frequency are listed below:
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Frequency
(Hz)
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63
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125
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250
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500
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1000
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2000
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4000
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8000
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Wavelength
(Ft.)
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17.9
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9.03
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4.51
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2.26
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1.13
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0.56
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0.28
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0.14
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What Is The Decibel Scale?
The
decibel (dB) is a dimensionless unit calculated using the ratio of a
measured value (p) to a reference value (pre). The values of sound pressure
of most interest range from the threshold of hearing at about 1 x 10-9
psi to the threshold of damage at about 15 psi. This range of energy
variation translates to 10 orders of
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magnitude with the high threshold at a level 1,000,000,000 times that of
the lower threshold. The use of a logarithmic scale compresses the unit of
measure to a manageable range in order to simplify calculations, computations
and quantitative manipulation of data.
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What is Sound Pressure?
Decibels of sound pressure (Lp) have a universally accepted reference
pressure of 2.0 x 10-5 Pascals (Pa).
Lp = 20 log10
Root Mean Square (RMS) Sound Pressure
2.0
x 10-5 Pa (Reference Pressure)
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What Is Sound Power?
Decibels of sound power (Lw) have a universally accepted reference value of
10-12 watts (1 picowatt).

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Are Sound Pressure And Sound Power Equal Values?
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No!
While both sound power levels (Lw) and sound pressure levels (Lp) are both
expressed in decibels, the referenced standards for each are different.
More importantly, the sound power level is the total acoustic energy output
of a noise source independent of environment. Sound pressure levels are
dependent on
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environmental
factors such as the distance from the source, the presence of reflective
surfaces and other characteristics of the room/building/area hosting the
source. Actual sound pressure levels will always be higher than sound power
levels.
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What Is The Difference Between dB and dBA?
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dB
sound pressure levels are unweighted. dBA levels are “A”
weighted according to theweighting curves
shown below to
approximate the way the human ear hears. For example, a 100 dB level at 100
Hz will be perceived to have a loudness equal to only 80 dB at 1000 Hz.
Other weighting scales (C and B) are also shown. The dBA scale is based on
a child’s hearing and was originally documented based on actual
hearing tests to characterize the human ear’s relative response to
noise.
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How Are Decibel Levels Added Together?
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The
tables at left show the additive effect for adding equal and unequal
decibel levels. Unless the two levels differ by 10 or more dB there will
always be some increase to the higher level. Frequency levels can also be
added together in a similar fashion to get overall dB levels.
Decibel
addition is illustrated in the following example: An industrial fan
radiates levels of 88 dBA from the fan housing, 86 dBA from the motor and
85 dBA from the belt drive assembly. To figure the overall dBA level we
find the difference between the fan housing and motor noise (88 – 86
= 2 dBA difference). The above table shows that 2 dBA is added to the
higher value resulting in 90 dBA (88 + 2). Considering the belt drive now,
adding 85 dBA to the 90 dBA (5 dBA difference) results in an overall level
of 91 dBA (90 + 1 = 91).
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How Are Frequencies Added Together?
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octave band
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center frequency (Hz)
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unweighted sound pressure (dB)
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A-weighting factor (dB)
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A-weighted sound pressure (dB)
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decibel addition
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overall resultant level
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1
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63
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94
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-26
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68
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72
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86
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91 dBA
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2
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125
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86
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-16
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70
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3
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250
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85
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-9
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76
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86
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4
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500
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89
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-3
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86
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5
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1,000
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89
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+0
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89
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89
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89
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6
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2,000
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77
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+1
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78
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7
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4,000
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75
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+1
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76
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79
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8
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8,000
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76
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+0
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76
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In
the example above, successive pairs of frequency dBA levels are added
together in accordance with the procedure outlined on the previous page.
Unweighted dB levels are “A” scale corrected prior to final
addition.
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Is A 5 dB Change Significant?
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Yes!
The pressure associated with the loudest known sound is more than one
billion times that assoc-iated with the faintest sound. Such a large range
is unmanageable for measurement purposes. Using a logarithmic scale
compresses the range to between 0 and 200 dB. At right, various sound level
changes are referenced to relative loudness and acoustic energy loss. A 5
dB change is more than a 50% change in acoustic energy!!!
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Broadband
noise has a frequency spectrum or signature where there are no discreet or
dominant tones. Sound pressure fluctuations of broadband noise are
non-periodic in nature with relatively random phase and amplitude. Although
devoid of discrete frequencies, the
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acoustical
energy of broadband noise may still be largely concentrated in one or more
areas of the spectrum. Examples of broadband noise are shop air blow-offs,
gas fired burners, jet engines and grinding tools.
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Tonal
noise is commonly referred to as discrete frequency noise and is
characterized by spectral tones that are pure tone in nature. Pure
tones are wave forms that occur at a single frequency. Tonal noise is
generated by rotating equipment at a predictable frequency relating to the
rotational speed of the shaft and the number of compressor
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vanes,
fan blades, engine pistons, gear teeth, etc. The fundamental tone (F) may
also manifest itself at progressively lower intensity levels at integer
harmonic multiples (2F, 3F, etc.). Tolerance levels for tonal noise are
generally at a lower threshold.
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Spectral
data measured using frequency filter sets is necessary to asses tonal
content. Characterizing the source noise frequencies in full octave
bands (see example above right for a transformer) does not provide the
degree of spectral definition of fractional 1/3 octave bandwidths (see
above left for same transformer example).
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Discernable
tones shown in 1/3 octave format can disappear in a full octave
analysis. Narrow band frequency analysis is used to precisely
identify tones. Examples of tonal noise include fans, rotary lobe blowers,
compressors, gears, transformers, saws, and piston driven engines. Noise
control treatment strategies for tonal noise sources must target the
discrete frequencies.
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What Is Impulse Noise?
Impulse noise is a short
duration transient acoustic event characterized by a sudden rise or spike
in sound pressure followed by a uniform or oscillatory decay (depends on
type of source equipment) lasting less than
½ second. Impulse noise usually exhibits a distinct spectral
signature across the frequency range without the presence of discrete
tones. Examples of impulse noise include gunshots, pulse cleaning
systems, punch presses, etc.
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What
Is The Audible Range?
At birth, the
audible frequency range is 20 Hz to 20,000 Hz. Generally speaking the
average audible range in humans is from 30 Hz to 17,000 Hz. Sound
pressure wave forms below and above this range are described as infrasonic
and ultrasonic. Infrasonic sound is experienced as a flutter while
ultrasonic sound produces no sensation of hearing.
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What Is Diffraction?
Diffraction of sound is “bending” of the pressure wave around
objects, obstacles and walls. Diffraction is greatest with low
frequency sound or where the wavelength is large compared to the object it
strikes. As illustrated above, diffraction of sound results in a less
pronounced acoustic shadow zone.
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| Some
words on this page are commonly misspelled: noise, noice, noize,
moise, moize, moice, no1se, noies, nosie, niose, onise. control,
contol, contrl, cntrol, cotrol, conrol, contror, contlol, contlor,
contro, contlo, contro1, comtrol, contrlo, contorl, conrtol, cotnrol,
cnotrol, ocntrol, ontrol. vibration, vbration, viration, vibation,
vibrtion, vibraion, vibratin, vibratiom, viblatiom, viblasion,
vibraton, vibrashun, vibrashon, viblation, viblaton, viblashun,
viblashon, vibrasion, v1brat1on, vibratino, vibratoin, vibraiton,
vibrtaion, vibartion, virbation, vbiration, ivbration, vibratio,
ibration. noise, noice, noize, moise, moize, moice, no1se, noies,
nosie, niose, onise. treatment, treatent, treatmnt, treatmet,
teatment, treament, tleetmiegnt, tratmiegnt, trheatmeignt, tleaitmant,
tleaitmiegnt, tretmiegnt, trheaitmeignt, tlheatment, tlheatmiegnt,
treetmiegnt, tleatmeignt, tlheatmant, treaitment, treaitmiegnt,
tlatmeignt, tlheaitment, treaitmant, trheatmiegnt, tletmeignt,
treatmeignt, trheatment, trheaitmiegnt, tleetmeignt, tratmeignt,
trheatmant, tleatmiegnt, tleaitmeignt, tretmeignt, trheaitment,
tlatmiegnt, tlheatmeignt, treetmeignt, trheaitmant, tletmiegnt,
treatmiegnt, treaitmeignt, tleaitment, tratmant, triatmiegnt,
tretmant, tratment, tliatmiegnt, treetmant, tretment, tleatmant,
treetment, triatment, tlatmant, tleatment, triatmant, tletmant,
tlatment, tliatment, tleetmant, tletment, tliatmant, tleetment,
triatmeignt, treatmant, tliatmeignt, treatnemt, treatmemt, treatmetn,
treatmnet, treatemnt, treamtent, tretament, traetment, teratment,
rteatment, treatmen, reatment. shock, shovk, shokc, shcok, sohck,
hsock. receive, rceive, reeive, recive, receve, receie, recieve,
leceive, lecieve, receiver, recever, receier, receivr, rceiver,
reeiver, reciver, reciebur, leceibur, reciever, leciebur, leceiver,
receivur, leciever, recievur, receivel, leceivur, recievel, lecievur,
leceivel, lecievel, receibur, rece1ver, receivre, receievr, recevier,
reeciver, rceeiver, erceiver, eceiver. equipment, equipmnt, equipmet,
eqwipment, ekipment, eqwapment, ekipmant, eqwipmant, euipment,
eqwapmant, eqipment, equpment, equiment, equipent, ekwipmant,
ekwapmant, equapment, ekwipment, ekwapment, equipmant, equapmant,
ekipmeignt, equapmeignt, equipmiegnt, ekwipmiegnt, eqwipmiegnt,
ekipmiegnt, equapmiegnt, equipmeignt, ekwipmeignt, eqwipmeignt,
ekipmeign, ekwipman, equipmen, equimen, equapmeign, eqwipman,
ekwipmen, equipen, equipmiegn, ekipman, eqwipmen, equipmn, ekwipmiegn,
equapmen, ekipmen, eqwipmiegn, equapmin, equipmin, ekipmiegn,
equapman, ekwipmin, equapmiegn, equipmeign, eqwipmin, euipmen,
ekwipmeign, ekipmin, eqipmen, eqwipmeign, equipman, equpmen, equ1pnemt,
equipnemt, equipmemt, equipmetn, equipmnet, equipemnt, equimpent,
equpiment, eqiupment, euqipment, qeuipment, quipment. |
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
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.
Option 5: Modify an active control headset
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.)
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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 Isolation Control (AIC) and Passive Mounting Systems
All mounting systems need to accomplish two basic functions:
constraining motion and providing vibration isolation.
“Constraining motion” refers to limiting the relative
motion between two structures. For example, an engine mounting
system must limit the relative motion between the engine and the
airframe structure created by thrust, “g” loads, weight,
and torque. “Providing vibration isolation” involves
minimizing the transmission of vibration from one structure to
another. For an engine mounting system, the goal is to reduce
the forces transmitted into the structure relative to rigidly
connecting the engine to the structure. To provide the first basic
function, the mounting system needs to be stiff to minimize motion.
In order to minimize transmitted vibration, however, the mounting
system needs to be dynamically soft.

The Fluidlastic® Torque Restraint (FTR) system, as shown in Figure 1,
is the state-of-the-art in passive mounting systems for turboprop engines (4).
FTR combines elastomer and fluid.
Fluid inertia is used to enhance vibration isolation by creating forces
that tend to counteract the vibratory forces transmitted vertically through
the elastomer. The overall effect is that the vertical dynamic stiffness
at the vibration frequency can be significantly less than the static
stiffness. (Fluidlastic® mounts used on turbofan engines also exhibit this
dynamic behavior). Further, the fluid in an FTR system reacts propeller torque loads, minimizing the
roll motion of the engine. This weight efficient system further enhances
vibration isolation by eliminating other connections which are typical of
torque tubes used in many aircraft today.
AIC systems utilize active mounts (1) with integrated
actuators. These systems are the ultimate solution for constraining motion
and providing vibration isolation. Active mounts can have virtually zero
dynamic stiffness at the vibration frequencies, and yet the static
stiffness can be quite high (1). Figure 2 shows an early prototype of an
active mount. The actuator is used to create dynamic pressure within the
fluid to cancel the transmitted force through the elastomer. Additionally,
active mounts can be used to create forces which directly control noise
sensed by microphones in the cabin. In this way, the actuator forces can
compensate for flanking paths, such as bleed airlines, linkages, fuel
lines, and hydraulic lines.
Figure 3 shows a graph that provides insight into the
selection of AIC or passive mounting systems. This figure is based on two
simplifying assumptions. The first is that the engine behaves like a
velocity source, meaning that engine vibration is unaffected by the dynamic
stiffness of the airframe or mounting system. Generally, the dynamic
stiffness of the engine is significantly higher than the combined dynamic
stiffness of the mounting system and the airframe (1). The second
assumption is that cabin noise and vibration is structure-borne. Testing
shows that the vibration transmitted through the mounting system often
creates up to 90% of the cabin noise and vibration. In figure 3, the dB
reduction in SPL (an vibration) relative to a hard mount is plotted versus
the normalized mount stiffness, which is the ratio r of the dynamic
stiffness of a passive mount to be roughly equivalent to the airframe
dynamic stiffness. The simple analysis used to produce Figure 3 predicts
that a normalized mount stiffness of r = 1.0 provides 6 dB reduction in
cabin noise (or vibration) relative to a rigid mount (also known as a “hard
mount”). With near zero dynamic stiffness at the engine vibration
frequencies, AIC systems can perform significantly better than passive
mounting systems.
Each transmission path must be considered in selecting the
optimal active or passive technology. To fully make this decision,
the designer must have a knowledge of the static and dynamic stiffness
properties of the airframe and engine, and engine vibration levels
in all directions at each mounting point. Also, and possibly more
difficult to acquire, an understanding is required of the dynamics
between airframe acceleration at the mounting points and cabin
noise. If the coupling for a given path is weak, AIC may not be
needed. Admittedly, aircraft designers rarely know all of this
information, even for existing aircraft. For aircraft being designed,
this understanding can be derived from data for existing aircraft.

In general, AIC systems will be hybrid, including active
and passive components. As an example, in many turbofan installations
actuators may be included in the front mounts and be oriented in the radial
and tangential directions relative to the engine coordinate system. The
radial and tangential directions will include elastomer to reduce the force
requirement of the actuators. Typically, the aft mounts and the fore/aft
direction of all the mounts will use passive RTM technology. Hybrid AIC
systems have been flight-tested and have reduced noise by 20 dB in the
cabin (5). In general, these systems reduce the noise globally throughout
the cabin.
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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.
Noise-Control Firms
The
most-used noise-control firm operates as design/build firm and product
manufacturer and distributor in the broad field of noise and sound control.
It should offer a wide range of products and services including:
- Technical publications, product specifications & project
planning guides
- Highly trained, highly skilled, experienced sales team
- Efficient cost estimating team
- Field sales engineering
- Product & system design engineering
- Standard catalogued & inventoried products, many available
for next day shipment
- A full line of "soft" goods: noise absorbing foams,
quilted blankets, baffles & noise blocking vinyls
- A full line of noise control curtain products & accessories
- A full line of metal products including modular panels, wall
panels, silencers, doors, windows and fully assembled rooms
- System engineering & application
- Custom product design
- New state of the art manufacturing facility
- Experienced field installation capability
- Turnkey design/build capabilities
- Technical distributor & contractor support
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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.
| Some
words on this page are commonly misspelled: engineer, egineer,
enineer, engneer, engieer, enginear, enginer, engineel, enginel,
engiegnel, engeigneer, iegngeigneer, eigngiegneer, eigngineer,
engeigner, iegngeigner, eigngiegner, eignginer, engeignear, iegngeignear,
eigngiegnear, eignginear, engeigneel, iegngeigneel, eigngiegneel,
eigngineel, engeignel, iegngiegneer, iegngineer, eignginel, engiegneer,
iegngiegner, iegnginer, eigngeigneer, engiegner, iegngiegnear,
iegnginear, eigngeigner, engiegnear, iegngiegneel, iegngineel,
eigngeignear, engiegneel, iegnginel, eigngeigneel, eniner, engner,
engier, enginr, eginer, 3ng1n3r, 3mg1n3r, eng1ner, enginre, enginere,
engiener, engnieer, enigneer, egnineer, negineer. source, soorce,
soulce, souce, sooce, sorce, solce, sourec, soucre, soruce, suorce,
osurce, sourc, soure, surce, ource.acoustic, aoustic, acoustc,
acousic, acoutic, acoustik, acostik, acustik, acoostik, aciustik,
acostic, acustic, acoostic, aciustic, acounstic, acounstik, acoonstik,
acoonstic, acousi, acousti, acounsti, acosti, acoosti, acoonsti,
aciusti, acusti, aousti, acouti, acoust1c, acoustci, acousitc,
acoutsic, acosutic, acuostic, aocustic, caoustic, coustic. shock,
shovk, shokc, shcok, sohck, hsock. product, prduct, prouct, prodct,
produt, produd, plodud, perdud, pordud, poduct, ploduct, perduct,
porduct, produc, ploduc, perduc, porduc, produtc, prodcut, proudct,
prdouct, rpoduct, roduct. testing, testint, teting, tesing, testng,
testig, tsting, testeigng, testeignt, testiegng, testiegnt, testin,
testen, testan, testeign, testiegn, test1ng, testimg, testign,
testnig, tesitng, tetsing, tseting, etsting, esting. measurement,
measuement, measurment, measureent, measuremnt, measuremet, meaurement,
measrement, miasulemiegnt, measuremiegnt, masuremeignt, masuremiegnt,
mesuremeignt, mesuremiegnt, meesuremeignt, meesuremiegnt, miasuremeignt,
miasuremiegnt, measulemeignt, miasurement, measulemiegnt, masulemeignt,
miasuremant, masulemiegnt, mesulemeignt, miasulement, mesulemiegnt,
meesulemeignt, miasulemant, meesulemiegnt, miasulemeignt, measuremeignt,
masuremant, mesuremant, masurement, meesuremant, mesurement, measulemant,
meesurement, masulemant, measulement, mesulemant, masulement,
meesulemant, mesulement, meesulement, measuremant, neasurememt,
measurememt, measuremetn, measuremnet, measureemnt, measurmeent,
measuerment, measruement, meausrement, mesaurement, maesurement,
emasurement, measuremen, easurement. |
| |
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
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Air flow
resistance
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Heat resistance
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Humidity resistance
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Flame and chemical
absorption resistance
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Ultraviolet protection
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Lightweight
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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
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Viscoelastic surface
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Pressure sensitive
adhesive backing
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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
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Light Weight
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Use under rigorous
environment
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Flame Resistance
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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
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Broad range of
applications
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Multiple layer of
damping/barrier and absorption
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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
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High transmission loss
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Very flexible
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Will not rot, shrink or
cause metal corrosion
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Attractive leather
surface
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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
| ENVIRONMENT
|
TYPICAL
EQUIPMENT
TO BE PROTECTED |
DESIRABLE
ISOLATOR
CHARACTERISTICS |
APPLICABLE
TPC RUBBER ISOLATORS |
Business
Machines
Computers &
Accessories
Copy Machines
Printers |
Memory drums
Electronic components
Structure of Machine
(reduce fatigue due to vibration)
Surrounding personnel
(noise reduction) |
Low natural
freq. (5-15 Hz)
Low cost
Ease of installation
Ozone resistant rubber |
Ring &
Bushing series
Compression Mounts
Bubble Mounts
Plate mounts
Stud/Plate Mounts
Mounting Feet
Pivotal Levelers |
Recreational
Vehicles
Boats, Snowmobiles, Golf Carts,
Trail Bikes, Jet Skis, Motorcycles, Motor Homes, Campers |
Passenger
(safety/comfort)
Structure of vehicle
(reduce fatigue due to vibration)
Surrounding personnel
(noise reduction)
Communication Equipment |
Low natural
freq. (5-15 Hz)
Low cost
Ease of installation
Soft bottoming (snubbing)
Chemical resistant rubber |
Ring &
Bushing
Plate mounts
Stud/Plate Mounts
Fail-Safe Compression
Dome Mounts
515 Series
Universal Mounts,
Stable-Flex Mounts |
Farm
Equipment
Tractors
Harvesters
Planters
Spreader s |
Passenger
(safety/comfort)
Structure of vehicle
(reduce fatigue due to vibration)
Communication Equipment
Mechanical, electrical, hydraulic, &
pneumatic operating equip.
Radiators
Engines |
Low natural
freq. (5-15 Hz)
Low cost
Ease of installation
Soft bottoming (snubbing)
High strength rubber |
Center Bushing
Mounts
Plate mounts
Stud/Plate Mounts
All-Attitude
Fail-Safe Compression
515 Series
Universal Mounts
Dome Mounts
Stable-Flex Mounts |
Construction
Equipment
On Highway/Off
Highway
Compressors
Generator Sets
Power Take Off
Engines
|
Passenger
(safety/comfort)
Structure of vehicle
(reduce fatigue due to vibration)
Surrounding personnel
Communication Equipment
Mechanical, electrical,
hydraulic, & pneumatic
operating equip. |
Attenuate
high frequency road & terrain shock & transient vibrations
High strength rubber |
Universal
Mounts
515 Series
Dome Mounts
Cupmounts
Stable-Flex
Center Bushing Mounts
All-Attitude Mounts |
| Railroads
|
Cargo &
Passengers
Indicators & controls
Mechanical, electrical,
hydraulic, & pneumatic
operating equip.
Communication equipment |
Very high
deflection for
shock attenuation (6" to 8")
For isolation of high freq. transient & high frequency steady-state
vibration, use 20 to 30 Hz isolator |
Cupmounts
Stable-Flex Mounts
515 Series
Dome Mounts
Compression Mounts
Stud/Plate Mounts
All-Attitude Mounts
Universal Mounts |
| Industrial
Machinery |
Metal forming
or cutting equipment, presses, brakes, shears, hammers, grinders,
compressors, machine controls |
Low natural
freq. (5-15 Hz)
Low cost
Ease of installation
Oil resistant rubber |
Leveling
Mounts
Dome Mounts
Compression Mounts
Pneumatic Spring Mnts.
Low Frequency Mnts.
Stud/Plate Mounts
Plate mounts
Ring & Bushing Mounts |
| Medical
Equipment |
Oxygen
Concentrators, Nebulizers, Bipap, Cpap, Low Air Loss Systems |
Low natural
freq. (5-15 Hz)
Low cost
Ease of installation
Ozone resistant rubber |
Bubble Mounts
Stud/Plate Mounts
Plate mounts |
General
Marine
& Small Boats |
Navigation
& communications gear
Sonar, Radar
Engines
Generators
Instruments, indicators, gages, etc. |
Low frequency
vibration
isolation to minimize
energy transmission
High frequency isolation for shock protection
& minimum noise transmission |
Fail-Safe
Compression
515 Series
Dome Mounts
Universal Mounts
Plate mounts
Stud/Plate Mounts
Cupmounts
Stable-Flex Mounts |
Vehicles
Truck
Bus |
Cargo &
passengers
Indicators & controls
Mechanical, electrical,
hydraulic, pneumatic
operating components
Communications equipment. |
Attenuate
high frequency road shock & transient vibrations |
Cupmounts
Stable-Flex
Fail-Safe Compression
Dome Mounts
515 Series
All-Attitude Mounts
Center Bushing Mounts |
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