Technologies for the Management of the Acoustic Signature of a Submarine

Technologies for the Management of the Acoustic Signature of a Submarine Carl Q. Howard School of Mechanical Engineering, The University of Adelaide, Adelaide, Australia ABSTRACT The stealth capability of a submarine differentiates it from other maritime platforms. The power plant for many submarines exceeds 1 Mega-Watt and the technical challenge is to reduce its acoustic signature to unperceivable levels. Novel vibration reduction technologies have been developed and tested at the University of Adelaide as part of a DSTO Capability Technology Demonstrator program that can reduce significantly the vibration from a submarine power-plant from reaching the hull, thereby reducing its acoustic signature. The noise radiated by the exhaust muffler attached to the power-plant can also be attenuated significantly by utilising a novel adaptive muffler system. This presentation will describe noise and vibration control technologies and methods that can be used to reduce the acoustic signature from a diesel engine power-plant. INTRODUCTION Submarines powered by diesel engines routinely surface to operate their diesel engines which rotates an electrical generator to recharge lead-acid battery banks. During this recharging operation the diesel engines operate under high load so as to recharge the batteries as quickly as possible. During the operation of the diesel engines, significant acoustic noise is generated within the submarine, and radiates from the submarine via the engine exhaust. In addition, the vibration generated by the engine must be prevented from reaching the hull of the submarine to reduce its acoustic signature. It is a significant engineering challenge to reduce the acoustic signature of such large power plants to unperceivable levels to remain undetectable. This paper describes some existing and emerging noise and vibration control technologies that can be used to reduce the acoustic signature of a submarine from the diesel engine and other rotating equipment. Figure 1 shows an illustration of some of noise generation equipment onboard a submarine. The most significant of these is the diesel engine power-plant used to recharge the lead-acid batteries. As an example, the Collins Class submarine contains 3 Garden Island-Hedemora HV (VB210) 18- cylinder diesel motors, each with a power rating of approximately 1.4 MW. Other noise generation mechanisms include flow induced noise of the water over the submarine exterior, radiation by the propeller, operation of onboard machinery, noise generation from fluid flowing through pipes, operation of valves and shutters, and others. Each noise generation mechanism has associated with it noise control technologies. Another important noise control technology to reduce the acoustic signature of a submarine is anechoic tiles that cover the exterior of the submarine. These tiles are designed to perform a dual function of absorbing incoming active sonar pulses, and attenuate sound originating from the submarine. Auxillary Machinery Diesel Engines Flow Generators Radiated Drive Motor Figure 1: generation mechanisms onboard a submarine. Propellor The focus this paper is the noise and vibration control technologies applied to diesel engines to reduce the acoustic signature of a submarine. The diesel engines must be vibration isolated from the hull of the submarine, to prevent the radiation of sound by the hull. It is unfortunate for submarines, that at the frequencies where diesel engines operate, the surrounding water provides little attenuation of sound with distance in a straight line, and the main mechanism for sound dissipation is the spreading of the acoustic energy into the water. In addition to the diesel engines, other vibrating equipment such as the main generators, drive motor, and numerous pieces of auxiliary equipment such as water pumps and pipe-work must also be vibration isolated from the hull. In the following section, three categories of vibration isolation systems for the diesel engines and other rotating equipment are described. The diesel engines also generate high amplitude acoustic noise during their operation that can be attenuated by an exhaust silencer. This paper describes three categories of ex- 1

haust silencers that can be used to attenuate the exhaust noise. VIBRATION ISOLATION OF A DIESEL ENGINE The diesel engines in the Collins Class submarine are vibration isolated from the hull using a two-stage vibration isolation system as shown in Figure 2. Active Vibration Isolation Systems Active vibration isolation systems involve the injection of vibrational energy into the system in order to achieve vibration reduction or cancellation. Often active vibration isolation systems are used in combination with passive vibration isolation systems, so that should the active system fail, the passive system will still provide some vibration attenuation. All active vibration systems utilise a control actuator to counter-act the disturbing vibration. The control actuator can be inserted into the isolation system in one of three methods as shown in Figure 3: inertial, parallel, and series. Tuned Vibration Absorber Diesel Engine Inertial Reaction Control Actuator Vibration Isolators Figure 2: Diesel engine and vibration isolator configuration in the Collins Class submarine (from Li et al. 2004). The two-stage vibration system comprises a heavy intermediate mass installed between upper and lower sets of vibration isolators. This two-stage configuration can result in greater vibration isolation than a single-stage configuration, although the intermediate masses add significant weight. As with all rotating equipment, including diesel engines, vibration is often generated at a fundamental frequency and harmonics. The vibration from the engine is attenuated by a passive vibration isolation system and can be further improved by the use of supplementary technology. The following sections describe three categories of vibration isolation systems namely: passive, active, and adaptive-passive. Machine (a) Inertial Machine Control Actuator (b) Parallel Machine Control Actuator Oscillating Oscillating Oscillating Passive Vibration Isolation The standard engineering practice is to use vibration isolators to attenuate a vibration source (the diesel engine), from a receiver (the submarine hull). There are various types of isolators, but essentially they utilise an elastic spring element such as a rubber block or metal spring. The term passive is used because no additional energy is injected into the isolation system to attenuate the vibration. This is in contrast to active systems where vibrational energy is injected into the system, and is described in the following section. The design of these passive vibration isolation systems always involves a trade-off between the amount of vibration reduction that can be achieved and the displacement of the object that is to be isolated - greater vibration attenuation requires low stiffness isolators, which results in potentially large excursions of the object to be isolated. This poses additional problems as designers also need to meet the displacement limits for shock loading in submarine applications. Passive vibration isolation systems are extremely robust and can provide consistent vibration isolation performance for many years. The most common reason for the degradation of performance is that the rubber material ages and can become stiff, thereby increasing the resonance frequency of the system and decreasing the vibration attenuation, or the rubber material can start to de-bond and disintegrate. (c) Series Figure 3: Three types of active vibration isolation systems: (a) inertial (b) parallel (c) series. The inertial type uses an inertial mass and shaker to create a reaction force. The parallel type uses a control shaker placed between the vibration source and the receiver. The series type uses a control shaker placed between the vibration source and an intermediate mass. The intermediate mass and its passive isolator support are used to isolate the control actuator and the vibration source from the dynamics of the flexible support structure, thus resulting in improved performance. The parallel type of vibration isolator is able to exert greater forces to the vibrating source and receiving structure, which means greater vibration attenuation can be achieved at lower frequencies than using the inertial type. The School of Mechanical Engineering at The University of Adelaide has investigated the use of active vibration isolation on a simulated Collins Class submarine engine vibration isolator (Li et al. 2004). Figure 4 shows the experimental setup where a large rigid mass was used to simulate a portion of the diesel engine, an intermediate mass was installed between two sets of rubber vibration isolators, and inertial electrodynamic vibration shakers were attached to the intermediate mass orientated vertically. The inertial shakers generated anti-vibration that 2

cancelled the simulated vibrations from the diesel engine. Accelerometers were attached to the intermediate mass that were used to measure the vibration. An electronic controller was used to determine the appropriate amplitude and phase of anti-vibration required to cancel the disturbing vibration. Sensors that detect vibration Actuators that generate anti-vibration Recorded vibration Figure 4: Active vibration isolation system. supported by one engine mount mass Isolators The results from the experiments demonstrated that the active vibration isolation system was able to reduce significantly the vibration of the intermediate mass along the translational vertical axis and two rotational axes, and therefore could reduce the vibration transmitted into a submarine hull. Although it was demonstrated that high levels of vibration attenuation could be achieved in the laboratory, the practicalities of such a system need to be addressed such as the robustness in the harsh submarine environment. The active vibration isolation system comprises electrodynamic shakers that have copper coil windings and a suspension system, similar to that found in loudspeakers. These devices are required to generate high amplitude forces to counter-act the forces generated by the diesel engines. It would be expected that these shakers would operate for years with little mainteance. The electrodynamic shakers operate most effectively at room temperature. In addition to the shakers, a power amplifier is required for each shaker, vibration sensors, and a control system is required to determine the appropriate antivibration signals. Each two-stage mount would require such a system, resulting in a complex system. A marine engine diesel manufacturing company MTU Friedrichshafen has developed a combined passive and active vibration isolation system for attenuating the vibration from large marine diesel generators (von Drathen, 2010). The active vibration isolation mount comprises three inertial electrodynamic shakers orientated along three axes. Their results indicate that significant vibration attenuation can be achieved. An alternative to the fully active vibration isolation system is called an adaptive-passive system that utilises passive vibration isolation technology, but it can be tuned. Adaptive Passive Vibration Isolation System The two-stage vibration isolation system used on the Collins Class submarine provides high levels of vibration attenuation and operates over a broad frequency range. The two-stage isolation is supplemented with fixed tuned vibration neutralisers that provide additional attenuation at a fixed frequency. These devices are illustrated in Figure 5 where masses on the end of cantilever arms resonate and cause a reduction in the vibration of the intermediate mass, and hence reduce the vibration transmitted into the submarine hull. When these devices are tuned precisely to the frequency of the disturbing vibration, they can provide very high levels of vibration attenuation. However, one of the problems with the use of fixed tuned vibration neutraliser is that if the frequency of the disturbing vibration changes slightly, such as if the engine speed changes, then the tuned vibration neutralisers become ineffective. In conjunction with The Defence Science and Technology Organisation, the School of Mechanical Engineering at the University of Adelaide developed an adaptive-passive tuned vibration neutraliser, as shown in Figure 5, that can reduce the vibration from the diesel engines from reaching the submarine hull, thereby reducing the acoustic signature of the submarine. The improvement in vibration attenuation is achieved passively, as opposed to an active vibration control system that requires electrically powered vibration shakers to counter-act the vibration from the submarine engine. The adaptive-passive system uses the same principle as the fixed tuned vibration neutralisers, only it can be tuned to the frequency of the disturbing vibration. The tuning is achieved by adjusting the length of the cantilever arm, which alters the effective stiffness and hence the resonance frequency of the device. The newly developed adaptive-passive vibration neutralisers automatically tune itself to the frequency of vibration of the diesel engine and can provide a significant reduction in the vibration that reaches the hull. The system only requires low power (12 volts) to operate a microcontroller and actuator. The actuators only operate when the device needs to be retuned by the automatic control system to match the frequency of the submarine engine. Laboratory tests were conducted on the device using a simlated Collins Class engine vibration isolation system as the test platform. It was demonstrated that the technology was able to reduce significantly the vibration of the intermediate mass. Transducer Accelerometers Adaptive Tuned Vibration Neutralisers Shaker Upper Vibration Isolators Ground Rigid Base Block Cantilever Arms Separation Distance Figure 5: Schematic of the adaptive-passive tuned vibration neutraliser. Discussion The three types of vibration isolation systems described here are all capable of providing high levels of attenuation when they are designed correctly. The design of the vibration isola- 3

tion system using only passive (non-adaptive) techniques is always a compromise between the amount of vibration isolation, the allowable excursion, weight and space in the case of two-stage isolation. Fully active vibration systems can vastly improve the vibration attenuation but there will be reliability issues of the actuators that need to be addressed. Adaptivepassive vibration isolation systems can offer the robustness of standard passive vibration isolation, and only require low power to adjust its vibration characteristics. As these systems can be automatically tuned to the frequency of the disturbing vibration, they can provide very high levels of vibration attenuation of tonal vibration. The drawback of adaptive passive systems is that they can only attenuate one frequency of vibration per resonator, whereas a fully active vibration isolation system is capable of attenuating broadband and harmonic frequencies. EXHAUST SILENCERS The diesel engine power-plant on a conventional submarine generates a significant amount of noise. It is important that this noise is attenuated if it is to remain undetected. However this is a significant engineering challenge. In a standard design of a conventional submarine, the diesel engine is installed near the rear of the vessel and the exhaust pipe-work passes through the pressure hull, up the fin (or conning tower) and into a retractable mast that can be extended during snort operations. As with all components on the submarine, space and weight are very limited. An exhaust silencer is installed along the pipework, usually close to the diesel engine. This device acts to attenuate the noise that is transmitted through the exhaust pipework, and hence it is beneficial to place the device as close to the engine as possible, rather than at the end of the ductwork as occurs in most automotive applications. There are three broad categories of exhaust silencers namely: passive, active, and adaptive-passive. Each of these categories is described below in the following sections. Passive Silencer Passive exhaust silencers are the most common type of silencer. They are installed in almost every automotive application, diesel engine generator sets, industrial applications that utilise large exhaust fans such as power stations, cement factories, dry milk powder factories, glass bottle manufacturing, etc. The term passive is used because the silencers are designed to absorb and or reflect acoustic energy, so that no additional energy sources are required for them to operate. This is in contrast to Active Control (ANC) silencers described in the next section. Passive silencers are extremely robust and can have a useful working life for over a decade. Problems occur when the exhaust contains particulates that can clog the porous material that is used to absorb the sound energy. For example in some coal fired power stations and cement manufacturing facilities, silencers that contain porous materials might only last 6 months. In these cases an alternative design configuration can be used, where instead of using porous materials to absorb sound, hard walled acoustic volumes are installed that act to reflect sound towards the source (Howard et al. 2000). As porous materials are not used, the performance of the device does not degrade over time. Technically, it changes the acoustic impedance of the acoustic duct so that sound does not propagate past the silencer. These resonator silencers are passive devices that provide high levels of acoustic attenuation at a single frequency or harmonic frequencies and do not require any power. One common type of resonator silencer designs is a sidebranch resonator, shown in Figure 6. Side-branch resonators have two common designs namely quarter-wave tubes and Helmholtz resonators. Figure 6: Type of resonator silencers. From Beranek, L.L. and Ver, I.L. (1992). These passive acoustic devices can attenuate tonal noise by more than 30dB when they are properly tuned to the frequency of the disturbing noise. However, when the frequency of the noise changes, or the temperature of the gas changes, a single device will become ineffective. However, by using multiple resonator devices tuned to different frequencies, it is possible to create a silencer that attenuates noise over a frequency range. Active Control Silencer The previous section described the conventional passive method of attenuating noise from exhausts, where noise is absorbed by porous materials, and or reflected by carefully designed acoustic components. It is also possible to achieve noise reduction using active means, where a cancellation or anti-noise is introduced into the exhaust pipe. The term active is used because energy is intentionally introduced into the system that acts to cancel the exhaust noise. A simplified system diagram is shown in Figure 7. The exhaust noise will propagate along the exhaust duct and will reach a loudspeaker that injects a noise that is equal in magnitude but 180 degrees out-ofphase with the exhaust noise. When these two acoustic pressures combine, the result is that the noise is cancelled. The resulting noise is measured by a microphone and this signal is used by an electronic controller that determines the appropriate anti-noise signal that is required. This type of system is called a feed-back arrangement and is commonly used on consumer active noise cancellation headsets. This description is intentionally simplified and some details are omitted for clarity, however the key concepts are conveyed. Another variant of this loudspeaker type active noise control system is where the controller utilises a feed-forward control structure. For this type of control system it is necessary to obtain a signal that is related to the unwanted engine noise before it reaches the loudspeaker. This could be either an upstream microphone that measures the actual unwanted noise in the exhaust stream, or alternatively a tachometer signal can be used to generate a sinusoidal signal that is related to the unwanted tonal exhaust noise. It is also possible to create a combination of both feed-back and feed-forward control architectures. These types of active noise exhaust silencers that utilise loudspeakers have been developed, tested, and shown some potential. For example, Everett et al. (1991) demonstrated this concept on a Detroit Diesel 6V-92TA industrial diesel engine used in a generator set application and achieved 13dB noise reduction greater than a conventional passive muffler. 4

Loudspeaker container noise and air-flow Loudspeaker introduces anti-noise + = Microphone Airflow Cooling Air Electronic controller Figure 7: Simple feedback noise control system acting as an exhaust silencer. Recently a company called Eberspächer developed an active muffler for automotive applications using a loudspeaker configuration (Kruger, 2009). They demonstrated noise reduction of around 0-8dBA with varying degrees of attenuation. The Acoustic Vibration and Control group at the University of Adelaide has installed an active noise control system in a large exhaust stack at a cement manufacturing factory (Hansen et al. 1996), and also a dry milk powder factory. In both cases, these systems were able to reduce significantly the tonal noise from the factories. The measured noise reduction of the tones generated by the large exhaust fans was about 20dB. Many journal and conference papers about active noise control systems will highlight the significant noise reductions achieved. This often leaves the reader to ponder why active noise control systems are not more widely installed. Unfortunately, authors neglect to note some of the practical difficulties of the systems and their robustness. The author makes the following comments from the practical experience of the ANC system in a cement factory. However, the comments are relevant to other applications in adverse environments. Active noise control of exhausts that utilise loudspeakers to generate anti-noise have reliability issues. The loudspeakers can be exposed to extreme temperature ranges from 0C up to 700C of the exhaust gas, whereas most loudspeakers are designed to operate at room temperature. gas streams can contain corrosive and abrasive contaminants and hence when a loudspeaker is installed in an exhaust duct, it must be protected from the adverse conditions by an artificial room temperature environment, free from contaminants. Figure 8 shows an installation of an active noise control system performed by the University of Adelaide s Acoustic Vibration and Control group in a dry milk powder factory. Hot exhaust air travels upwards through the duct. A conventional consumer loudspeaker was attached to the exhaust duct inside a protective container. Cooling air must flow through the loudspeaker container to maintain it at room temperature. There is significant heat load on the loudspeaker from the exhaust gases, and also the loudspeaker is required to operate at high electrical power levels which also generates heat. Figure 8: Active noise control installation in a dry milk powder factory. An alternative configuration to using a loudspeaker attached to the duct was presented by Boonen and Sas (1999) that utilised a butterfly valve that rotates back-and-forth within the exhaust pipe, as shown in Figure 9. The concept was demonstrated on a cold-engine simulator. noise and air-flow Butterfly valve Figure 9: Active exhaust silencer utilising a rotating butterfly valve. In their laboratory experiments using their cold-engine simulator, they demonstrated a 13dB noise reduction with only a 3kPa increase in back-pressure. As with loudspeaker type active noise control systems, this butterfly valve configuration utilises a continuously moving element that must withstand the harsh temperatures and exhaust gas contaminants, which presents robustness issues. Adaptive Passive Silencer Another technology for exhaust silencers is called an adaptive-passive resonator silencer. The term adaptive means that the silencer is capable of re-tuning itself to the frequency of the disturbing noise, thereby maintaining acoustic attenuation for changes in the frequency of the disturbing noise. This can be achieved by designing the device so that the length of the tube (length L in Figure 6) can be altered, which has the effect of altering the resonance frequency of the resonator. The term passive means that no additional power is required to reduce the acoustic noise. As they are passive devices, they cannot become unstable, which is a possibility with an active noise control system. The only power source required is a low-voltage supply to power a micro-processor and acoustic sensors (typically a couple of Watts) and an actuator to change the length of a tube in the resonator silencer. 5

The following section describes the features of the quarterwave tube resonator silencer. Quarter-Wave Tube Resonator Silencer The side-branch resonator shown in Figure 6 can be attached to an engine exhaust system using a flange. Note that the side branch does not have to be perpendicular to the main duct, and could be installed parallel to the existing exhaust system as shown in Figure 10. The backwards orientation of the silencer is important to reduce flow induced noise, to prevent the side branch from singing. Side-Branch Resonator Piston There are other variants of this type of adaptive passive silencers such as a variable Helmholtz resonator, and an Adaptive Herschel-Quincke Tube. Helmholtz Resonator A Helmholtz resonator is a passive noise control technology that comprises a sealed cavity volume attached to the main exhaust duct with a narrower neck, as shown in Figure 12. The device can be tuned to the frequency of the tonal noise by altering the geometry of the volume or neck. Figure 12 shows an adaptive-passive configuration where the volume can be altered by changing the position of a piston. This has the effect of changing the resonance frequency of the device, and can be integrated into a control system to attenuate the noise in the exhaust duct. Low Duct Piston Figure 10: Schematic of the attachment of an adaptivepassive side-branch resonator silencer to an exhaust duct. The quarter-wave tube type of resonator silencer provides noise attenuation at odd multiples of the fundamental resonance frequency. This means that the device will attenuate noise at a fundamental frequency and its harmonics, which is a very desirable feature for attenuating the noise from a diesel engine. Figure 11 shows the transmission loss in decibels of this type of resonator silencer, where k = 2πf / c is the wavenumber, f is the frequency in Hertz, c is the speed of sound of the gas in m/s, L is the length of the quarter-wave tube, S1 is the cross sectional area of the main exhaust duct, S2 is the cross sectional area of the side-branch, and N = S2 / S1 is the ratio of the cross-sectional areas of the side-branch to the main duct. Figure 11 shows that greater attenuation is achieved as this ratio N increases. Volume Figure 12: Schematic of a Helmholtz resonator. Herschel-Quincke Tube Neck Low Duct A Hershel-Quincke tube comprises a side branch attached to a main duct as shown in Figure 13, where the acoustic energy from the main duct is directed into a side branch and propagates over longer path, then recombines with the main duct. Side-Branch Resonator Low Duct Figure 13: Typical Hershel-Quincke tube. Figure 11: Transmission loss for a side-branch resonator (quarter-wave tube). From Beranek, L.L. and Ver, I.L. (1992). An adaptive quarter-wave tube resonator silencer can be used to reduce significantly the tonal noise from the exhaust system. This can be realised by varying the length of the quarterwave tube using a piston. Acoustic sensors are required to measure the noise level and frequency within the exhaust duct and these signals are provided to an automatic control system that tunes the adaptive quarter-wave tube resonator silencer. The length of the tube is selected such that the resulting phase angle of the noise is opposite to that of the noise in the main duct, and hence the sound is attenuated when the noise sources from the two branches are recombined. An attractive feature of the Herschel-Quincke Tube is its simple design and that it will attenuate sound at multiple frequencies. This device can also be configured as an adaptive passive silencer, where the length of the side branch is altered (Byrne and Moe, 2006), or a variable membrane can be inserted into the side branch that alters its acoustic impedance (Griffin et al. 1999). CONCLUSIONS This paper has described technologies for the management of the acoustic signature of a submarine s diesel engine, which is perhaps the noisiest piece of machinery onboard. It is a significant engineering challenge to attenuate the acoustic noise and vibration generated by the diesel engines from reaching the submarine hull, where unfortunately underwater sound propagates effectively. Three types of technology have 6

been discussed for amelioration of noise and vibration namely passive, active, and adaptive-passive. Passive noise and vibration control technologies provide adequate attenuation when designed properly. This technology provides consistent results and can be designed to be robust to harsh operating environments. The attenuation offered by this technology can be further improved by utilising other technologies. Active noise and vibration control technology can provide very high levels of attenuation however the actuators used in these systems must be extremely reliable and robust to their harsh operating environment. The actuators for active control technologies must continually operate in order to provide attenuation and this presents reliability issues. Adaptive-passive technologies can offer the benefit of the robustness of passive technology in operating in harsh environments, and when tuned properly can provide high levels of attenuation. These adaptive passive devices also require actuators to tune, but they only operate intermittently and hence have less risk of failure than an active control system. Howard, C.Q., Cazzolato, B.S. and Hansen, C.H. (2000), stack silencer design using finite element analysis, Control Engineering Journal, 48 (4), p113-120. Kruger, J. (2009), Silence is golden: Active exhaust silencers in vehicles, dspace Magazine, March, p28-33. Li, X., Howard, C.Q., Hansen, C.H., Winberg, M. (2004), Feasibility of Active Vibration Isolation of Diesel Engines in Collins Class Submarines, Navy Engineering Bulletin, March, p25-26. Von Drathen, Arndt (2010), Propulsion system solutions for present and future naval vessels, presented at Pacific 2010, Sydney Australia, 27-29 January 2010. Passive technologies will always be utilised to control noise and vibration. However the demand for ensuring the stealth capability of a submarine means that designers will always be looking for technological advancements to achieve greater attenuation of noise and vibration, and this is where active and adaptive-passive technologies have potential. REFERENCES Beranek, L.L. and Ver, I.L. (1992), and vibration control engineering: principles and applications, John Wiley and Sons, New York, USA. Boonen, R. and Sas, P., (1999) Development of An Active Silencer for Internal Combustion Engines Using Feedback Control, Proceedings of the Society of Automotive Engineers, & Vibration Conference & Exposition, May 1999, Traverse City, MI, USA, Paper Number: 1999-01- 1844. Byrne, Stuart J, and Moe, Jeffrey, W. (2006), Assembly and Method for Fan Reduction from Turbofan Engines Using Dynamically Adaptive Herschel-Quincke Tubes European Patent EP 1 322 844 B1. Everett, A., Frazer, W., and Hoge, W.J.J. (1991), Development of a prototype active muffler for the Detroit Diesel 6V- 92 TA industrial engine, Proceedings of the 1991 & Vibration Conference, 13-16 May, SAE Preprints, n244, p57-67. Griffin, Steve, Huybrechts, Steve, and Lane, Steven A. (1999), An Adaptive Herschel-Quincke Tube, Journal of Intelligent Material Systems and Structures, 10 (12), p956-961, doi:10.1106/j2fl-17l3-b7jv-8492. Hansen, C.H., Howard, C.Q., Burgemeister, K.A., Cazzolato, B.S. (1996), Practical implementation of an active noise control system in a hot exhaust stack, Acoustics 1996: Making Ends Meet: Innovation and Legislation, Brisbane, Queensland, Australia, 13-15 November. 7