QUIETING THE ENVIRONMENT WITH
NOISE CANCELLATION TECHNOLOGY
by
Jeffrey N. Denenberg, PhD.
INTRODUCTION
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 as 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 or 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.
Practical application of this technology still had to wait as the electronic technology available at that time was not sufficient 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 noise cancellation. This technology advance has made it feasible to apply Active Noise Cancellation to previously unsolvable problems in low frequency environmental noise at a reasonable cost.
RANDOM NOISE
Noise Sources
Sources of noise exist throughout the environment. One type of noise is due to turbulence and is therefore totally random and impossible to predict.
Engineers like to look at signals, noise included, in the frequency domain. That is, "How is the noise energy distributed as a function of frequency?" These turbulent noises tend to distribute their energy evenly across the frequency bands and are therefore referred to as "Broadband Noise". Examples of broadband noise are the low frequency noise from jet planes and the impulse noise of an explosion.
Figure 1: A Broadband Noise Spectrum
A large number of environmental noises are different. These "Narrow Band Noises" concentrate most of their noise energy at specific frequencies. When the source of the noise is a rotating or repetitive machine, the noise frequencies are all multiples of a basic "Noise Cycle" and the noise is approximately periodic. This "Tonal Noise" is common in the environment as man made machinery tends to generate it (along with a smaller amount of broadband noise) at increasingly high levels.
Figure 2: A Narrow Band Noise Spectrum
Examples of sources of narrow band noise include:
Internal Combustion Engines: in transportation and as auxiliary power sources.
Compressors: as auxiliary power sources and in refrigeration units.
Vacuum Pumps: used to transfer bulk materials in many industries (also carpet cleaning).
Rotating Machines: Imbalances cause vibration and secondary acoustic noise
Power Transformers: Strong magnetic fields cause vibration at harmonics of the power line frequency and secondary acoustic noise.
PASSIVE NOISE CONTROL
Good Engineering Practice
The first line of defense against noise is good design. Machines should be well balanced. Symmetry in design and careful manufacturing can significantly reduce vibration and noise. Turbulence can be reduced by good aerodynamics. High "Q" resonances in structures and gas flows should be avoided.
Field noise problems are often due to poor design and correcting them is difficult as a retrofit.
Energy Absorption
The second line of defense is to absorb noise and vibration energy and control its propagation using passive materials. The use of sound absorbing and rigid materials to reduce noise levels is an effective approach at high frequencies. Below 500 Hz, however, the cost of, weight of, and inefficiencies due to passive sound attenuation often makes this approach ineffective or impractical. Another technique for noise control is therefore required.
NOISE CANCELLATION TECHNOLOGY
Anti-Noise
The idea to create a copy of the noise and use it as "Anti-Noise" to cancel the original dates back to the early part of this century. Figure 3 shows the relationship, in time, of a noise signal, an anti-noise signal and the residual noise that results when they meet.
FIGURE 3: Noise Cancellation
Note that Active Noise Cancellation does not mask the noise; it removes a significant portion of the noise energy from the environment.
Digital Feed Forward
This form of Active Noise Cancellation is shown in Figure 4 as used to reduce the noise in an air duct. This is the classical example application for active noise cancellation and is widely discussed in the technical literature.
FIGURE 4: Feed Forward Cancellation
Here a microphone is placed "upstream" in the duct to get a reference sample of the noise. The effect of the duct on the noise is modeled to produce an anti-noise waveform at the output speaker. A residual microphone is placed downstream in the duct to determine how well the system is operating and the duct model is continuously adjusted to maintain peak cancellation. Feedback Compensation is also required since the anti-noise waveform also propagates backwards along the duct making the reference signal inaccurate. Incorrect feedback compensation results in unstable operation.
These systems can be effective:
Broadband Cancellation: They can cancel broadband noise. This requires causality (the reference signal must give a sufficiently advanced indication of the approaching noise). Noise that correlates with the reference will be cancelled.
Stability Limits: Can readily
achieve
Synchronous Feedback
This advanced technique, developed by G. B. B. Chaplin in the mid 1970's, is very effective on repetitive noise and does not rely on causality. Here, instead of the reference microphone, 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 cancelling these known noise frequencies.
Figure 5 shows the configuration of such a system applied to reduce engine exhaust noise. Its basic operation is described in this application context in the next section.
FIGURE 5: Narrow Band Noise Cancellation
(An Active Muffler)
APPLICATIONS
At Source: Active Mufflers
Passive mufflers are in wide use to control exhaust
noise. Applications include internal combustion
engines (transportation and auxiliary power generators) and vacuum pumps (as
used for transfer of lightweight bulk materials). There are two classes of
these mufflers:
Absorptive - These are the straight through or "glass pack" mufflers and consist of a length of pipe with holes surrounded by fiber glass (or other sound absorbent material) which is enclosed by a larger diameter pipe. They 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.
Reactive/Dispersive - These are the classical passive mufflers found in automobiles today. They work by creating a tortuous path for the exhaust, dissipating some noise energy while spreading the remaining noise energy across frequencies through turbulent flow. A significant amount of exhaust back pressure is created in this process. This back pressure decreases engine efficiency (about 2.5% lost efficiency per inch of mercury (HG) back pressure in a typical diesel engine).
Reactive mufflers can be effective in noise reduction at low frequencies at a reasonable size but only at the cost of high back pressure. They can have less back pressure, but then are also impractically large and heavy.
Figure 5 shows an active muffler system as it would be applied to an engine exhaust. The passive element is a simple straight through glass pack muffler which controls high frequency noise (above 500 Hz). The active muffler is a speaker cabinet which is concentric to the exhaust pipe and 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 the low frequency noise (500 Hz and below). 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 to zero at the noise frequencies. The tachometer signal drives a harmonic generator to internally provide pure tones at the harmonics of the engine's basic cycle (two full revolutions in a "four cycle" engine). This sets up the whole system to concentrate its efforts on the noise from the engine.
THE FILTERED - X LEAST MEAN SQUARE
(LMS) ALGORITHM
Most 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*). The result is then multiplied by an
adaptation rate constant and 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
which 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.
* For a rigorous discussion of the mathematics of LMS see - Widrow
and Stearns, Adaptive Signal
Processing, Prentiss - Hall (1985).
The following table shows actual measured results on a 450 HP, six cylinder, two cycle diesel engine configured for use as an auxiliary electrical power generator.
TABLE 1:
Active Muffler Performance
|
CONDITION |
NOISE |
BACK PRESSURE |
NOTES |
|
Straight Pipe |
123 dbA |
None |
2 meters from exhaust |
|
Conventional Muffler |
89 dbA |
1.5 inches HG |
12 ft long |
|
Active Muffler System |
81 dbA |
None |
2% fuel savings 20% smaller |
Along The Transmission Path: Active Mounts
Controlling vibration at low frequencies in a passive mount requires soft materials. Since the other purpose of the mount is to statically maintain the engine in position it is often impractical to use soft materials. A solution to this problem is to provide an active mount that is compliant (soft) only at the vibration frequencies. Figure 6 shows synchronous cancellation applied to create an Active Engine Isolation Mount accomplishing this goal.
FIGURE 6: An Active Engine Mount
The anti-vibration continuously works against the stiffness of the mount to keep the mount out of the way of the engine at harmonics of the "vibration cycle". No vibration forces are then passed through the mount from the engine and the supporting frame is vibration free. This can greatly reduce secondary acoustical noise which was being generated by vibrating surfaces associated with the supporting frame and is a good solution for both transportation engines and stationary rotating machinery.
At The Ear
HEADSETS
Standard passive hearing protectors are used in many industries when the environmental noise exceeds 85 dbA. They only start to work, however, at frequencies above 250 HZ and are effective at 500 HZ and above. Since much of the noise from rotating machines is below 500 Hz and the passive headset impedes worker communications, a better solution is needed.
The addition of a microphone, inverting amplifier, and speaker in each ear cup of a passive ear defender can extend the range of hearing protection down to 20 HZ. This simple form of noise cancellation headset can be very effective but it has some disadvantages:
Comfort: They are heavy and uncomfortable to wear.
Safety/Productivity: They may actually be too effective. Human speech and normal warning signals are also eliminated.
The use of Synchronous Noise Cancellation in a headset offers some additional benefits in those cases where most of the noise is tonal. Only the tonal noise from the rotating machine is cancelled. The selectivity provided by this approach allows the wearer to still hear warning signals and co-worker speech while reducing the tonal components to the broadband noise floor. If that is a sufficient reduction of the noise level and no additional passive attenuation is needed and the headset becomes light in weight and comfortable to wear. Figure 7 shows a selective noise cancelling headset configuration.
FIGURE 7: A Noise Cancelling Headset
QUIET ZONES
Another approach to "Personal Quieting" is to position a set of speakers and microphones in a region and attach a multi-channel version of the synchronous cancellation system. This can, if carefully engineered, significantly reduce the low frequency tonal noise in that region and create a "Quiet Zone."
A localized quiet zone at a worker's work station can be cost effectively produced by integrating speakers and microphones at head height in a "Silent Seat". Figure 8 shows a Silent Seat with its two anti-noise speakers and residual microphones arranged to provide a quiet zone around the head of the seated individual.
Figure 8: A Silent Seat
Jeffrey N. Denenberg, PhD.
Dr. Denenberg has over 20 years of experience in the electronics, communications, and computer industries. He has worked for Motorola, Bell Laboratories, and ITT prior to joining Noise Cancellation Technologies as Vice President of Engineering and Chief Technology Officer. Dr. Denenberg received his B.S. from Northwestern University in 1966 and both an M.S. and Ph.D. from the Illinois Institute of Technology in 1968 and 1970 respectively, all in Electrical Engineering. He holds 11 patents and is a Senior Member of the IEEE.