Improving loudspeaker performance for active noise control applications

Hybrid noise control is the combination of both active noise and passive noise control. For high-frequency noise, we generally use passive noise control technique, and for low-frequency noise of less than 200 Hz, we use active noise control technique. Passive noise control is achieved by introducing passive techniques such as enclosures, physical barriers, and silencers to attenuate the undesired noise, whereas active noise control (ANC) is achieved by introducing a cancelling “anti-noise” signal generated by a processor or electronics circuit. Digital filters can be used for this purpose with an appropriate adaptive algorithms such as FXLMS, FXNLMS, FBLMS, SE-FXLMS, and FULMS. The development of an ANC simulation model can be divided into two parts: the modelling of primary noise and the modelling of secondary or counter noise. The anti-noise generated by the ANC system can be modelled only if we have the information of primary noise. For successful simulation of secondary noise, we have to model the control system and its interaction with the surrounding acoustical space. This article describes the practical application of active noise control as well as passive noise control to a server room, greatly reducing the level of noise in the whole area through appropriate adaptive algorithms. The results obtained using different algorithms are compared and analyzed.

Commercial active noise and vibration control systems for vehicles are gradually developing, mainly in Japan, for both active noise control of interior sound and active control engine mounts [1]. There have been four Japanese commercial applications: • Active control for booming noise by Nissan in 1991, • Active vibration mount for idle noise and vibration by Toyota in 1997, • Active vibration mount for diesel engines by Nissan in 1998, • Active control for low-frequency road noise by Honda in 2000. Other manufacturers have, of course, developed similar systems, but these have not progressed to production applications. Additionally, active control engine mufflers using cancelling loudspeakers have been successfully demonstrated. A new type of active muffler, using a controlled oscillating butterfly valve in the exhaust line has been developed recently. The future of active control in vehicles is dependent on customers who are willing to pay the higher price for additional noise and vibration control, so that cost reductions are the essential factor in further commercial products. Trends towards fuel efficiency and lighter vehicles run counter to reducing noise and vibration. This has given new interest in active noise control and active control engine mounts as a solution to these conflicting issues.

In most practical applications of active noise control, acoustic feedback is a major problem that often interferes with the operation of the control system and even renders it unstable. The optimal collocated positions of secondary source and two microphones are studied and the theoretical equations that reduce the influence of acoustic feedback are developed based on plane-wave transmission theory of sound in a duct. The original signal of primary noise is obtained from the measured signals of reference microphone and error microphone, which are transformed to frequency domain by FFT and operated. In this study, the performance of the proposed method is compared with other traditional time-domain methods by means of simulation and analysis using LabVIEW programming language. The results showed that the proposed method can effectively reduce the influence of acoustic feedback. In practical application of active noise control in a duct, the adaptive control or other control method incorporating the proposed method in this paper will improve the stability and performance effectively.

Application of active noise control for engine related cabin noise

A new efficient filtered-x affine projection sign algorithm for active control of impulsive noise

This paper focuses on acoustic modeling for active noise control applications. The desired model must be finite dimensional and efficient numerically to meet implementation requirements. It must also be parameterized by the position considered inside the cavity. This additional characteristic is desirable for addressing specific active noise control applications such as optimizing microphone placement, estimating acoustic pressure at different positions of the considered cavity, and finally attenuating noise level in a subarea of the cavity. The main contribution of this paper is to propose a finite-dimensional, low-order, and parameterized acoustic model of a cavity, suitable for active noise control applications. The resulting model is defined as a gray box that combines a one-dimensional analytical model of acoustic propagation, which handles the physical parameterization, and a black-box model that copes with actuator and sensor dynamics as well as modeling errors. The parameters of the proposed model are optimized in order to reproduce the frequency behavior of the real system (LS2N active noise control platform) in a frequency range. This model allows one to accurately reproduce the dynamics at any position in the considered cavity. The prediction performance of the proposed model was compared to a classical black-box model (usually used for active noise control applications) and validated experimentally using the LS2N active noise control platform. The analysis highlighted that the proposed gray-box model can predict the acoustic behavior in a great range of positions.

Application of active noise control (ANC) was discussed as method to prevent the occurrence of an inner acoustic mode inside the semi-closed cover placed in the diffuse acoustic field. The effect was validated by the experimental measurement using a reverberant room. The results show that the acoustic mode formed by semi-closed cavity has locally-coherency at its occurrence frequency and the inside cover acoustic mode can be controlled by applying ANC. It has been confirmed that noise reduction of ANC for the peak level caused by the acoustic mode is approximately 5dB.

The application of active noise control has been extended from single noise source, one-dimensional acoustical fields to complicated multiple noise source, three-dimensional acoustical fields. The application to a multiple noise source environment usually requires multiple reference sensors to generate a complete vector of reference signals. A widely adopted control configuration feeds each reference signal into a different control filter. This approach suffers from the problem of ill-conditioning when the reference signals are correlated. The problem of ill-conditioning results in slow convergence rate and high sensitivity to measurement error especially when the FXLMS algorithm is applied. A practical method is presented to pre-process the reference signals based on the Wiener filter theory and Gram-Schmidt orthogonalization which overcomes these problems. Simulations and experiments have been conducted to verify the issues associ- ated with multiple reference ANC when the signals are correlated. The results presented are consistent with the theoretical analysis. [Work supported by ONR.]

In practical active noise control (ANC) applications, various constraints are usually required, for example, the disturbance enhancement constraint, the robust stability constraint, and the controller output power constraint. One commonly used approach for designing a constrained ANC filter is to formulate a constrained optimization problem using an H / H framework, which requires significant computational power to solve. Recent work has shown that such an ANC filter design problem can be reformulated into a convex optimization problem and then further reformulated to a cone programming problem to reduce the required computational time by several orders. However, the standard cone programming reformulation procedure leads to a large number of free variables, which, in many applications, can adversely influence the numerical behavior of the optimization algorithm. In the current work, the ANC filter design problem structure is exploited in its dual conic form, which allows an elimination of free variables and can result in a numerically more stable solving process for the filter design problem while keeping the noise control performance unchanged. It is demonstrated that when compared with the reformulation using the standard procedure, the proposed formulation significantly improves its numerical stability and computational efficiency.

The authors present a delayed least mean square (LMS) algorithm which introduces delay in the reference input for active periodic noise control applications. Results show a reduction in out-of-band overshoot and coefficient quantization noise for fixed-point implementation. Computer simulations show an average of 5 dB overshoot constraint and 10 dB quantization noise reduction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SUMMARYIn real‐world active noise control (ANC) applications, disturbance can be picked up by error sensors and significantly degrade the steady‐state ANC performance. This study proposes two techniques in combination with a least‐mean‐square (LMS) based ANC algorithm, named normalized filtered‐x LMS/commutation error (NFxLMS/CE) algorithm, to deal with the disturbance that is independent of a reference signal. A new stochastic method to analyze convergence properties of the NFxLMS/CE algorithm under influence of the disturbance is first established. Given that the reference signal is persistently exciting of sufficient order, exponential convergence of the algorithm is derived with a step‐size condition. An exponential‐decay step size (EDSS) is then proposed to obtain a new ANC algorithm referred to as EDSS‐NFxLMS/CE algorithm. In addition, a disturbance‐compensation (DC) technique is developed for the EDSS‐NFxLMS/CE algorithm to obtain an EDSS‐NFxLMS/CE_DC algorithm such that the influence of the disturbance can be reduced. It is shown that the EDSS‐NFxLMS/CE_DC algorithm is exponentially convergent. Moreover, computer simulations show that the EDSS‐NFxLMS/CE_DC algorithm can achieve a better ANC performance in terms of convergence rate and level of noise reduction as compared with that using the EDSS‐NFxLMS/CE algorithm without DC and that using NFxLMS/CE_DC algorithm of constant step sizes. These results support the effectiveness of the proposed techniques and EDSS‐NFxLMS/CE_DC algorithm. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A new type of piezoelectric air transducer has been developed for active noise control and other air acoustics applications. The transducer is based on the composite panel structure of a bimorph-based double amplifier, that is, two parallel bimorphs or bimorph arrays with a curved cover plate as an active face attached to the top of the bimorphs. The electro- mechanical and electro-acoustic properties of the double amplifier structure and the transducer are investigated in this paper. The displacement of the cover plate of the double amplifier structure can reach millimeter scale with a relatively low driving voltage, which is more than ten times larger than the tip displacement of bimorphs. The sound pressure level (SPL) of the transducer can be larger than 90 dB (near field) in the frequency range from 50 to 1000 Hz and be larger than 80 dB (far field) from 200 Hz to 1000 Hz, with the largest value more than 130 dB (near field). Because of its light weight and panel structure, it has the potential to be used in active noise control.

Control structures and optimal sensor/actuator allocation: application in active noise control

A novel design strategy for iterative learning and repetitive controllers of systems with a high modal density: Theoretical background

Experimental study of active noise control for a full-scale plenum window in a domestic apartment