Fundamentals and Applications of Active Noise Control
(Grundlagen und Anwendungen des
„Aktiven Schallschutzes“)
erstellt am 1.11.2002
Institut für Entsorgungs- und Deponietechnik (IED) Montanuniversität Leoben
Vorgelegt von: Betreuer:
Rainer Mirau, 9535148 O.Univ.Prof.Dipl.-Ing.Dr. Karl E. Lorber Emil Kraft Gasse 13/2 Dr. Karl-Heinz Gresslehner
2500 Baden
Leoben, am 5.11.2002
TABLE OF CONTENT
1 INTRODUCTION... 4
2 AIMS AND SCOPE... 6
3 HISTORICAL REVIEW... 7
3.1 Overview... 7
3.2 Development of ANC... 7
4 FUNDAMENTALS... 13
4.1 Fundamentals of acoustics/sound... 13
4.1.1 Sound waves... 14
4.1.2 Frequency analysis... 15
4.1.3 Sine and Cosine waves... 15
4.1.4 Fourier Analysis... 16
4.1.5 Harmonics... 17
4.2 Fundamentals of Vibration... 19
4.2.1 Simple freely vibrating spring-mass system... 19
4.2.1.1 Without damping... 19
4.2.1.2 Viscous damping... 21
4.3 Fundamentals of ANC... 23
4.3.1 Structures of ANC... 25
4.3.1.1 Single channel ANC system... 25
4.3.1.2 ANC with combined error/reference microphone... 26
4.3.1.3 Multichannel ANC... 27
4.3.2 Interference... 27
4.3.2.1 Constructive Interference... 27
4.3.2.2 Destructive Interference... 28
4.3.2.3 Impedance coupling... 29
4.3.3 Feedforward and Feedback... 30
4.3.3.1 Feedforward... 31
4.3.3.2 Feedback... 33
4.3.4 Active Vibration Control... 34
4.3.4.1 Fundamentals... 34
4.3.4.2 Smart materials and structures... 35
5 APPLICATION... 37
5.1 Introduction... 37
5.2 Transformer noise reduction... 40
5.2.1 Introduction... 40
5.2.2 Measurement of sound field... 41
5.2.2.1 Control achieved by sound intensity minimization... 42
5.2.2.2 Control achieved by squared pressure minimization... 44
5.2.2.3 Effect of error sensing strategies on the control... 44
performance 5.2.3 Experimental results... 45
5.2.3.1 Evaluation of control performance at the monitor sensors... 45
5.2.3.2 Effect of the number of control sources on the control... 47
results 5.2.4 Control mechanism... 48
5.2.5 Conclusions... 49
5.3 Applications for cars... 50
5.3.1 Introduction... 50
5.3.2 How far is automobile industry?... 50
5.3.2.1 Electronic Controlled Active Silencer Exhaust System for... 50
Car Engines 5.3.2.2 Active noise control inside a car with advanced speakers... 52
5.3.2.2.1 Introduction... 52
5.3.2.2.2 Active control of power train noise... 53
5.3.2.2.3 Active control of simulated road noise... 58
5.3.2.2.4 Conclusions... 61
5.3.3 Some commercial approaches of the car industry... 61
5.3.3.1 System by Siemens VDO in the air intake... 61
5.3.3.2 ANC as standard equipment in Honda cars... 62
5.4 Industrial applications... 64
5.4.1 Active Exhaust Silencer... 64
5.4.2 ANC in a large exhaust stack... 66
5.4.2.1 Introduction... 66
5.4.2.2 Physical system design... 68
5.4.2.2.1 Control source location... 69
5.4.2.2.2 Control source equipment... 70
5.4.2.2.3 Error sensor location... 71
5.4.2.2.4 Error signal equipment... 71
5.4.2.2.5 Reference signal considerations... 72
5.4.2.2.6 Reference signal equipment... 73
5.4.2.2.7 Electronic controller... 73
5.4.2.3 ANC trials... 74
5.4.2.4 Conclusions... 77
6 SUMMARY... 78
7 GLOSSARY... 79
7.1 Dictionary... 79
7.2 Internet links... 82
7.2.1 Research... 82
7.2.2 Companies... 84
7.2.3 More noisy links... 85
7.3 References... 86
7.4 Figures... 88
7.4 Tables... 91
1 Introduction
Noise is unwanted sound and is more and more perceived as an environmental pollutant.
This shift in attitude is reflected in stricter legislation and has wide ranging economic consequences. Traditionally, noise was reduced by passive means like damping plates, sound absorbing materials, double-glazing windows, noise barriers etc. All these passive means are mostly suitable for high frequencies and have various disadvantages. In the last decade, active control of sound and vibration (at audio frequencies) has emerged as a viable technology to fill the low-frequency technology gap.
The challenge has been and still is to make active noise control a cost-effective solution to noise problems. This challenge has been far more difficult than many people expected.
When a technology is more expensive than its benefits are generally worth, only a small market exists for the technology.
New technologies are usually expensive to start out. Early users of a technology pay a higher price for being first, with a competitive advantage as the benefit. Technology developers gain valuable experience working with early customers. This experience and know-how yields product improvements that provide more economical solutions for future customers. In this way, the technology spirals into wider and wider use, as the cost for implementation falls.
So, what happened to active noise control? Why don't we see active noise control technology in our daily lives? The answer lies in the fact that most consumers don’t pay more than 100 to 2000€ for noise reduction solutions. This cost range has been difficult to achieve with effective active solutions. The notable exceptions are noise canceling headphones and some dishwashers.
The result is that other applications of active noise control have been in industrial situations, where the benefit for active noise control is driven by hearing safety concerns. In some cases, improving speech communication has been important enough to use active noise control . Even in industrial markets, active noise control technology spirals have been rare.
So, where has active noise control been successfully applied? A partial list:
• Exhaust mufflers for internal combustion engines
• Headphones for aircraft use
• Ambulances
• Dishwashers
• Vibration isolators for engines
• Vibration isolation to protect electronics from vibration and shock
• Aircraft cabin noise reduction
• Furnaces and boiler
These products are successful because they have taken active noise control technology and reduced it to practice. Another common element is that with the exception of cabin noise reduction, the solution is 'simple'. The solution does not require an extensive number of microphones, sensors etc. In many cases the controller that produces anti-sound commands can be built with analog circuits, leading to lower costs and portability.
So where should we go in ANC technology development? In my opinion continued development of simple solutions is best done through education. Present commercial efforts are blocked in patent issues and return on investment questions. Through education, engineers can be trained in the use of active noise control. When these engineers are confronted with design problems, they will be equipped to find niche solutions using active noise control.
Researchers in this field should focus on two approaches. The first is the development of integrated active control solutions for complex noise problems (industrial noise) and the second are automobile audio systems, with integrated personal communications systems.
Active noise control technology could improve our environment as well as driving enjoyment and safety.
2 Aims and scope
This study describes the fundamentals and development of active noise and vibration control as well as some new technical applications which took place in recent years. In Austria ANC is not widely known, though several applications could help solving problems caused by noise and/or vibration.
This work is not a practical scientific study, the aim is to show the worldwide state of the art in active noise technology, especially under the environmental point of view. Which opportunities gives us ANC? Where is it used today commercially? Where does this technology find its new applications? Where can I find more information about ANC? These are some of the questions answered in the following chapters.
3 Historical Review
3.1 Overview
Active Control is an old concept, but it is mainly in the past 10-15 years that developments in digital signal processing have made it a commercial reality. Some milestones in the development of active control are [2]:
Early 1930's:
Coanda (France) and Lueg (Germany) publish the first patents on active control.
Mid 1950's:
Olsen (USA) gives a wide range of proposed applications and the results of experiments. Conover (USA) applies active control to transformer noise. Active head- sets developed.
1960's
Jessel (France) and Kido (Japan) develop the theoretical basis of active control and carry out experiments.
1970's:
Leventhall, Swinbanks and Chaplin, all in the UK, develop practical systems. Chaplin makes early use of microprocessors.
1980's:
Eriksson (USA) develops commercial digital-based systems. First DSP chips become available. Several companies are formed to provide active control systems. First commercial installations.
1990's:
Steady progress and growth of interest. Many new participants come to the the topic.
Rapid development in availability of hardware. Considerable drop in cost. active control becomes a product for OEMs.
3.2 Development of ANC
The first patent on an ANC system (interference and absorption) got the German Lueg in 1936. He was the first who mixed acoustic waves with result of constructive or destructive interference, which cause intensification and weakening of the sound field. Lueg tried to manipulate the principle of superposition (see chapter 7.1) so that the destructive interference of sound waves could be used to cancel unwanted noise. He introduced the
concept of active attenuation of sound by using artificially generated acoustic waves mixed with the unwanted sound so that the waves were in anti–phase and destructive interference resulted by design. In Figure 3.1 Lueg shows the problem of cancelling sound in a duct.
Fig. 3.1: Cancelling sound in a duct (Lueg)
Here acoustic noise propagates along the duct. The microphone detects the sound and converts it to an electrical signal. The electrical signal passes through an amplifier and then to the loudspeaker. Lueg shows a single frequency in the duct, the phase reversal of which is accomplished by considering the electronic system as a transmission line with a given time delay. The length of the line is adjusted to give the neccassary time delay that results in 180°
phase shift relative to the sound wave detected at the microphone so that cancellation of that particular wave (frequency) results.
In Figure 3.1 Lueg illustrates the basic physical phenomenon which provides the possibility of ANC for this case[3]:
‘An acoustic wave with a specific frequency has a relatively much lower speed than an electrical signal of the same frequency. This implies that while a sound wave is travelling from a point where it is to be attenuated, there is enough time available within the electronic circuit to process the signal and activate the control elements, to a greater or lesser degree, depending on the frequency, type of noise, and physical extent of the system’.
Lueg found out this basic phenomenon and is also shown in the structure of Figure 3.1. The relative distance of the detector from the primary source is less than that of the secondary source relative to the primary source. In applying the above mentioned physical phenomenon, Lueg shows that within the time interval required for the passage of an acoustic wave from the detection point (microphone) to the controler point (loudspeaker) sound can be detected by the microphone, passed through the amplifier (controller) and fed to the loudspeaker. In general, to achieve good noise cancellation has to have the required phase correction as well as amplitude correction characteristics. Unfortunately, the electronic technology of the 1930s was not sufficiently advanced to meet the requirements of ANC systems and practical results were not realized.
Amplifier Microphone
Loudspeaker Source
Interests in ANC disappeared for about twenty years until Olsen introduced his ‘electronic sound absorber’ (Olsen and May 1953). In several papers Olsen proposed localized sound reducers for occupants of vehicles and for machine operators, machinery noise control, noise reducing headsets and duct noise reduction. Olsons absorber is shown in Figure 3.2. The unwanted noise is detected by the microphone and passed through the amplifier. The amplifier drives the loudspeaker so that the soundpressure at the microphone location is reduced. This effectively creates a zone of cancellation in front of the absorber. Olson’s device shows considerable sophistication for the technology in the 1950s. The problem of the backwave is solved by using the absorbing material in the cabinet. The phase problem is minimized by locating the microphone (detector) close to the loudspeaker and ensuring linearity in the electronics. It was possible to control the phase with a good degree of accuracy over a broader frequency range. The basic feedback structure of the system with other system errors limits the utility of broadband noise cancellation and the physical extent of cancellation.
Fig. 3.2: Olsons absorber
Simshauser and Hawley proposed the development of an ‘active ear defender’ in the mid- 1950s. A device to reduce ambient noise at the ear by using a headset to generate a sound pressure equal in magnitude and opposite in phase to the noise. In a noisy environment where the interference from the ambient noise especially at low frequencies, could be at a significant level, conventional ear defenders which use passive techniques of noise reduction are incapable of providing sufficient noise attenuation. Additional attenuation at low frequencies can be obtained by feeding an anti-phase signal to the ear defender. The device developed by Simshauser and Hawley is a two channel system consisting of two microphones mounted on either earphone of a conventional military headset. Each channel has an amplifier and a phase-shifting network. Noise is detected by the microphone, passed through the amplifier and phase-shifting network and applied to the earphone. Simshauser and Hawley tested the device in a pure tone field with proper amplitude and phase adjustments for minimum loudness. The result of their calculations, based on experimental
Amplifier
Cabinet
Absorbing material
Microphone Loudspeaker
measurements, indicated that such a device would provide an average reduction of 10dB more than is provided by the earcap alone, in the range of 100Hz to 1200Hz. This application is currently one of the more succesful one.
At about the same time Conover discussed the active control of transformer noise. He worked with a relatively large 15MVA tranformer installation. Unlike Simshauser and Hawley he did not confine his work to the laboratory but was rather succesful in that he developed his work in the field. Conover’s scheme was to place loudspeakers near the tranformers surface and cancel the pressure radiation in the near field. He argued that sound radiates from the transformer due to vibrations of the core (caused by magnetostriction) and is transmitted through the core mountings and the fluid cooling medium to the tank. The tank couples these vibrations to the surrounding air. This produces a periodic spectrum with harmonics at even multiples of 100Hz. Furthermore an important characteristic of transformer noise is that the first few harmonics are usually the most important contributors to the sound level. A substantial reduction of one or more of the first three components (i.e.
100/200/300Hz) is usually sufficient to achieve a reduction in the noise.
After adjusting each component in amplitude and phase the signals were recombined, amplified and fed to the speaker placed at the centre of one of the flat faces of the transformer. Measurements at 30m radial distance from the transformer showed a reduction of more than 6dB within an angular zone of about 11.5° on either side. Conover was the first to attempt the attenuation of transformer noise by active methods. Since then this has become a classic problem investigated directly or indirectly by Kido (1975), Hesselman (1978), Ross (1978), Jessel and Angevine (1980).
It is now clear that the noise of a physical source may be reduced everywhere in the far field, as long as the source is much smaller than the wavelength of the maximum frequency component of the radiated sound, and the cancelling source and radiating source are located less than one third or one quarter of the maximum wavelength apart (Hesselman 1978, Warnaka 1982). When the size of the source is much less than the wavelength of the radiated sound , the vibration of the surface is generally in phase. In this case the combined acoustic pressures of the source and the out-of-phase cancelling signal tend to cancel each other near the source. On the other hand, if the physical size of the source is large compared to the wavelength of the radiated sound the surface no longer movers in a simple uniform phase relationship. The movement is rather in a more complicated pattern since the phase varies over different regions of the source. As a consequence, the radiation pattern of the source is causing local reduction of the sound pressure without having much effect on the total radiated sound energy. Certain zones are created with reduced sound level whereas in other locations the sound is intensified.
The accelerating interest in ANC began in the late 1960s with the publications of Jessel and his co-workers in France, and Kido in Japan. Jessel’s work has been mainly concerned with duct noise. He and his co-workers have also made significant contributions to the theory of active attenuators and general consideration of the field. Jessel and Kido both realized early
that the primary advantage of ANC systems is in their ability to attenuate low-frequency noise. This is an area of great interest because of the pervasiveness of low-frequency sources and the high cost and relative inefficiency of current passive hardwear in low- frequency applications. An advantage in the control of the one-dimensional propagation of duct noise lies in the fact that active noise silencers produce no back pressure.
Jessel and others discovered some of the problems associated with reducing duct noise.
Longitudinal duct modes leading to acoustic feedback, caused by reflected components, tend to confuse the controllers as to the exact level of noise itself, since the detector microphone cannot distinguish between the noise and the reflected components. This leads to system instability and/or no noise reduction in some bands of frequency. In order to solve the longitudinal mode problem, so that the detector microphone detects the unwanted noise only, loudspeaker arrays can be used. Two or three loudspeakers used to form an acoustic dipole or an acoustic tripole can be phased to produce acoustic waves travelling in one direction.
The acoustic monopole system was originally considered by Lueg. In this system a standing wave is produced upstream which interferes with the detection of the unwanted noise. This makes the position of the microphone extremely sensitive to noise reduction. The acoustic tripole and acoustic dipole were developed by Jessel and his co-workers (Jessel and Mangiante 1972) and Swinbanks (1973). They attempt to provide a cancelling signal in the duct that propagates only in the downstream direction. The three types of active attenuators for duct noise are shown in Figure 3.3.
C
C
C
Loudspeaker
Loudspeaker Microphone
Microphone a)
b)
Fig. 3.3: Types of active attenuators for duct noise: a) monopole (Lueg)
b) tripole (Jessel)
c) dipole (Swinbanks)
Multiple loudspeaker systems (dipole and tripole) have essentially been attempted to overcome the problems associated with frequency dependent controller design. These systems try to isolate the detector microphone from secondary source radiation and/or produce undirectional radiation, thereby avoiding problems of acoustic feedback and instability in the system. Furthermore, they have been unable to cope with reflected waves and have geometry related limitations. The control problem is also much more complex in such systems. The third limitation of these systems is the so called ‘tuning effect’ due to the physical spacings of the microphone and loudspeakers relative to each other. By altering these spacings the system is tuned to a different centre frequency, with no significant improvement in the bandwidth of attenuation. For these reasons, the current trend is to use the capabilities of modern electronic systems to implement complex controllers and hence give preference to controller complexity over geometrical complexity. With proper control the monopole is capable of cancelling noise over a broad frequency range. The control problem is relatively simple with a single source. The task of large attenuation over a broad band primarily lies in the design of the controler. Required is a proper ANC system design based on sound analytical conditions for general phase cancellation.
A general trend since the late 80s is now seen towards the application of digital signal processing techniques to the active control of noise. The current advance in digital technology makes the implementation of digital controlers feasible at low cost.
C
C Loudspeaker
Microphone c)
Types of active attenuators for duct noise: a) monopole (Lueg), b) tripole (Jessel), c) dipole (Swinbanks)
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