| INDEX | NOISE CONTROL FAQ | GLOSSARY OF TERMS | SOUNDPROOFING |
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.
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.
What are the benefits
of active control?
The many practical benefits of active noise control technology are not all obvious
at first glance. The main payoff, of course, is low-frequency quieting that
would be too expensive, inconvenient, impractical, or heavy by passive methods
alone. For example, the lead-impregnated sheets used to reduce aircraft cabin
propeller noise impose a severe weight penalty, but active control might perform
as well with a much smaller weight penalty.
Other possible benefits reflect the wide range of problems on which active
control can be applied. For instance, with conventional car mufflers the
engine spends extra energy to push exhaust gasses through the restrictive
muffler passages. On the other hand, an active control muffler can perform
as well with less severe flow restrictions, thus improving performance
and/or efficiency. Additional benefits include:
Of these, the potential for reduced maintenance and increased material fatigue life have received new emphasis in 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.
Sound Perception
and Sound Pressure Level
Air-conveyed sound is defined as variation in air pressure, which at sea
level is an average of 101325 Pascal (Pa). The human ear can perceive
pressure variations between the hearing threshold p0 =0.000'02 Pa and
the pain threshold of 20 Pa. The ear can therefore cope with a pressure
range of 1:1 million! The sound pressure level Lp is the log of the ratio
of the amplitude p of a sound event and the hearing threshold p0, and
is expressed in decibels (dB): Lp = 20 log(p/p0). The human hearing range
between 0 and 120 dB.
Sound-measurements
Sound-measuring instruments consist of a microphone, which converts sound
pressure into an electrical signal, and electronics for processing the
signal and displaying the measured values. An instrument which combines
both of these functions is the simple sound level meter. Instruments which
are equipped with additional functions for time average value display,
are called integrated sound level meters. It is also of interest in sound
measurements to display the composition of a sound in terms of its individual
frequency components. Third-octave and octave filters are used for this
measurement. And finally, sound analysers are instruments which combine
the above-mentioned functions with additional characteristics in the one
instrument. For the calibration of acoustic measuring equipment, sound
sources (calibrators) are used, which emit a constant sound pressure level
at one or several frequencies.
Pattern
Evaluation and Approval
Measuring instruments for acoustics are needed principally for combating noise,
product control and measurement of human hearing acuity. Instruments used for
official purposes, i.e. for the implementation of Federal Ordinances, must comply
to international requirements. The Euronoisecontrol
- GEIE tests such instruments or accepts the test results of equivalent
foreign laboratories. If the test results meet the requirements, The Euronoisecontrol
- GEIE issues a conformity certificate for the respective instrument model.
In addition, sound measuring instruments must be verified before their release
for use (initial verification) and subsequently every 2 years. The verifications
are done by authorised verification laboratories or by Euronoisecontrol
- GEIE.
An
information sheet issued by Euronoisecontrol
- GEIE provides information concerning
the official approval and verification of acoustic measuring instruments.
The requirements on acoustic measuring equipment are stipulated in the recommendations of the International Electrotechnical Commission (IEC) and the scope of periodic verifications in the recommendations of the Organisation Internationale de Métrologie Légale (OIML). The reference values for the calibration of headphones and bone-conduction attenuators used in audiometers are stipulated in the ISO standards.
Device
Approval Verification:
Microphones IEC 61094 Serie
Soundlevel meters IEC 60651 OIML RI 58
Integrating soundlevel meters IEC 60804 OIML RI 88
Filters IEC 61260 OIML (draft)
Calibrators IEC 60942 OIML RI 102
Audiometers IEC 60654 Serie OIML RI 104
IEC reference couplers (1-inch microphone) for the calibration of headphones.
IEC Publication 60303
IEC artificial ear, broadband type (1/2 inch microphone). IEC 60318
IEC mechanical earphone coupler for the measurement of bone-conduction
transducers. IEC 60373
Standard reference zero for the calibration of pure-tone air conduction
audiometers ISO 389
Standard reference zero for the calibration of pure-tone bone conduction
audiometers ISO 7566
Realisation
of the Unit of Sound Pressure
The determination of the relationship between the sound pressure on the
membrane of a microphone and the output voltage of its electrical connector
creates the basis for sound measurement. The transmission constant of
a laboratory standard microphone is determined at METAS by means of the
so-called Reciprocity Method (IEC 61094, Part 2). The acoustic unit, the
pascal, is derived from mechanical and electrical units. For the dissemination
of the pascal, sound calibrators and standard microphones are used, which
by means of purpose-built measuring systems are traceable back to the
national sound pressure standard.
Some technical terms:
Acoustics
Acoustics Laboratory
Sound Pressure
Sound Measurements
Sound Measuring Instruments
Sound Perception and Sound Pressure Level
acoustical
treatment,acoustical treatments,acoustic silencer,acoustic
silencers,acoustic panel,acoustic panels,soundproofing, sound absorbent,sound
proof
booth,sound proof booths,acoustic planning,noise control,acoustic
system,acoustic systems,industrial sound-proofing,anti-noise,acoustic
pollution,decibel,acoustic protection, noise-proofing,noise reduction,noise,panel,sound
absorbent panel,sound absorbent panels,isolation,isolations,sound-proofing
EURONOISECONTROL
- GEIE
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