Energy Savings Through the Use of
Active Noise Cancellation
by
Jeffrey
N. Denenberg, Ph.D. and Jonathan M. Charry, Ph.D.
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ABSTRACT
Active noise cancellation technology has already resulted
in significant reductions of the noise
level from a wide range of equipment. A
surprising benefit of this technology is often a significant improvement in
energy efficiency. Active noise control
allows the removal of inefficiencies in equipment that resulted in the original
design due to the effort to control low frequency noise through passive
techniques. This promise of energy
savings in industrial and commercial equipment has resulted in Active Noise
Cancellation being included in the Energy Policy Act of 1992.
The application of Active Noise Cancellation (the use of an
equal, but opposite, anti-noise to eliminate noise) to the control of low
frequency noise was proposed over 50 years ago. But at that time it was quite impractical due to the complexity
of the electronics required. Since that
time, Digital Signal Processing technology has had the same rapid advances as
those in general purpose computer technology.
As a consequence, Active Noise Cancellation is now a practical solution
to many previously difficult problems in environmental noise. Several demonstrations of this advance are
now being prepared for commercial use and have demonstrated up to 25% energy
savings. Other applications are already
in commercial use. Applications
described in this paper include: Industrial Silencers (for positive
displacement vacuums/blowers and stationary internal combustion engines),
Transportation Engine Exhaust Mufflers, HVAC Equipment and Active Engine
Mounts.
INTRODUCTION
The Energy Policy Act of 1992 (see inset) directs in section
173 that active noise and vibration cancellation technologies using fast
adapting algorithms be studied to assess their performance, cost effectiveness,
and impact on energy efficiency. This
is due to indications that these technologies can lead to significant energy
savings when compared to passive noise and vibration control techniques in many
commercial applications.
Environmental noise has been a problem since the industrial
revolution. Noise affects our health
and safety, interferes with our communications, and adversely impacts our
quality of life. High frequency noise
and vibration are amenable to passive controls. However, it is difficult to passively control low frequency (500
HZ and below) noise and vibration.
Efforts to control low frequency noise and vibration has required
engineering compromise and often results in heavier, bulkier, less efficient,
and poorer performing equipment. Active
Noise Cancellation allows the reevaluation of those design decisions and can
therefore provide quiet and efficient equipment.
Active Noise Cancellation is not a new idea. Creating a copy of the noise and using it to
cancel the original dates back to the early part of this century. The first systems used a simple "delay
and invert" approach and showed some promise, but the variability of real
world components limited their effectiveness.
In the mid 1970's a major step forward took place with the
application of adaptive filters to generate the anti-noise. This greatly enhanced the effectiveness of
the systems because they could continuously adapt to changes in their external
world as well as changes in their own components. A second breakthrough in the mid 1970's was the recognition that
many noise sources, particularly those produced by man-made machines, exhibit
periodic (tonal) noise. This tonal
noise allows a more effective solution as each repetition of the noise is
similar to the last and the predictability of the noise allows creation of an
accurate anti-noise signal.
ENERGY POLICY ACT OF 1992
Public Law 102-486 --- Oct. 24, 1992
Section 173 --- Study and Report on Vibration Reduction Technologies
(a) In General. --- The Secretary shall,
in consultation with the appropriate industry representatives, conduct a
study to assess the cost effectiveness, technical performance, energy
efficiency, and environmental impacts of active noise and vibration
cancellation technologies that use fast adapting algorithms.
(b) Procedure.
--- In carrying out such study, the Secretary shall ---
(1) estimate
the potential for conserving energy and the economic and environmental
benefits that
may result from implementing
active noise and vibration abatement technologies in demand side
management; and
(2) evaluate
the cost-effectiveness of active noise and vibration cancellation
technologies as
compared to other alternatives
for reducing noise and vibration.
(c) Report.
--- The Secretary shall transmit to the Congress, not later than 12 months
after the date of the enactment of this Act, a report containing the findings
and conclusions of the study carried out under this section.
(d) Demonstration.
--- The Secretary may, based on the findings and conclusions of the study
carried out under this section, conduct at least one project designed to
demonstrate the commercial application of active noise and vibration
cancellation technologies using fast adapting algorithms in products or
equipment with a significant potential for energy efficiency.
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Practical
application of this technology still had to wait since the electronic
technology available at that time was not mature enough for implementation of
Active Noise Cancellation systems. Now
digital computer technology has evolved to the point where cost effective
Digital Signal Processing (DSP) microcomputers can perform the complex
calculations involved in Active Noise Cancellation. This technology advance has made it feasible to apply Active
Noise Cancellation to cost effectively solve previously difficult problems in
low frequency environmental noise.
NOISE
Industrial noise sources tend to simultaneously generate
noise with two distinct characteristics.
One characteristic noise is due to turbulence or grinding. This noise is distributed evenly across the
frequency bands and is referred to as “Broadband Noise.” “Narrowband Noise” concentrates most of its
noise energy at specific frequencies.
When the source of this noise is a rotating or repetitive machine, the
noise frequencies are all multiples of a basic “Noise Cycle” and the noise is
approximately periodic and consists of a set of tones. This “Tonal Noise” is common in the
environment as man made machinery tends to generate it (along with some broadband noise) at
increasingly high levels.
FIGURE
1: A Typical Noise Spectrum (A Positive
Displacement Blower)
Equipment noise often has multiple sources. Noise sources in equipment such as a truck
include:
A two
stroke engine produces a noise like that in Figure 1, but with many tonal
components at multiples of the rotational rate (multiples of one half that rate
with a four stroke engine). This noise
is radiated from the exhaust outlet as well as directly from the engine
itself. The exhaust also contains a
significant amount of broadband noise since many mufflers generate turbulence
as part of the process of reducing the low frequency pulsations in the exhaust.
The engine cooling fans are another source of noise. This noise includes tonal components at harmonics of the blade
passage rate as well as broadband noise due to turbulent air flow.
When the truck is at highway speed, tire noise can be a significant
factor. It can contain tonal components
due to the tread design as well as broadband energy due to a rough road
surface.
Many of these noise sources can be reduced by active noise
cancellation.
ACTIVE NOISE CANCELLATION
The idea of creating a copy of the noise and using it
as “Anti-Noise” to cancel the original dates back to the early part of this
century. Figure 2 shows the
relationship in time of a noise signal, an anti-noise signal and the residual
noise that results when they meet.
FIGURE
2: Noise Cancellation
Note that
Active Noise Cancellation does not mask the noise; it removes a significant
portion of the noise energy from the environment.
The Synchronous Feedback noise cancellation technique,
developed by G. B. B. Chaplin in the mid 1970's and recently improved and
productized by Noise Cancellation Technologies, is very effective on repetitive
noise and does not rely on causality.
In this approach a tachometer signal is used to provide information on
the rate of the noise. Since all of the
repetitive noise energy is at harmonics (or multiples) of the machine's basic
rotational rate, the DSP microcomputer can dedicate its resources to canceling
these known noise frequencies.
AN ACTIVE CANCELLATION SYSTEM
Figure 3 shows an active
muffler system. The passive element is
a simple straight through glass pack muffler that controls high frequency noise
by absorbing the noise energy above 500 Hz.
The active element is a speaker cabinet mounted at the end of the
exhaust pipe. It outputs the anti-noise
in a ring around the end of the exhaust.
The symmetry of the noise and anti-noise sources in this arrangement
provides for global cancellation of low frequency noise (500 Hz and
below). The active and passive noise
control technologies are therefore complementary and together form a practical
solution. A microphone in the exhaust
noise sound field feeds back the RESIDUAL noise (after cancellation) so that
the ADAPTER (usually a LMS adaptation algorithm -- see inset) can continuously
adjust the cancellation to drive the RESIDUAL noise toward zero at the noise
frequencies. The tachometer signal
drives a harmonic generator to internally provide pure tones at the harmonics
of the basic cycle of the noise source (usually two full revolutions). This sets up the whole system to concentrate
its efforts on the noise from the source and ignore other sounds.
FIGURE 3: Synchronous
Cancellation in an Active Muffler
ENERGY SAVINGS
Energy savings can be obtained through the use of Active
Noise Cancellation technology. Savings
are due to the removal of inefficiencies and/or weight that were designed into
the equipment in an attempt to control low frequency noise. Discussion of particular applications best
illustrates the energy savings that can be expected.
Mufflers and Silencers
An active muffler (called silencers in industrial
applications) was used in the introduction to describe active noise
cancellation technology. It can provide
significant energy savings through the reduction of back pressure or flow
resistance in the exhaust pipe.
Classical mufflers quiet the exhaust pulsations from an engine (or
blower) by forcing the flow through a torturous path. The resulting turbulent flow tends to dissipate some of the noise
energy and result in a more acceptable output noise level. The resistance to exhaust flow must be
overcome by the engine (or blower) doing work.
This is wasted energy. The
amount of energy wasted varies with application. The electrical energy used in the cancellation process is only a
small fraction of the energy saved.
Some general guidelines are:
Diesel Engine - The energy wasted in a
typical diesel engine due to muffler back pressure is 0.7% for each 10 inches
of water pressure drop. This commonly
results in a 2% energy loss that can be recovered through the use of an active
muffler system.
Automobile Engine - The typical savings in an American car due
to reduced back pressure are 5% for city driving (due the times the engine is
pushed to peak power) and 1% to 2 % under highway driving conditions.
Positive Displacement
Blowers - These blowers are used in many industrial applications (See the
Appendix at the end of this paper for a specific example). Here the air flow is the primary work output
of the machine and any flow restriction has a larger impact on energy
efficiency. Savings of up to 20 %
have been obtained.
Fans
and HVAC Equipment - Energy savings here come from two mechanisms.
Reduced flow restrictions
- Passive duct noise control involves lining ducts with sound absorptive
material. It must be thick to control
low frequency noise. Since most
installations are space limited, the resulting ducts have a small cross
sectional area and produce a pressure drop.
This makes the fan work harder and wastes energy.
Impeller Design - Fan
blades can be a significant source of noise.
For years fans have been used that produce acceptable noise but are
inefficient movers of air. Efficient
fan blade (or impeller) designs tend to produce irritating tonal noise
components at low frequencies. Since
this noise can be controlled by an active cancellation system, an efficient
impeller design can now be used. Energy
savings of up to 30 % have been obtained in prototype systems.
ACTIVE NOISE
CANCELLATION ELECTRONICS
The cancellation
algorithm is executed in a modern DSP computer that fits on one 250 cm2
printed circuit board.
Included on this board are:
A digital signal processing computer (such as the ADSP-2101)
capable of 10 million
operations per second.
Two Low Pass filters set at 500 Hz to avoid aliasing, the
confusion of high frequency
signals with low
frequency signals due to sampling.
An A/D converter to measure the noise
remaining after cancellation.
A D/A converter to output the
Anti-Noise.
The electronics often
includes an audio power amplifier to generate the required anti-noise
power. Since the anti-noise is at low
frequencies, a “Class D” (switching)
amplifier can be used to further enhance energy efficiency.
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Engine Vibration Control
A primary
consideration in modern automobile design is fuel economy. The new engines are therefore smaller and
have fewer cylinders than the engines of a few years ago. Limiting this trend is the desire for a
smooth running car. It is difficult to
balance the vibrational forces in an engine (especially the component at twice
the engine rotational rate) with a small number of cylinders. One option is to use counter rotating shafts
inside the engine that have a slight imbalance and spin at twice the engine
RPM. The forces generated by the extra
shaft can be designed to cancel the undesired vibration. The drawback to this approach is that it
uses a significant percentage of the engine power (up to 10% has been estimated
in some engines) by adding weight and friction losses.
An
alternative is to use an Active Engine Mount.
Here an active vibration
control system dynamically adjusts the dimensions of each engine mount so that
engine vibrations are isolated from the car chassis. Again the energy needed by the electronics is much smaller than
the energy that would be lost using the passive solution and a more energy
efficient car is the result.
THE
CANCELLATION ALGORITHM
Active Noise
Cancellation Systems employ a variant of the LMS algorithm known as
“Filtered-X”. The basic LMS algorithm
correlates an error signal (the Residual Noise in this case)
with a reference signal (called “X” by Widrow and Stearns[JND1] ). The result is then multiplied by an
adaptation rate constant and then used to adjust the relevant parameter of
the adaptive filter. This is done
repeatedly for each filter parameter with the objective being convergence to
an operation that minimizes the average power in the error signal.
In
real world systems, however, the LMS algorithm does not converge due to the
delay and gain effects of the physical path taken by the anti-noise
signal. Using a compensating filter
on the
reference signal (hence the name “Filtered-X”) restores stability and
produces a well-behaved system.
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Aircraft Cabin Quieting
Turboprop aircraft have noisy interiors. The noise is generated by pressure pulses
coming off the propellers and striking the skin of the aircraft. The airframe skin vibrates and in turn
generates low frequency noise at harmonics of the propeller blade rate (80 to
100 Hz). The airframe manufacturers
have tried to solve this problem by either stiffening the airframe or adding
harmonic dampers at strategic points on the airframe. These solutions add weight to the airframe (125 lbs to 200 lbs
for a 39 passenger plane). Since the
additional weight (and volume) matches that of a passenger, it therefore
significantly increases the fuel usage per passenger. Active cabin noise quieting systems have been flown on several
different aircraft that demonstrate that the noise can be reduced 10 dB by this
technique. Since such a system would
weigh about 25 Kg (electronics, cables and speakers) the result is a more
energy efficient aircraft.
ENVIRONMENTAL IMPACT
A significant secondary gain from improving performance of
operating equipment is the resultant improvement in air quality that is
directly tied to decreased fuel consumption. Modeling the impact on C02
reduction using an active electronic muffler on a diesel powered vacuum pump
yields some interesting results. When
the electronic muffler is tied to a more strategic approach to conserve energy
by using a cleaner driver (i.e. an electric motor vs. a diesel combustion
engine), numerous benefits occur. CO2
reduction (tons/yr) increases dramatically, and performance economies accrue to
users of the technology even though the actual demand for electric power into
the grid system increases. This is
because the amount of fuel required to centrally generate electricity is far
less than that required for individual stationary or mobile equipment. (see
Figure 4, below).
|
|
Electrical Power
Savings
kwhr / year
|
Net Fuel
Savings
BOE / year
|
Net CO2
Reduction
Tons / year
|
|
Electric Blower
|
4,200
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7
|
3
|
|
Diesel Driven Blower
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N / A
|
24
|
12
|
|
Switch From Diesel to
Electric Drive
|
N / A
|
74
|
37
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Figure 4: Environmental Effects of Different
Strategies for
Using Active Noise Cancellation
Technologies
in the Blower Application
Assumptions:
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Motor Power
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50 hp
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Fuel Usage
|
4 gal / hr
|
|
Electric Motor Size
|
35 kW
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Active Muffler Savings
|
20%
|
|
Duty Cycle
|
60%
|
BTU / gallon of Fuel
|
5,800,000
|
|
Usage / year
|
1000 hrs
|
BTU per BOE
|
10,000
|
|
lbs of CO2
per BTU
|
0.00017
|
|
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PRODUCTIVITY GAINS
Another factor to consider in the use of Active Noise
Cancellation Technology is the productivity improvement that comes along with
the energy savings. These improvements
accrue to both worker cost and the capital equipment costs.
Personnel are no longer subjected to the effects of
excessive noise and often the need for hearing protection can be reduced. The reduced fatigue and enhanced safety of
the work environment lead to higher worker productivity.
The capital equipment performs better after applying active
noise cancellation technology. The
industrial blower detailed in the example application is an expensive
machine. It now can do 20 % more work
in a day. This leads to significant
savings in depreciation costs per year.
These savings can even show up in a vehicle as the car or truck can now
accelerate faster and handle hills better.
CONCLUSION
Active noise cancellation is now an effective approach
to solving many environmental noise problems created by man-made
machinery. Where standard passive
approaches are ineffective, costly or energy inefficient, Active Noise Cancellation can significantly
reduce noise and vibration while, in many cases, improving the overall energy
efficiency of the machine.
The rapid advances in computer technology during the
last decade have now made it feasible to do the sophisticated calculations
required to implement effective noise cancellation using a low cost digital
signal processing computer. Many
applications are now practical.
Applications that have been shown to save energy are:
Active industrial silencers and active
mufflers for transportation engines -- by reducing back pressure
Active mounts for industry and
transportation -- by reducing weight and the need for balance shafts
Active fan silencers -- by allowing the
use of efficient impellers and/or blade designs
Active HVAC noise control -- by allowing
larger cross section air ducts in space limited installations.
Active noise cancellation products for these applications as
well as others in which noise reduction, energy use, and air pollution have
been a problem are now moving from the laboratory into commercial products.
ADDITIONAL READING
C. Ross, “Quieter Air Travel Takes off With Active
Noise Control Technology,” Noise and Vibration Worldwide, scheduled to be
published shortly.
D. P. Mendat, et. al., “Active Control of
Centrifugal Fan Noise,” Fan Noise - An International INCE Symposium, CETIM
Senlis, France, pp. 455-462 (1-3 September, 1992).
J. M. Charry, “Cutting Industrial Energy Use and
Emissions with Noise,” Demand Side
Management: Completing the Picture - Highlights of a National Conference on
Energy Efficiency, Environment, Profits and Growth with Electrotechnologies, June 17, 1992, Denver Colorado.
J. N. Denenberg, “Anti-Noise, Quieting the
Environment with Active Noise Cancellation Technology,” IEEE Potentials, Volume 11, No. 2, pp. 36-40
(April, 1992).
Kh. Eghtesadi, M. McLoughlin, and E. W. Ziegler,
“Development of the Simulation Model of the Multiple Interacting Sensors and
Actuators (MISACT) for an Active Control System”, Recent Advances in Active
Control of Sound and Vibration, Edited by C. A. Rogers and C. R. Fuller,
Virginia Polytechnic Institute, pp.
246-257, (15-17 April, 1991).
C. Ross, “The Control of Noise Inside Passenger
Vehicles”, Recent Advances in Active Control of Sound and Vibration,
Edited by C. A. Rogers and C. R. Fuller, Virginia Polytechnic Institute, pp. 671-681, (1517 April, 1991).
Proceedings of NOISE-CON 90, “Reducing the
Annoyance of Noise”, University of Texas at Austin (October, 1990).
Air Movement and Control Association, Inc., Fans and Systems, ACMA Publication 201-90, (1990)
Kh. Eghtesadi and J. W. Gardner, “Experimental
Results of the Non-Restrictive Electronic Muffler on Internal Combustion
Engines”, Acoustics Letters, Volume 4, Number 4, pp. 70-73 (1990).
P. A. Nelson, “Causal Constraints in the Active
Control of Sound”, IEEE Conference on Acoustic Speech and Signal Processing,
Volume 1 (April, 1987).
B. Widrow and S. D. Stearns, Adaptive Signal
Processing, Prentiss-Hall (1985).
G. B. B. Chaplin, “Anti-Noise - the Essex
Breakthrough”, Chartered Mechanical Engineering, Volume 30, pp. 41-47
(1983).
APPENDIX A
A Diesel Driven Vacuum Transfer
System
Custom designed for CSX, a
large rail transportation company on
the east coast of the United States, the MasterVac is a vacuum transfer
system. It is used to load and unload dry
bulk material (such as grain and plastic pellets) between railroad cars and
trucks at the CSX Transportation Bulk Inter modal Distribution Services
(CSXT/BIDS) terminals. The MasterVac
features a 100 horsepower diesel engine that drives a powerful, positive
displacement, vacuum pump attached to a long, flexible, 6 inch diameter,
stainless steel pipe. The air flow
propels the material into a separation bucket that is periodically emptied into
the target vehicle.
The CSXT
MasterVac
The
system, as manufactured, has excessive noise.
At the operator's position (next to the diesel engine) the noise was
measured at 115 dBA. Even with
hearing protection, OSHA safety rules limited the operator exposure to the
machine to 15 minutes a day. It
takes about an hour to fill or empty a railroad car with the MasterVac as
originally designed.
The MasterVac Active Silencer reduces the three major noise
tones by over 30 dB. The overall
results as measured at the operator's position (near the diesel engine) are
shown in Table 1.
TABLE
1: CSXT MasterVac Active
Silencer Performance
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CONDITION
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NOISE LEVEL
(operator’s position)
|
BACK PRESSURE
|
NOTES
|
|
Original System
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115 dBA
|
3 inches HG
|
Passive silencer on vacuum exhaust
|
|
With Passive
Engine Enclosure and Active Silencer System on Blower Exhaust
|
91 dBA
|
Negligible
|
24% fuel savings
20% faster operation
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The Solution
There
were opportunities to improve the MasterVac using passive techniques. Since a good noise control system is usually
a combination of active and passive techniques, passive treatments were applied
before considering active noise cancellation.
The diesel engine was originally enclosed
in a sheet metal housing. This housing
was rebuilt using sound absorbing materials. This reduced the noise from the
engine block as well as that from the cooling fan
The muffler on the diesel engine exhaust
was ineffective and was replaced with a high performance, low back pressure
unit.
With these changes the diesel engine noise was reduced to a
manageable level, leaving the primary noise source: the vacuum outlet.
A standard passive silencer was used on the vacuum exhaust
in the original design in an attempt to control the vacuum outlet noise. It was a baffled unit similar to those
commonly found on engine exhausts (but significantly larger). Baffler silencers are not effective enough
at low frequencies and create additional turbulent noise that makes active
noise cancellation more difficult, so we looked at other possibilities.
The best alternative was a straight through or “glass pack”
muffler. It is similar to those used in
the “Hot Rods” of the 50's. This
absorptive silencer consists of a length of pipe with holes wrapped by fiber
glass (or other sound absorbent material) which is then enclosed by a larger
diameter pipe. Absorptive silencers are
very effective at high frequencies, reduce turbulence in the exhaust, and
produce little or no back pressure.
They have little effect, however, on the low frequency tonal noise in
these applications unless the muffler is made impractically large and heavy.
We therefore chose a high quality absorptive silencer to
handle the high frequency and turbulent noise in the outlet air stream and left
the three major tonal components for the Active Cancellation System.
The Active Silencer System
The three speaker active
silencer used on the MasterVac is shown below.
Each 12 inch speaker is driven by a 100 Watt audio amplifier to produce
the required peak anti-noise to match the noise from the blower at full
load. Approximately one horsepower of
engine power is used to generate the required electrical energy. This is far less than the approximately
20 horsepower saved by eliminating the back pressure in the original
passive silencer.
The MasterVac Active Silencer
This solution to the MasterVac noise problem was first
demonstrated by Noise Cancellation Technologies, Inc. in the Fall of 1990 at
CSXT/BIDS terminal in Baltimore Maryland. The use of minimal hearing protection
now allows a worker to accomplish a full shift without the threat of hearing
damage. An operator's enhanced ability
to hear other sounds also makes the work site safer. Roger Posey, Operations Manager for CSXT/BIDS says, “The noise
from the original MasterVac can be likened to a jet plane during takeoff ...
now, with the NCT Active Industrial Silencer, the MasterVac is much quieter,
performs better, and we are seeing significant fuel savings. Workers are happier and we have eliminated
the possibility of noise complaints from the surrounding community.” Active Industrial Silencers are now installed on 20 MasterVacs and are planned
for all 80 units owned by CSX Transportation.
