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  • 1. Andrén, Linus
    et al.
    Johansson, Sven
    Winberg, Mathias
    Claesson, Ingvar
    Active Noise Control Experiments in a Fork-lift Truck Cabin2004Conference paper (Refereed)
    Abstract [en]

    High comfort for the driver in working vehicles is an important feature as well as a demand from the drivers. Low noise level is an essential factor for the manufacturer to maintain a high standard and comfort of vehicles. In many cases the noise inside the cabin can be related to the engine orders. Hydraulic pumps and fans are also related to the engine but not necessarily integers of the engine order. Passive absorbers are not suitable for the lowest frequencies and one approach is to use an active noise control system to solve the noise problem at low frequencies. In the present experiment loudspeakers were mounted inside the cabin of a fork lift-truck to produce the secondary noise field. To sense the residual noise, microphones were installed close to the driver's head. The aim is to create a zone of reduced noise around the head. Since a large portion of the noise inside the cabin can be related to the engine, an active control system based on a feedforward solution is possible. Experimental results from a feedforward solution of active noise control in a fork-lift truck cabin show that the noise level in the low frequency region can be reduced significantly.

  • 2. Borgh, Markus
    et al.
    Johansson, Sven
    Blekinge Institute of Technology, School of Engineering, Department of Signal Processing.
    From, Åsa
    Lindström, Fredric
    A Personal Voice Analyzer and Trainer2010Conference paper (Refereed)
    Abstract [en]

    This paper presents a personal voice analyzer and trainer that allow the user to perform four daily exercises to improve the voice capacity. The system grades how well the user is performing the exercises by analyzing the duration, the intensity and the pitch of the user’s voice.

  • 3. Håkansson, Lars
    et al.
    Johansson, Sven
    Claesson, Ingvar
    Chapter 78 Machine Tool Noise, Vibration and Chatter Prediction and Control2005In: HANDBOOK OF NOISE AND VIBRATION CONTROL / [ed] Crocker, Malcolm J., New York: To be published by by John Wiley & Sons , 2005Chapter in book (Other academic)
    Abstract [en]

    A frequent problem in the manufacturing industry today is the vibrations or chatter induced by metal cutting, e.g. turning, milling and boring operations. Vibrations in boring operations or internal turning operations, for example,are inevitable and constitute a major problem for the manufacturing industry. Tool vibrations in metal cutting affect the result of machining, particularly the surface finish. Furthermore, tool life is correlated with the degree of vibration and acoustic noise introduced. Generally, tool vibrations are related to a low-order bending mode of, for example, the tool holder shank in external turning, the boring bar in internal turning, spindle-cutter assembly in milling, etc. Tool chatter or vibration problems in internal turning or milling may be reduced, for example, by using boring bars and milling adapters with passive tuned dampers. These are usually manually tuned to increase the dynamic stiffness of the boring bar or milling adapter at one of the eigenfrequencies of its low-order bending modes. Active control approaches for the attenuation of the bending motion of boring bars, tool holder shanks and spindle-cutter assembly in milling have been developed; such approaches involve both adaptive and time-invariant feedback control. In addition, prediction and control methods for controlling cutting data to maintain stable cutting, i.e. to avoid cutting data resulting in chatter, have been developed.

  • 4. Håkansson, Lars
    et al.
    Johansson, Sven
    Claesson, Ingvar
    Machine Tool Noise, Vibration and Chatter Prediction and Control2007In: HANDBOOK OF NOISE AND VIBRATION CONTROL / [ed] Crocker, Malcolm J., New York: John Wiley & Sons , 2007Chapter in book (Other academic)
  • 5. Håkansson, Lars
    et al.
    Johansson, Sven
    Dahl, Mattias
    Sjösten, Per
    Claesson, Ingvar
    NOISE CANCELLING HEADSETS FOR SPEECH COMMUNICATION2002In: Noise Reduction in Speech Applications / [ed] Davis, Gillian M., Boca Raton, Florida: CRC Press , 2002, p. 305-327Chapter in book (Other academic)
    Abstract [en]

    Headsets for speech communication are used in a wide range of applications. The basic idea is to allow hands-free speech communication, leaving both hands available for other tasks. One typical headset application is aircraft pilot communication. The pilot must be able to communicate with personnel on the ground and at the same time use both hands to control the aircraft. Communication headsets usually consist of a pair of headphones and a microphone attached with an adjustable boom. Headphone design varies widely between different manufacturers and models. In its simplest form, the headphone has an open construction providing little or no attenuation of the environmental noise. In headsets designed for noisy environments, the headphones are mounted in ear cups with cushions that provide some attenuation. The microphone is primarily designed to pick up the speech signal, but if the headset is used in a noisy environment, the background noise will also be picked up and transmitted together with the speech. As a consequence, speech intelligibility at the receive end will be reduced, possibly to zero. To increase the speech-to-noise ratio, it is common to use a directional microphone that has a lower sensitivity to sound incident from other directions than the frontal direction. In addition to this, the microphone electronics are usually equipped with a gate function that completely shuts off the microphone signal if its level drops below a threshold value. The purpose of the gate is to open the channel for transmission only when a speech signal is present. Headsets are frequently used in noisy environments where they suffer from problems of speech intelligibility. Even if an ear-cup type headset is used, the attenuation is relatively poor for low frequencies. Low frequency noise has a masking effect on speech, which significantly reduces the speech intelligibility. Several cases have been reported in which the sound level of the communication signal was increased to hazardous levels by the user to overcome this low frequency masking effect [1,2]. Ear exposure to the communication system resulted in hearing damage, such as hearing loss, tinnitus and hyperacusis.

  • 6. Johansson, Sven
    Active Control of Lateral Vibrations in a High-Speed Train2003Conference paper (Refereed)
    Abstract [en]

    As trains are continually designed for higher and higher speeds the problems of railway carriage vibration are on the increase. Lateral vibrations in a railway carriage are noticeable to passengers if the vibration frequencies are lower than approximately 20 Hz. Below this frequency discomfort is a common problem for the passengers and below approximately 1 Hz motion sickness is a problem. The passive solution of stiffening the carriage chassis to shift the vibrational frequencies higher up results in inflated manufacturing and running costs, and opposes higher travel speeds due to increased weight. Semi-passive solutions such as modifying the structural dynamics of the carriage body by decoupling heavy under-floor equipment does not come with a weight penalty, but do not reduce the vibrations sufficiently. However, by using an active vibration control system it is possible to improve the attenuation of the lateral train vibrations. This paper addresses two simple active vibration control systems for reducing lateral vibrations: one feedforward system and one feedback. Computer simulations based on signals derived from a dynamic computer model of a train indicates that the incorporation of an active control system, in addition to the semi-passive approach, is likely to introduce an improved attenuation of the low-frequency lateral vibrations. Active control results illustrate an attenuation of the lateral carriage vibration by up to 15 dB.

  • 7. Johansson, Sven
    Active Control of Propeller-Induced Noise in Aircraft: Algorithms & Methods2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the last decade acoustic noise has become more and more regarded as a problem. In cars, boats, trains and aircraft, low-frequency noise reduces comfort. Lightweight materials and more powerful engines are used in high-speed vehicles, resulting in a general increase in interior noise levels. Low-frequency noise is annoying and during periods of long exposure it causes fatigue and discomfort. The masking effect which low-frequency noise has on speech reduces speech intelligibility. Low-frequency noise is sought to be attenuated in a wide range of applications in order to improve comfort and speech intelligibility. The use of conventional passive methods to attenuate low-frequency noise is often impractical since considerable bulk and weight are required; in transportation large weight is associated with high fuel consumption. In order to overcome the problems of ineffective passive suppression of low-frequency noise, the technique of active noise control has become of considerable interest. The fundamental principle of active noise control is based on secondary sources producing ``anti-noise.'' Destructive interference between the generated and the primary sound fields results in noise attenuation. Active noise control systems significantly increase the capacity for attenuating low-frequency noise without major increase in volume and weight. This doctoral dissertation deals with the topic of active noise control within the passenger cabin in aircraft, and within headsets. The work focuses on methods, controller structures and adaptive algorithms for attenuating tonal low-frequency noise produced by synchronized or moderately synchronized propellers generating beating sound fields. The control algorithm is a central part of an active noise control system. A multiple-reference feedforward controller based on the novel actuator-individual normalized Filtered-X Least-Mean-Squares algorithm is introduced, yielding significant attenuation of such period noise. This algorithm is of the LMS-type, and owing to the novel normalization it can also be regarded as a Newton-type algorithm. The new algorithm combines low computational complexity with high performance. For that reason the algorithm is suitable for use in systems with a large number of control sources and control sensors in order to reduce the computional power required by the control system. The computational power of the DSP hardware is limited, and therefore algorithms with high computational complexity allow fewer control sources and sensors to be used, often with reduced noise attenuation as a result. In applications, such as controlling aircraft cabin noise, where a large multiple-channel system is needed to control the relative complex interior sound field, it is of great importance to keep down the computational complexity of the algorithm so that a large number of loudspeakers and microphones can be used. The dissertation presents theoretical work, off-line computer experiments and practical real-time experiments using the actuator-individual normalized algorithm. The computer experiments are principally based on real-life cabin noise data recorded during flight in a twin-engine propeller aircraft and in a helicopter. The practical experiments were carried out in a full-scale fuselage section from a propeller aircraft.

  • 8. Johansson, Sven
    Active Noise Control in Aircraft: Algorithms and Applications1998Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The thesis consists of five papers which are divided into three main parts. Parts A and B deal with active noise control in a propeller aircraft application, whereas Part C deals with active noise control in a helicopter application. Part A presents a comparison between single- and multiple-reference controllers, while Part B deals with different multiple-reference, multiple-channel algorithms. Finally, Part C presents a hybrid headset. The five papers comprise: Part A1: Comparison between a Single-- versus a Twin--Reference Controller in Narrowband ANC Applications. Part A2: Performance of a Multiple- versus a Single-Reference MIMO ANC Algorithm based on a Dornier 328 Test Data Set. Part B1: A Novel Multiple--Reference, Multiple--Channel, Normalized Filtered--X LMS Algorithm for Active Control of Propeller-Induced Noise in Aircraft Cabins. Part B2: Evaluation of Multiple--Reference Active Noise Control Algorithms on Dornier 328 Aircraft Data. Part C: A New Active Headset for a Helicopter Application. The test data sets used in the computer evaluations are based on real--life data throughout. The data was recorded in a twin-engine propeller aircraft (Dornier 328) and a Super Puma helicopter during flight.

  • 9. Johansson, Sven
    et al.
    Claesson, Ingvar
    Active Noise Control in Propeller Aircraft2001Conference paper (Refereed)
    Abstract [en]

    A noisy environment dominated by low frequency noise can often be improved through the use of active noise control. This situation arises naturally in propeller aircraft where the propellers induce periodic low frequency noise inside the cabin. The cabin noise is typically rather high, and the passenger flight comfort could be improved considerably if this level were significantly reduced. This paper addresses same design aspects for multiple-reference active noise control systems based on the feedforward strategy. The paper also discusses the operation of narrowband feedforward active noise control system and presents results from experiments.

  • 10. Johansson, Sven
    et al.
    Claesson, Ingvar
    Nordebo, Sven
    Sjösten, Per
    Evaluation of Multiple Reference Active Noise Control Algorithms on Dornier 328 Aircraft Data1999In: IEEE transactions on speech and audio processing, ISSN 1063-6676 , Vol. 7, no 4, p. 473-477Article in journal (Refereed)
    Abstract [en]

    This correspondence presents an evaluation of multiple-reference adaptive algorithms. Two LMS-types and a Newton-type algorithm are considered. The special structure of the adaptive filtering problem implies that the Newton-type algorithm can be implemented with the same numerical complexity as LMS-type algorithms. The concept of a fast filtered-x Newton algorithm is thus introduced.

  • 11. Johansson, Sven
    et al.
    Håkansson, Lars
    Claesson, Ingvar
    Aircraft Cabin Noise and Vibration Prediction and Active Control2007In: HANDBOOK OF NOISE AND VIBRATION CONTROL / [ed] Crocker, Malcolm J., New York: John Wiley & Sons , 2007Chapter in book (Other academic)
  • 12. Johansson, Sven
    et al.
    Håkansson, Lars
    Claesson, Ingvar
    AIRCRAFT CABIN NOISE AND VIBRATION PREDICTION AND ACTIVE CONTROL2005In: HANDBOOK OF NOISE AND VIBRATION CONTROL / [ed] Crocker, Malcolm J., New York: To be published by John Wiley & Sons , 2005Chapter in book (Other academic)
    Abstract [en]

    The active control of noise and vibration in passenger aircraft cabins generally rely on adaptive multi channel feedforward control of disturbing acoustic cabin modes based on a set of control sources and error sensors with optimized locations, or on feedback control in active headsets or in quiet seats. The structural aircraft fuselage vibrations produced by the propulsion system and turbulence pressure fluctuations acting on the external fuselage generally excite the interior cabin sound field. Cabin noise in turboprop aircraft and jet aircraft with tail-mounted engines are usually dominated by tonal noise related to the fundamental blade passage frequency and the jet engine spool imbalance frequencies respectively. The jet noise -engine exhaust noise- and turbulent boundary layer noise, on the other hand, have broadband characteristics. The active control of tonal noise in aircraft is of global character and involves multi channel active noise control or multi channel active structural acoustic control. The locations of the secondary sources and error sensors are optimized for the acoustic noise attenuation over a range of flight conditions and are selected from a set of potential configurations. Generally, the active control systems for reducing the tonal cabin noise are based on variants of the multi channel Filtered-X LMS algorithm, and both time domain and frequency domain algorithms are used. Feedback control in active headsets or in silent seats addresses the broadband cabin noise.

  • 13. Johansson, Sven
    et al.
    Håkansson, Lars
    Persson, Per
    Claesson, Ingvar
    Active Control Methods for Reducing Lateral Vibrations in High-Speed Trains2002Conference paper (Refereed)
    Abstract [en]

    The problem of low-frequency lateral train-car vibration is an important issue surrounding the design of high-speed trains. Passive vibration control solutions such as stiffening the car chassis are impractical because of the weight increase, and modifying the structural dynamics of the carboy through nonrigid coupling of heavy underfloor equipment does not come with a weight penalty, but do not sufficiently reduce the vibrations. This paper addresses two simple active vibration control systems for reducing lateral vibrations: one feedforward system and one feedback. Computer simulations based on signals derived from a dynamic computer model of a train car indicates that the incorporation of an active control system, in addition to the passive approach, is likely to introduce an improved attenuation of the low-frequency lateral vibrations. Active control results illustrate an attenuation of the lateral train-car vibration by up to 14 dB.

  • 14. Johansson, Sven
    et al.
    Håkansson, Lars
    Persson, Per
    Claesson, Ingvar
    Active Control of Lateral Vibrations in a Railway Carriage2002In: International Journal of Acoustics and Vibration, ISSN 1027-5851, Vol. 7, no 4, p. 195-210Article in journal (Refereed)
    Abstract [en]

    As trains are continually designed for higher and higher speeds the problems of railway carriage vibration are on the increase. Lateral vibrations in a railway carriage are noticeable to passengers if the vibration frequencies are lower than approximately 20 Hz. Below this frequency discomfort is a common problem for the passengers and below approximately 1 Hz motion sickness is a problem. The passive solution of stiffening the carriage chassis to shift the vibrational frequencies higher up results in inflated manufacturing and running costs, and opposes higher travel speeds due to increased weight. Semi-passive solutions such as modifying the structural dynamics of the carriage body by decoupling heavy underfloor equipment do not reduce the vibrations sufficiently. However, by appending a multi-reference feedforward active vibration control system, one way expect a substantial reduction in the lateral vibration level. Using a dynamic computer model of a railway carriage simulating the lateral vibration, and using as input bogie acceleration data measured on a running train, multiple-input/single-output coherence spectra were shown to constitute a suitable set of reference signals for an active control system. Control simulations based on the Feedforward Multiple-Input/Single-Output Filtered-x LMS Algorithm are carried out using different reference signal combinations. The control results indicate lateral vibration attenuation on the order of 15dB at the objective frequency of 10Hz.

  • 15. Johansson, Sven
    et al.
    Håkansson, Lars
    Persson, Per
    Claesson, Ingvar
    THE MULTIPLE-INPUT/SINGLE-OUTPUT FEEDFORWARD CONTROL OF LATERAL VIBRATION IN A HIGH-SPEED TRAIN CAR2002Conference paper (Refereed)
    Abstract [en]

    The problem of low- frequency lateral train-car vibration is an important issue surrounding the design of high-speed trains. Passive solutions such as stiffening the car chassis are impractical because of the weight increase. Semi-passive solutions, such as modifying the structural dynamics of the carboy through non-rigid coupling of heavy underfloor equipment does not come with a weight penalty, but do not sufficiently reduce the vibrations. However, computer simulations based on signals derived from a dynamic computer model of a train car indicates that the incorporation of a feedforward active control system, is likely to introduce a substantial reduction in the lateral vibration level. A simple controller structure is used and it is based on the multiple- input/single-output filtered-x LMS algorithm. Active control results illustrate an attenuation of the lateral train-car vibration by up to 14 dB.

  • 16. Johansson, Sven
    et al.
    Håkansson, Lars
    Persson, Per
    Claesson, Ingvar
    The Multiple-input/Single-output Feedforward Control of Lateral Vibration in a High-speed Train Car2002Conference paper (Refereed)
    Abstract [en]

    The problem of low- frequency lateral train-car vibration is an important issue surrounding the design of high-speed trains. Passive solutions such as stiffening the car chassis are impractical because of the weight increase. Semi-passive solutions, such as modifying the structural dynamics of the carboy through non-rigid coupling of heavy underfloor equipment does not come with a weight penalty, but do not sufficiently reduce the vibrations. However, computer simulations based on signals derived from a dynamic computer model of a train car indicates that the incorporation of a feedforward active control system, is likely to introduce a substantial reduction in the lateral vibration level. A simple controller structure is used and it is based on the multiple- input/single-output filtered-x LMS algorithm. Active control results illustrate an attenuation of the lateral train-car vibration by up to 14 dB.

  • 17. Johansson, Sven
    et al.
    Lagö, Thomas L
    Borchers, Ingo
    Renger, Klaus
    Performance of a Multiple versus Single Reference MIMO ANC Algorithm based on a Dornier 328 test data set1997Conference paper (Refereed)
  • 18. Johansson, Sven
    et al.
    Lagö, Thomas L
    Claesson, Ingvar
    ANVC Performance and Robustness for Different Error Sensor Disturbance Signals1997Conference paper (Refereed)
    Abstract [en]

    When designing and using an ANVC system it is of interest how the system will be affected by different disturbances in the error microphones/accelerometers. With the use of a reference signal adaptive feedforward systems can attenuate a primary field substantially given that the noise field is tightly correlated with the reference signal. Questions that often arise in these applications are therefore how disturbances affect the performance: Will random noise in the error microphones affect the algorithm? Will error microphone tonal components affect the algorithm? Will speech in the error microphones affect the algorithm? How is convergence speed reduced by disturbances? In this paper, a filtered-X LMS feedforward controller has been investigated. Different types of noise sources have been injected and convergence speed as well as global reduction have been analyzed, given different contamination levels.

  • 19. Johansson, Sven
    et al.
    Lagö, Thomas L
    Claesson, Ingvar
    PERFORMANCE OF A SINGLE- VERSUS A TWIN-REFERENCE CONTROLLER IN A NARROWBAND ACTIVE NOISE CONTROL APPLICATION1999Conference paper (Refereed)
    Abstract [en]

    In many applications of noise control, the greatest annoyance is caused by periodic low frequency noise. Successful reduction of such noise can often be achieved by using an Active Noise Control (ANC) system with narrowband feedforward control. If several noise sources contribute to the sound field, a multiple--reference control system is usually required. This allows the reference signals from each noise source to be processed individually within the controller, thereby enabling individual control of the sound field from each noise source. The present paper addresses the problem of controlling noise from two sources that are more or less synchronized. A typical application is the control of propeller-generated noise within a twin propeller aircraft. To find out whether a multiple-reference controller is necessary, or if a single--reference controller can do the job, the performance of a single- versus twin-tacho control algorithm was evaluated in a comparative study. The study is a computer simulation using real life data recorded in a Dornier 328 under different flight conditions. The results demonstrate that the twin-tacho algorithm performs better than the single-tacho whenever there is a slight deviation in the rotational speed of the two propellers.

  • 20. Johansson, Sven
    et al.
    Lagö, Thomas L
    Nordebo, Sven
    Claesson, Ingvar
    CONTROL APPROACHES FOR ACTIVE NOISE CONTROL OF PROPELLER-INDUCED CABIN NOISE EVALUATED FROM DATA FROM A DORNIER 328 AIRCRAFT1999Conference paper (Refereed)
    Abstract [en]

    In many applications of noise control, the greatest annoyance is caused by periodic low frequency noise. Successful reduction of such noise can often be achieved by using an Active Noise Control (ANC) system with narrowband feedforward control. If several noise sources contribute to the sound field, a multiple-reference control system is usually required. This system allows the reference signals from each noise source to be processed individually within the controller, thereby enabling individual control of the sound field from each noise source. This paper presents a mukiple-reference control algorithm and addresses the problem of controlling noise from two sources that are more or less synchronized. A typical application is the control of propeller-generated noise within a twin propeller aircraft. To find out whether a multiple-reference controller is necessary, or if a single-reference controller can do the job, the performance of a single- versus twin-tacho control algorithm was evaluated in a comparative study. The study is a computer simulation using real life data recorded in a Domier 328 under different flight conditions. The results demonstrate that the twin-tacho controller performs better than the single-tacho whenever there is a slight deviation in the rotational speed of the two propellers.

  • 21. Johansson, Sven
    et al.
    Nordebo, Sven
    Claesson, Ingvar
    Convergence Analysis of a Twin-Reference Complex Least-Mean-Squares Algorithm2002In: IEEE transactions on speech and audio processing, ISSN 1063-6676, E-ISSN 1558-2353, Vol. 10, no 4, p. 213-221Article in journal (Refereed)
    Abstract [en]

    In many noise control applications, the noise is dominated by low frequencies and generated by several independent periodic sources. In such situations the tonal noise may be suppressed by using a narrowband multiple-reference feedforward controller. The performance characteristics of the control system, e.g., the convergence behavior and noise reduction are directly related to the controller adaptation rate as well as the frequency separation of the tonal components in the noise, i.e., the beat frequency. This paper treats the convergence performance of a complex least-mean-squares (LMS) algorithm using two reference signals. An analysis of its convergence behavior is presented as well as the results from computer simulations validating the convergence behavior. The convergence of the filter weights and the decrease rate of the squared error (the learning curve) for noise control applications are also discussed.

  • 22.
    Johansson, Sven
    et al.
    Blekinge Institute of Technology, Department of Signal Processing.
    Nordebo, Sven
    Blekinge Institute of Technology, Department of Signal Processing.
    Lagö, Thomas L
    Blekinge Institute of Technology, Department of Signal Processing.
    Sjösten, Per
    Blekinge Institute of Technology, Department of Signal Processing.
    Claesson, Ingvar
    Blekinge Institute of Technology, Department of Signal Processing.
    A Novel Multiple-Reference, Multiple-Channel, Normalized Filtered-X LMS Algorithm for Active Control of Propeller-Induced Noise in Aircraft Cabins1998Conference paper (Refereed)
    Abstract [en]

    The dominating cabin noise in propeller aircraft consists essentially of strong tonal components at harmonics of the Blade Passage Frequency (BPF) of the propellers. In order to efficiently reduce such low frequency periodic noise, it is advisable to employ an Active Noise Control (ANC) system based on a feedforward controller. This paper presents a set of normalized complex Filtered-X Least-Mean-Square (FX LMS) algorithms. By using different variants of normalization factors the convergence rate, the tracking performance and the steady-state noise attenuation can be improved. The algorithms presented are based either on a single normalization factor for the whole control system (global normalized FX LMS algorithm), or several individual normalization factors (reference-individual FX LMS algorithm) or the novel actuator-individual FX LMS algorithm. The evaluation is performed on noise recorded during flight in the cabin of a Dornier 328, a twin-engine propeller aircraft.

  • 23. Johansson, Sven
    et al.
    Nordebo, Sven
    Lagö, Thomas L
    Sjösten, Per
    Claesson, Ingvar
    Algoritmer för aktiv bullerundertryckning i propellerflygplan1997Conference paper (Refereed)
    Abstract [en]

    I många praktiska tillämpningar generas buller av roterande maskiner, t.ex. av kompressorer, fläktar, motorer och propellrar. Detta buller består huvudsakligen av periodiska komponenter, grundton med tillhörande övertoner. Vid denna typ av tillämpningar där bullret är lågfrekvent och periodiskt är det lämpligt att använda aktiv bullerreglering. Aktiva system baserade på framkopplad reglerteknik har visat sig ge en bra undertryckning av denna typ av buller. Ett framkopplat reglersystem behöver en synkroniseringssignal från den buller alstrande källan. Denna signal används sedan för att generera referenssignaler innehållande de frekvenser man vill undertrycka. Med hjälp av referenssignalerna generar reglersystemet ett ljudfält som är lika starkt som bullret och i motfas med detta. På detta sätt erhålls en reducering av bullret. För att ställa in den adaptiva regulatorn så att bästa möjliga undertryckning erhålls används reglermikrofoner som mäter det erhållna ljudtrycket. För att erhålla god undertryckning är det viktigt att referenssignalerna är väl korrelerade med bullret. I applikationer där endast en eller flera korrelerade källor bidrar till bullret räcker det att använda ett reglersystem baserat på en synkroniseringssignal, dvs ett enreferenssystem. Är det flera okorrelerade bullerkällor är det emellertid nödvändigt att använda en synkroniseringssignal från varje källa för att erhålla effektiv bullerundertryckning, dvs ett flerreferenssystem. I propellerflygplan härrör bullret huvudsakligen från propellrarna och de dominerande frekvenskomponenterna kan relateras till propellrarnas bladpassagefrekvenser (BPF) samt deras övertoner. Idag är de flesta tvåmotoriga flygplan utrustade med en synkroniseringsenhet, som synkroniserar propellrarnas varvtal. När propellrarna är synkroniserade kan de betraktas som fullständigt korrelerade, vilket medför att ett enreferenssystem kan användas för att åstadkomma en effektiv bullerundertryckning. I situationer där propellrarna är osynkroniserade skiljer sig propellrarnas varvtal åt och propellrarna kan nu betraktas som två okorrelerade källor. I dessa fall bör ett flerreferenssystem användas, eftersom denna regulatortyp kan följa varvtalsförändringar hos båda propellrarna. I denna presentation görs en jämförelse mellan prestanda för ett enreferenssystem och ett flerreferenssystem. Utvärderingen görs för två olika flygsituationer, synkroniserade respektive osynkroniserade propellrar.

  • 24. Johansson, Sven
    et al.
    Nordebo, Sven
    Lagö, Thomas L
    Sjösten, Per
    Claesson, Ingvar
    Borchers, Ingo
    Renger, Klaus
    Evaluation of a Multiple- versus a Single-Reference MIMO ANC Algorithm on Dornier 328 Test Data Set1998Conference paper (Refereed)
    Abstract [en]

    In many applications, such as in propeller aircraft, the dominating noise is periodic. Successful reduction of the periodic noise components can be achieved by using an Active Noise Control (ANC) system based on feedforward techniques. In this paper, a comparison between the performance of single--reference (single-tacho) and multiple--reference (twin-tacho) feedforward control systems is presented. The comparison is made for two different flight conditions, both with and without synchronized propellers. The evaluation results show that a multiple--reference controller provides better performance than a single--reference controller when a slight deviation exists in the propeller synchronization.

  • 25. Johansson, Sven
    et al.
    Nordebo, Sven
    Sjösten, Per
    Claesson, Ingvar
    A Novel Multiple-Reference Algorithm for Active Control of Propeller-Induced Noise in Aircraft Cabins2000Report (Other academic)
    Abstract [en]

    The cabin noise inside propeller aircraft is essentially dominated by strong tonal components at harmonics of the blade passage frequency of the propellers. In order to achieve an efficient reduction of such a periodic low frequency noise, it is advisable to use an active noise control system based on adaptive narrowband feedforward techniques. The feedforward controller presented in this paper exploits narrowband assumptions by using complex-valued filtering and complex modeling of control paths. This paper introduces a multiple reference controller based on the novel actuator-individual normalized Filtered-X Least-Mean-Squares (FX LMS) algorithm. This algorithm combines low computational complexity with high performance. The algorithm is of the LMS-type. However, owing to the novel normalization of the algorithm it can also be regarded as a Newton-type algorithm. A comparison between the actuator-individual normalized FX LMS algorithm and the ordinary normalized FX LMS algorithm is presented. The results demonstrate better performance in terms of convergence rate and tracking properties when the Newton-like actuator-individual normalized FX LMS algorithm is used as compared with the conventional normalized LMS algorithm. The evaluation was performed using noise signals recorded inside the cabin of a twin engine propeller aircraft during flight. The paper also discusses variants of the actuator-individual normalized FX LMS algorithm.

  • 26. Johansson, Sven
    et al.
    Nordebo, Sven
    Sjösten, Per
    Winberg, Mathias
    Claesson, Ingvar
    Active Control of Sound using the Actuator-Individual Normalized Filtered-X LMS Algorithm2001Conference paper (Refereed)
    Abstract [en]

    The control algorithm is a central part of an active noise control system. The computational power of the DSP hardware is limited, and therefore algorithms with high computational complexity allow fewer control sources and sensors to be used, often with reduced noise attenuation as a result. This paper presents results from practical experiments using an ANC system based on the actuator-individual normalized Filtered-X LMS algorithm. The algorithm combines low computational complexity with high performance. For that reason the algorithm is suitable for use in large multi-channel systems in order to reduce the computional power required by the control system.

  • 27. Johansson, Sven
    et al.
    Persson, Per
    Claesson, Ingvar
    ACTIVE CONTROL OF PROPELLER-INDUCED NOISE IN AN AIRCRAFT MOCK-UP1999Conference paper (Refereed)
    Abstract [en]

    A noisy environment dominated by low frequency noise can often be improved through the use of active noise control. This situation arises naturally in propeller aircraft where the propellers induce periodic low frequency noise inside the cabin. The overall interior sound pressure level inside a turboprop aircraft is typically rather high, and the passenger flight comfort could be improved considerably if this level was significantly reduced. This paper discusses the behaviour and robustness of a narrowband twin--reference feedforward active noise and vibration control system in a SAAB 340 mock-up. The resulting sound pressure level were recorded and is presented as the narrowband attenuation versus time. The spatial distribution of the sound pressure level is also presented. The results show that the twin-reference controller exhibits good performance with respect to attenuation and robustness for both stationary and non-stationary conditions.

  • 28. Johansson, Sven
    et al.
    Persson, Per
    Claesson, Ingvar
    Lagö, Thomas L
    Evaluation of the Performance of an Active Noise Control System in an Aircraft Mock-up2000Conference paper (Refereed)
    Abstract [en]

    This paper treats the convergence behavior and robustness of a narrowband twin-reference feedforward active control system using different kinds of actuators and control sensors, and various actuator/sensor configurations. Results from a practical experiment carried out in a fuselage section of a SAAB 340 aircraft, using beating and non-beating sound fields, are presented. The results show that the proposed multiple-reference controller based on the actuator-individual normalizedjfiltered-x LMS algorithm exhibits high performance with respect to robustness and noise suppression, for the different configurations as well as for beating and non-beating sound fields. A slight degradation of the performance was noticed for beating sound fields.

  • 29. Johansson, Sven
    et al.
    Sjösten, Per
    Nordebo, Sven
    Claesson, Ingvar
    Comparison of Multiple- and Single-Reference MIMO Active Noise Control Approaches Using Data Measured in a Dornier 328 Aircraft2000In: International Journal of Acoustics and Vibration, ISSN 1027-5851, Vol. 5, no 4, p. 77-88Article in journal (Refereed)
    Abstract [en]

    In many applications of noise control, the greatest annoyance is caused by periodic low frequency noise. Successful reduction of such noise can often be achieved by using an active noise control system with narrowband feedforward control. If several noise sources contribute to the sound field, a multiple-reference control system is usually required. This type of system allows the reference signals from each noise source to be processed individually within the controller, thereby enabling individual control of the sound field from each noise source. The present paper addresses the problem of controlling noise from two sources that are more or less synchronized. A typical application is the control of propeller-generated noise within a twin propeller aircraft. To find out whether a multiple-reference controller is necessary, or if a single-reference controller is sufficient, the performance of a single- versus twin-reference control algorithm is evaluated in a comparative study. The study is performed as a computer simulation (off-line evaluation) using real-life data recorded in a Dornier 328 under different flight conditions. The results demonstrate that the twin-reference controller performs better than the single-reference whenever there is a slight deviation in the rotational speed of the two propellers. The paper also treats the generation of reference signals. The approach presented is based on a fixed sampling rate and uses a sliding FFT filtering technique.

  • 30. Johansson, Sven
    et al.
    Sjösten, Per
    Persson, Per
    Claesson, Ingvar
    Experimental Performance Evaluation of a Multi-Reference Algorithm for Active Control of Propeller-Induced Cabin Noise2000Report (Other academic)
    Abstract [en]

    A noisy environment dominated by low frequency noise can often be improved through the use of active noise control. This situation arises naturally in propeller aircraft where the propellers induce periodic low frequency noise inside the cabin. The cabin noise is typically rather high, and the passenger flight comfort could be improved considerably if this level were significantly reduced. This paper discusses the operation and robustness of a narrowband feedforward active noise control system in a practical installation. The ANC system used 8 control sources and 11 control microphones, and the control algorithm was the multi-reference actuator-individual normalized filtered-x least mean squares algorithm. The experiment was performed in a full-scale fuselage section of a SAAB 340 aircraft. To produce the propeller noise, loudspeakers mounted in a ring around the fuselage were used. Results are presented from a series of experiments on the active control of propeller-induced cabin noise. Among the ``flight" conditions evaluated were: conditions where the ``propellers" were completely synchronized, and conditions with constant as well as time-varying frequency beat between ``left and right propellers."

  • 31. Johansson, Sven
    et al.
    Winberg, Mathias
    Claesson, Ingvar
    A New Passive/Active Hybrid Headset for a Helicopter Application1999Conference paper (Refereed)
    Abstract [en]

    In helicopters, the low frequency noise generated by the rotors and engines often masks and jeopardizes safe communication. In addition, pilots are likely to suffer from damage to their hearing due to the high sound levels in the headset produced in compensating for the noise caused by increased loudspeaker levels. A feasible approach is to reduce the low frequency noise by using active techniques combined with a method for reducing the noise in the intercom microphone signal, with lower loudspeaker levels as a result. In order to achieve an efficient attenuation of the primary noise inside the headset, a combination of a digital feedforward controller and an analog feedback controller is employed. Spectral Subtraction is used to suppress the background noise in speech signals. This paper evaluates a combination of the techniques and their application to real data.

  • 32. Johansson, Sven
    et al.
    Winberg, Mathias
    Claesson, Ingvar
    Svensson, Börje
    Lönn, Jan
    Active Control of Interior Noise in a Fork-Lift Truck2003Conference paper (Refereed)
    Abstract [en]

    Modern fork-lift trucks incorporate a wide range of sophisticated passive techniques to reduce the noise inside the cabin. By using a combination of passive and active noise control technologies it may be possible to further reduce the noise level, thereby improving the comfort for the truck driver. The purpose of the work presented in this paper is to examine whether it is possible to reduce the low-frequency cabin noise by using active noise control technology. The noise situation inside the truck cabin is discussed as well as the active noise control strategy for a practical fork-lift truck installation. The paper presents sound field plots of the cabin noise and preliminary active noise control results from computer experiments based on recorded cabin noise.

  • 33. Johansson, Sven
    et al.
    Winberg, Mathias
    Lagö, Thomas L
    Claesson, Ingvar
    A New Active Headset For a Helicopter Application1997Conference paper (Refereed)
    Abstract [en]

    In helicopters, the low frequency noise generated by the rotors and engines often masks and jeopardizes safe communication. Additionally, pilots are likely to suffer from hearing damages due to the higher sound levels in the headset produced when compensating for the noise by increased speakerlevels. A feasible approach is to reduce the low frequency noise using active techniques, thereby enabling lower speakerlevels. In many Active Noise Control (ANC) applications the primary noise field is either periodic or broadband which simplifiesthe choice of algorithm. In our application, noise up to 100Hz is dominated by tones and in the range from 100 Hz to 400 Hz the noise characteristicsis more broadband. In order to achievean efficient attenuation of the primary noise, a combination of a digital feedforward controller and an analog feedback controller is employed. The feedforward controller is tachometer based and reduces the tonal components, while the feedback controller attenuates the more broadband noise. In this paper, a combination of these two techniques is evaluatedon real data.

  • 34. Johansson, Sven
    et al.
    Winberg, Mathias
    Lagö, Thomas L
    Claesson, Ingvar
    A New Active Headset for a Helicopter Application1997Conference paper (Refereed)
  • 35. Johansson, Sven
    et al.
    Winberg, Mathias
    Sjösten, Per
    Claesson, Ingvar
    ACTIVE HEADSET FOR ENHANCED SPEECH INTELLIGIBILITY IN INTERCOM SYSTEMS2001Conference paper (Refereed)
    Abstract [en]

    Low frequency noise has a masking effect on speech, which significantly reduces the speech intelligibility. Usually, in order to overcome the masking effect the sound level of the communication signal is increased. Exposure to the communication system then causes hearing damage. A feasible approach is to reduce the low frequency noise by using active techniques combined with a method for reducing the noise in the intercom microphone signal. In order to achieve an efficient low frequency noise attenuation inside the headset an active hybrid headset is proposed, and in order to suppress the background noise in speech signals spectral subtraction is used. Improved low frequency noise attenuation enables lower communication levels and reduces the risk for hearing damage.

  • 36.
    Khan, Imran
    et al.
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Moazzam, M
    Rabbani, S.M.
    Johansson, Sven
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Håkansson, Lars
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    PERFORMANCE EVALUATION OF CONTROL ALGORITHMS IMPLEMENTED ON A REMOTELY CONTROLLEDACTIVE NOISE CONTROL LABORATORY2013Conference paper (Other academic)
  • 37.
    Khan, Imran
    et al.
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Muhammad, Moazzam
    Rabbani, Muhammad Shoaib
    BTH.
    Johansson, Sven
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Håkansson, Lars
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Performance evaluation of control algorithms implemented on a remotely controlled active noise control laboratory2013In: Active Noise and Vibration Control in Practical System Implementations, 2013, article id 731Conference paper (Other academic)
    Abstract [en]

    The remotely controlled laboratory setup for Active Noise Control (ANC) developed by Blekin-ge Institute of Technology, Sweden provides an efficient learning platform for the students to implement and learn ANC algorithms with real world physical system, hardware and signals. The initial laboratory prototype based on a Digital Signal Processor (DSP) TMS320C6713 from Texas Instruments (TI) was successfully tested with Filtered-x Least Mean Square (F-XLMS) algorithm applied to control noise in a ventilation duct. The resources of the DSP platform used in the remote laboratory setup enable testing and investigating substantially more challenging and computationally demanding algorithms. In this paper, we expand the horizon of the laboratory setup by testing more advanced and complicated single channel feed forward ANC algorithms. Filtered-x versions of algorithms such as the normalized least mean square (N-LMS), leaky least mean square (L-LMS), Filtered-U recursive least mean square (FURLMS) and recursive least square (RLS) algorithm etc. have been implemented utilizing the remote web based client provided in the remote laboratory. A comprehensive performance comparison of the aforementioned algorithms for the remote laboratory setup is presented to demonstrate the viability of the remote laboratory.

  • 38.
    khan, Imran
    et al.
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Muthusamy, Dineshkumar
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Ahmad, Waqas
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Gustavsson, Ingvar
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Zackrisson, Johan
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Nilsson, Kristian
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Johansson, Sven
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Håkansson, Lars
    Blekinge Institute of Technology, School of Engineering, Department of Electrical Engineering.
    Remotely Controlled Active Noise Control Laboratories2012Conference paper (Refereed)
    Abstract [en]

    Remotely controlled laboratories in educational institutions are gaining popularity at an exponential rate due to the multidimensional benefits they provide. The Virtual Instrument Systems in Reality (VISIR) project by Blekinge Institute of Technology (BTH) Sweden has successfully implemented remotely controlled laboratories, with remotely controlled real instruments and experimental setups. Currently these laboratories provide students the opportunity to conduct experiments in the field of electronics, antenna theory and mechanical vibration measurements. In this paper a prototype system of a remotely controlled laboratory for active noise control (ANC) is introduced. The proposed lab will focus on addressing the problem of a ventilation duct noise. The laboratory is informative and to a great extent introduces a student to the general steps in ANC when it is suggested as a plausible solution for a noise problem. The student can perform an investigation concerning feasibility of active control, design, configuration and implementation of an active control system. The laboratory is based on a modern and relevant DSP platform with the corresponding software development environment controlled remotely. In addition, it may be utilized remotely both for lab assignments in acoustics courses and digital signal processing courses.

  • 39. Lagö, Thomas L
    et al.
    Johansson, Sven
    Hellström, Per-Anders
    Analysis of Helicopter Sound for the Development of A New Generation Active Headset1997Conference paper (Refereed)
    Abstract [en]

    Helicopters generate substantial noise levels, especially at low frequency. These noise levels are normally not harmful for the ear. However, the low frequency content masks the speech. Therefore, pilots tend to set the communication system at maximum level so that the sound levels reach severe amplitudes for the ear. These high sound levels are exposing the ear to fatigue and hearing loss. The low speech intelligibility caused by the background noise is also a safety issue since it is most important that all commands can be understood correctly. The risk of noise induced hearing loss can normally be evaluated by sound pressure level measurements. However, in some cases, the standardized methods for these measurements does not take into account some secondary effects of the frequency distribution of the sound pressure levels. This paper addresses a noise exposure situation where the noise level itself is not harmful for hearing but is still the reason for increased risk for noise induced hearing loss. Military helicopter crews in Sweden have recent shown an increase in noise induced hearing loss. A combined active passive technique has been chosen as a possible solution. Therefore, the dominant sound sources have to be identified and the number of harmonics for each source determined. An investigation of the coupling between the structure borne sound and the air borne sound was also performed. This analysis is the basis for the development of a new generation active headset, and is presented in this paper.

  • 40. Lagö, Thomas L
    et al.
    Winberg, Mathias
    Johansson, Sven
    AVIIS, ACTIVE VIBRATION ISOLATION IN SHIPS: SOURCE IDENTIFICATION AND ANVC APPROACH1997Conference paper (Refereed)
    Abstract [en]

    Engine induced sound and vibration levels in boats for professional and leisure use is in many cases unacceptably high in term of comfort and the environment. Classical methods for passive treatment are normally less effective due to the low frequency content and often leads to an increase in weight. This contradicts the requirements for lower weight for increased speed. More efficient vibration damping methods must therefore be found. With active engine mounts, it is possible to achieve a decrease in the vibrations even when the hull is not very stiff. This is especially important in marine applications since the engines are mounted on weak and light structures. The AVIIS project aims at investigating the effects of a combined passive/active engine mount for use in boats. A Storebro 36 Royal Cruiser with two Volvo Penta engines has been used in the project. Four different approaches have been appraised, the results of which are presented here: 1. passive engine mounts, with and without thrust bearings, 2. optimized passive engine mounts, 3. passive engine mounts, rigidly mounted, 4. A combined active/passive engine mount. This paper reports the key data from the measurements and how the different primary sources have been estimated from the analysis. This analysis has then been used to select the ANVC approach.

  • 41. Larsson, Martin
    et al.
    Johansson, Sven
    Claesson, Ingvar
    Håkansson, Lars
    A Module Based Active Noise Control System for Ventilation Systems, Part I: Influence of Measurement Noise on the Performance and Convergence of the Filtered-x LMS Algorithm2009In: International Journal of Acoustics and Vibration, ISSN 1027-5851, Vol. 14, no 4, p. 188-195Article in journal (Refereed)
    Abstract [en]

    Low noise level is an essential feature when installing ventilation systems today. To achieve attenuation over a broad frequency range, the passive silencers traditionally used to attenuate ventilation noise can be combined with an active noise control (ANC) system. To insure reliable operation and desirable levels of attenuation when applying ANC to duct noise, it is highly important to be able to suppress the contamination of the microphone signals due to the turbulent pressure fluctuations arising as the microphones are exposed to the airflow in the duct. This paper is the first in a series of two, that treats the problem of turbulence induced noise originating from the airflow inside the ducts. Part I is concerned with theoretical and experimental investigations of the influence of the turbulence induced noise on the adaptive algorithm in the ANC system. Part II is concerned with design and investigations of microphone installations for turbulence suppression and results concerning the performance of an ANC system with the different microphone installations are presented. Some of the results were obtained at an acoustic laboratory according to an ISO-standard. The attenuation achieved with ANC was approximately 15-25 dB between 50-315 Hz even for airflow speeds up to 20 m/s.

  • 42. Larsson, Martin
    et al.
    Johansson, Sven
    Claesson, Ingvar
    Håkansson, Lars
    A Module Based Active Noise Control System for Ventilation Systems, Part II: Performance Evaluation2009In: International Journal of Acoustics and Vibration, ISSN 1027-5851, Vol. 14, no 4, p. 196-206Article in journal (Refereed)
    Abstract [en]

    To utilize the full noise attenuation potential of an active noise control (ANC) system applied to duct noise, it is important to be able to minimize the turbulence induced noise in the microphone signals. This is the second paper in a series of two, that treats the problem of turbulence induced noise originating from the airflow inside the ducts, when applying ANC to ducts. Part I contains theoretical and experimental investigations of the influence of the turbulence induced noise on the filtered-x LMS algorithm used in the ANC system. Part II (the present paper) is concerned with design and investigations of microphone installations which produces a sufficient amount of turbulence suppression while also meeting industry requirements. These requirements are, for example, that the microphone installations should be based on standard ventilation parts, and that they should be easily installed and maintained. Furthermore, results concerning the performance of an ANC system with different microphone installations are presented. Some of the results were obtained at an acoustic laboratory according to an ISO standard. The attenuation achieved with ANC was approximately 15-25 dB between 50-315 Hz even for airflow speeds up to 20 m/s.

  • 43. Larsson, Martin
    et al.
    Johansson, Sven
    Claesson, Ingvar
    Håkansson, Lars
    Influence of measurement noise on the performance and convergence of the filtered-x LMS algorithm; Theory and simulations2010Conference paper (Refereed)
    Abstract [en]

    In active noise control (ANC) applications the sensor signals feeding the controller may be contaminated by signal components not linearly related to the actual noise to be controlled, e.g. measurement noise. An example is the noise in the microphone signals generated by airflow turbulence when applying ANC to ducts. The measurement noise may significantly degrade the performance of an ANC system. This paper is concerned with theoretical investigations of the influence of measurement noise on the performance and convergence of the filtered-x LMS algorithm. Further, computer simulations have been performed to verify the theoretical results. In the theoretical investigations, the convergence of the filtered-x LMS algorithm is derived for different cases with measurement noise affecting the different sensor signals. These cases are compared to the ideal case with no measurement noise present at neither sensor signal. The results from both the theoretical investigations and the simulations show that measurement noise can, depending on the SNR of the sensor signals, degrade the performance of the filtered-x LMS algorithm regarding both the filter coefficients the algorithm converges to in mean, and the convergence rate of the algorithm.

  • 44. Larsson, Martin
    et al.
    Johansson, Sven
    Håkansson, Lars
    Claesson, Ingvar
    A FEEDFORWARD ACTIVE NOISE CONTROL SYSTEM FOR DUCTS USING A PASSIVE SILENCER TO REDUCE ACOUSTIC FEEDBACK2007Conference paper (Refereed)
    Abstract [en]

    Ventilation systems installed in buildings usually generate low-frequency noise because the passive silencers commonly used to attenuate the ventilation noise are not effective in the low-frequency range. A method proven to effectively reduce low-frequency noise in a wide variety of applications is active noise control (ANC). A feedforward ANC system applied to duct noise normally uses a reference microphone, a control unit, a loudspeaker to generate the secondary noise created by the controller, and an error microphone. The secondary noise generated by the loudspeaker will travel both downstream canceling the primary noise, and upstream to the reference microphone, i.e. acoustic feedback. The acoustic feedback may result in performance reduction and stability problems of the control system. Common approaches to solve the feedback problem result in more complex controller structures and/or system configurations than the simple feedforward controller, e.g. introducing a feedback cancellation filter in the controller in parallel with the acoustic feedback path, or using a dual-microphone reference sensing system. This paper presents a simple approach to reduce the acoustic feedback by using a basic feedforward controller in combination with a passive silencer. Simulations show that efficient acoustic feedback cancellation is achieved by using a passive silencer. In the experimental setup another advantage with using a passive silencer is that the frequency response function of the forward path, which is to be estimated, is smoother, i.e. most of the dominant frequency peaks in the frequency response function when not using a passive silencer is reduced. This in turn results in an acoustic path that is less complex to estimate with high accuracy using an adaptive FIR filter steered with the LMS algorithm.

  • 45. Larsson, Martin
    et al.
    Johansson, Sven
    Håkansson, Lars
    Claesson, Ingvar
    A SYSTEM IMPLEMENTATION OF AN ACTIVE NOISE CONTROL SYSTEM COMBINED WITH PASSIVE SILENCERS FOR IMPROVED NOISE REDUCTION IN DUCTS2007Conference paper (Refereed)
    Abstract [en]

    This paper presents a feedforward active noise control system combined with passive silencers for reducing acoustic noise propagating through ventilation ducts. It is investigated if the passive silencers can increase the noise attenuation potential of the active noise control system and experimental results are presented. The results show that installing a passive silencer results in less pronounced standing waves in the duct and hence to performance increase of the active noise control system. Evaluating measurements regarding the performance of the active noise control system have also been conducted in an acoustic laboratory according to an ISO-standard.

  • 46. Larsson, Martin
    et al.
    Johansson, Sven
    Håkansson, Lars
    Claesson, Ingvar
    Experimental Investigations of Different Microphone Installations for Active Noise Control in Ducts2006Conference paper (Refereed)
    Abstract [en]

    A request on ventilation systems today is the feature of a low noise level. A common method to attenuate ventilation noise is to use passive silencers. However, such silencers are not suitable for the lowest frequencies and one solution is to use active noise control (ANC) to increase the noise attenuation in the low frequency range. Normally when using a feedforward ANC system to attenuate duct noise, both the reference microphone and the error microphone are exposed to airflow. As the airflow excites the diaphragm of the microphones, the microphone signals become contaminated by uncorrelated pressure fluctuations that are not part of the sound propagating in the duct. By reducing the flow velocity around the microphones, these uncorrelated pressure fluctuations can be reduced and the noise reduction improved. One way to reduce the flow velocity around the microphones is to place the microphones in outer microphone boxes connected to the duct via a small slit. In this paper a new practical design for the reduction of flow velocity around the microphones is presented; the microphone installation is based on a T-duct, and therefore it makes maintenance and especially construction easier, compared to the microphone box with a slit. Furthermore, comparative results concerning the performance of an ANC system for the two different microphone installations, the T-duct configurations and the microphone boxes with varying slit width, are presented. The results show that the active noise control performance is almost equal when using the suggested microphone installation as compared to when using a microphone box with a slit.

  • 47. Larsson, Martin
    et al.
    Johansson, Sven
    Håkansson, Lars
    Claesson, Ingvar
    Microphone Windscreens for Turbulent Noise Suppression when Applying Active Noise Control to Ducts2005Conference paper (Refereed)
    Abstract [en]

    Low noise level is an essential feature when installing ventilation systems nowadays. The traditional noise control approaches use passive silencers to attenuate the undesired ventilation noise. These silencers have a high attenuation over a broad frequency range. However, traditional passive silencers are ineffective at low frequencies and tend to be relatively large and bulky when they are used for low frequencies. An approach to improve the low frequency noise attenuation and to reduce the size of a low frequency silencer is active noise control (ANC). A problem when applying ANC to attenuate noise in ducts is that both the reference microphone and the error microphone are placed in an air flow. Accordingly, the microphones sense the sound propagating through the duct as well as the turbulent fluctuations generated by the wind passing over the microphones. The turbulent flow noise reduces the coherence between the reference microphone and the error microphone, resulting in reduced performance of a feedforward ANC system. For improving the performance it is important with as little corruption from turbulent flow noise as possible. The coherence can be improved by reducing the flow velocity around the microphones by using some kind of windscreen. This paper presents comparison results for microphone installations based on different windscreens for suppression of the turbulent wind noise. The presented measurements are carried out in the frequency range 0-400 Hz - the plane wave propagation region for the ducts in use - and for flow speeds up to 5,9 m/s. The results show that with appropriate screens and placement the attenuation and frequency range of attenuation can be significantly improved.

  • 48. Larsson, Martin
    et al.
    Johansson, Sven
    Håkansson, Lars
    Claesson, Ingvar
    Performance Evaluation of a Module Configured Active Silencer for Robust Active Noise Control of Low Frequency Noise in Ducts2008Report (Other academic)
    Abstract [en]

    Low noise level is an essential feature when installing ventilation systems today. Since the passive silencers traditionally used to attenuate ventilation noise tend to become bulky, impractical, and expensive when designed for low frequency attenuation, other solutions for the reduction of the low frequency duct noise often present in ducts are of interest. Active noise control (ANC) is a well known method for attenuating low frequency noise and much research has been performed to successfully apply ANC to duct noise. To insure reliable operation and desirable levels of attenuation when applying ANC to duct noise, it is of highest importance to be able to suppress the contamination of the microphone signals due to the turbulent pressure fluctuations arising as the microphones are exposed to the airflow in the duct. The work presented in this report is concerned with analysis of the influence of the turbulence induced noise on the adaptive algorithm in the ANC system, and design of microphone installations which produce sufficient turbulence suppression while also meeting industrial requirements. These requirements are, for example, that the installations should be based on standard ventilation parts, and that they should be easily installed and maintained. Furthermore, results concerning the performance of an ANC system with different microphone installations are presented. Some of the results were obtained at an acoustic laboratory according to an ISO standard. The attenuation of duct noise achieved with ANC was approximately 15-25 dB between 50-315 Hz, even for airflow speeds up to 20 m/s.

  • 49. Larsson, Martin
    et al.
    Johansson, Sven
    Muddala, S.M.
    Gafar, A.E. Mohamed
    Håkansson, Lars
    Tarkka, Juhani
    Sandor, Mats
    An Initial Study on Applying Active Noise Control to an Insulated Box Fan Used in Ventilation System Applications2009Conference paper (Refereed)
    Abstract [en]

    In many different applications and buildings fans are used to remove unwanted and used air. These fans often generate broadband and tonal noise. Commonly, passive resistive silencers are used to attenuate noise generated by different types of fans installed in ventilation systems. Passive silencers tend to become bulky and impractical when designed for low frequency attenuation. However, active noise control (ANC) is a technique known for its ability to produce high levels of attenuation in the low frequency range, even with a relatively moderate sized ANC system. This paper presents an initial study performed to investigate the possibilities of applying ANC to a radial fan installed inside a box, an insulated box fan. The box is connected to a duct system and can for example be used as a waste air fan. The primary interest in this application, when the fan is used as a waste air fan, is to attenuate the noise generated on the suction side, since that side generates noise into a particular room. Investigations were carried out to determine where the ANC system should be installed, e.g. inside the box, in the duct connected to the box etc. Factors considered were for example, turbulence, standing waves, the type of noise generated by the fan (tonal, broadband, or a combination), and space limitations. The noise generated by the fan was found to be dominated by a tonal component, but also to have broadband energy in the low frequency range. Further, a feedforward ANC system was applied on the suction side, producing approximately 28 dB attenuation of the tonal component, and 5-10 dB attenuation of the broadband noise between 50 and 200 Hz.

  • 50. Larsson, Martin
    et al.
    Nilsson, Kristian
    Johansson, Sven
    Claesson, Ingvar
    Håkansson, Lars
    An Active Noise Control Approach for Attenuating Noise Above the Plane Wave Region in Ducts2011Conference paper (Refereed)
    Abstract [en]

    In a narrow duct, a relatively simple single-channel feedforward ANC system may be used to attenuate noise propagating as plane waves. However, for ducts with larger dimensions the cut-on frequencies for one or several higher-order acoustic modes may be within the frequency range where ANC is applied. In such situations it is generally necessary to use a multiplechannel feedforward ANC system with several secondary sources, error sensors, and perhaps reference sensors. Such a system has a significantly higher complexity than a single-channel ANC system. In this paper another approach is described. Instead of using a multiple-channel feedforward ANC system on a duct of large dimension, the idea is to divide the duct into several more narrow parallel ducts. In this way the complexity of the ANC system may be reduced. In the experiments conducted for this paper, a duct was divided into two more narrow ducts. The noise propagating in each duct was controlled by a feedforward ANC system based on the leaky filtered-x LMS algorithm, where different reference- and error microphone configurations were used. The different configurations were compared to a configuration where the noise in respective narrow duct was controlled using a basic single-channel ANC system per duct. The results preliminary show that high attenuation of low-frequency noise in a duct of large dimension may be achieved using this approach.

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