Energy Savings Through the Use of
Active Noise Cancellation




Jeffrey N. Denenberg, Ph.D.
Vice President, R & D
Noise Cancellation Technologies, Inc.


Jonathan M. Charry, Ph.D.
Active Acoustical Solutions, Inc.



This is a preprint of a chapter from the upcoming (Summer, 1993) Fairmont Press publication, The Guide To The Energy Policy Act of 1992 edited by
L. Good and D. Williams.








800 Summer Street,  Stamford,  CT,  06901

Tel.  (203) 961-0500,  FAX.  (203) 348-4106

Energy Savings Through the Use of
Active Noise Cancellation


Jeffrey N. Denenberg, Ph.D.
Vice President, R & D
Noise Cancellation Technologies, Inc.

Dr. Denenberg has over 20 years of experience in the electronics, communications, and computer industries.  He has worked for Motorola, Bell Laboratories, ITT, and Prodigy Services Co. prior to joining Noise Cancellation Technologies in 1990 as Vice President, R&D and Chief Technology Officer.  Dr. Denenberg received his BS. from Northwestern University in 1966 and both a M.S. and a Ph.D. from the Illinois Institute of Technology in 1968 and 1970 respectively, all in Electrical Engineering.  He is a Senior Member of the IEEE and holds 11 U. S. patents.


Jonathan M. Charry, Ph.D.
Active Acoustical Solutions, Inc.

Dr. Charry has more than 15 years of experience in assessing environmental and economical impacts of new technology in the energy industry.  He served as President and CEO of Environmental Research Information, Inc. for over ten years, prior to that was an assistant professor at The Rockefeller University and before that a Rockefeller Foundation Fellow in Environmental Affairs.  He has been a principal investigator on research grants and contracts from the National Institute of Health and the Electric Power Research Institute.  Dr. Charry is Past President and Director of the American Institute of Medical Climatology as well as Past Chairman of the Section on Air and Other Environmental Ion Technology of the American Society of Testing of Materials.  A Member of several professional societies and associations, including the New York Academy of Sciences and The Rockefeller University chapter of Sigma Xi, Dr. Charry received a Ph.D. from New York University and an A.B. from Tufts University.





800 Summer Street,  Stamford,  CT,  06901

Tel.  (203) 961-0500,  FAX.  (203) 348-4106

Energy Savings Through the Use of
Active Noise Cancellation


Jeffrey N. Denenberg, Ph.D. and Jonathan M. Charry, Ph.D.


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.


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.

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.

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.


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.


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.


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 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.


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.

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.



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.

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.


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
BOE / year

Net CO2 Reduction
Tons / year

Electric Blower




Diesel Driven Blower

N / A



Switch From Diesel to Electric Drive

N / A



Figure 4:  Environmental Effects of Different Strategies for
 Using Active Noise Cancellation Technologies
 in the Blower Application


Motor Power

50 hp

Fuel Usage

4 gal / hr

Electric Motor Size

35 kW

Active Muffler Savings


Duty Cycle


BTU / gallon of Fuel


Usage / year

1000 hrs



lbs of CO2
per BTU




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.


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.


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).

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


(operator’s position)



Original System

115 dBA

 3 inches HG

Passive silencer on vacuum exhaust

With Passive Engine Enclosure and Active Silencer System on Blower Exhaust

91 dBA


24% fuel savings
 20% faster operation

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.


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