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Träfflista för sökning "WFRF:(Larsson Lars) ;pers:(Håkansson Lars)"

Sökning: WFRF:(Larsson Lars) > Håkansson Lars

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  • Larsson, Martin, et al. (författare)
  • 2007
  • Ingår i: ICSV 14.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • 2007
  • Ingår i: Fan Noise.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • An Active Noise Control Approach for Attenuating Noise Above the Plane Wave Region in Ducts
  • 2011
  • Ingår i: International Congress on Sound and Vibration, ICSV.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • An Initial Study on Applying Active Noise Control to an Insulated Box Fan Used in Ventilation System Applications
  • 2009
  • Ingår i: The Sixteenth International Conference on Sound and Vibration, ICSV 16.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • Experimental Investigations of Different Microphone Installations for Active Noise Control in Ducts
  • 2006
  • Ingår i: Thirteenth International Congress on Sound and Vibration.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • Influence of measurement noise on the performance and convergence of the filtered-x LMS algorithm; Theory and simulations
  • 2010
  • Ingår i: ICSV17.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • Microphone Windscreens for Turbulent Noise Suppression when Applying Active Noise Control to Ducts
  • 2005
  • Ingår i: Twelfth International Congress on Sound and Vibration, ICSV12.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
  • Larsson, Martin, et al. (författare)
  • Performance Evaluation of a Module Configured Active Silencer for Robust Active Noise Control of Low Frequency Noise in Ducts
  • 2008
  • Rapport (övrigt vetenskapligt)abstract
    • 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.
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