Noise Control Methods Research Articles

A preprocessing method for multichannel feedforward active noise control

A multichannel filtered-x least mean square (LMS) algorithm is an efficient feedforward algorithm in an active noise control (ANC) system, whose convergence rate is known to be limited by many factors, such as a secondary path model and the correlation between reference signals. In this paper, we introduce an adaptive blind preprocessing method for reducing the eigenvalue spread of the correlation matrix of reference signals, which is often ignored in a typical multichannel ANC algorithm. Two blind adaptive decorrelation algorithms are derived for different reference path models. Numerical experiments verify the robust performance of the proposed preprocessing methods, including reducing the mean square error and improving the convergence speed.

LMI design of frequency domain LMS filters for active noise control

Recently, a frequency-domain adaptive filter design method for active noise control has been proposed based on the so-called least mean square algorithm. However, this method does not sufficiently exploit the degree of freedom of the step parameter of the recursive rule in the case where a priori information on an uncertain plant is available. This paper proposes a design method of the step parameter such that noise cancellation is guaranteed against the plant uncertainty. The design problem of the step parameter is reduced to a semidefinite programming problem which is efficiently solvable. Experimental results are provided to illustrate the effectiveness of the proposed method.

효율적 실내 소음 저감을 위한 흡음재 분포 위치 결정 방법

Absorptive material arrangement method for effective interior noise control is proposed. Sound field with arbitrary boundary condition is formulated by Kirchhoff-Helmholtz integral equation. A simple example such as a rectangular cavity will present physical meaning between changing boundary condition and control of sound field. The effect of changing boundary condition is expressed in modal admittance. From this formulation. an admittance map is presented. The admittance map is the figure to represent position where absorptive material is attached for effective interior noise control. The admittance map can be assigned to each resonant frequency. There. however, may be common area of those maps. Then, frequency robust arrangement of absorptive material in noise control will be presented.

Structural Stator Spacers—A Solution for Noise Reduction of Switched Reluctance Motors

This paper presents a powerful new design aspect to reduce acoustic noise and vibration of electromagnetic origin for electrical machines, by introducing improved slot wedges referred to as "structural stator spacers." These spacers, by using a very stiff dielectric and nonmagnetic material, a modified shape, and small modifications to the stator laminations, not only secure the windings and reduce windage losses, but also make it possible to increase the stiffness of the stator structure significantly, thereby reducing the generation of audible noise. This new method does not incur the significant drawbacks usually associated with other noise reduction methods or interdict other noise control methods. Design models and practical prototypes are detailed which are used to verify the effectiveness of the spacers.

Active structural acoustic control of road noise in a passenger vehicle

An active noise control system for a passenger vehicle is presented in this paper. The noise control method, Active Structural Acoustic Control, is discussed and a brief comparison is drawn with other popular methods of active noise control. The primary focus of the research was to develop a cost-effective active noise control system for passenger vehicles for the reduction of road noise. The approach relies on concentrating the controller and actuator effort in a narrower but significant region in the spectrum of the road noise, which contains frequency content mainly between 80 and 350 Hz. This improved the acoustic comfort within the vehicle cabin, while keeping the actuator and processor costs low. The paper presents different progressively developed control strategies that lead from a single tone controller to counter sine wave type excitation from a 50 lb shaker to a more practical controller that was demonstrated with the car on a chassis dynamometer.

自動車用ラジエターファンのNZ音予測法の開発(J13-3 空力音III,J13 流体関連の騒音と振動)

Recently, as the noise of the automobile is decreasing, a radiator fan included in it is also needed to be quiet. However, the noise of the radiator fan is mainly aero-dynamic noise around fan blades, generally called BPF (Blade Passing Frequency) noise, and it is easy to generate in a narrow flow channel in an engine room of automobile. Especially CRFM (Condenser-Radiator Fan Module) of MHI is proud on its compact body, so noise control method to predict or prevent BPF noise is very important in this compact module. In this paper, we show the noise prediction technique for a radiator fan of CRFM using unsteady CFD result and assessment result compared with an experimental data.

Hot topics in noise

Our world continues to be a noisy place and the challenge to ‘‘increase and diffuse knowledge of noise propagation, passive and active noise control, and the effects of noise’’ remains. In the last several years, noise in the classroom has emerged as one of the hotter topics: Considerable progress has been made in the underpinning research, the formulation of recommendations, and the process of educating society on the social and personal impact of inadequate acoustical conditions in classrooms. The establishment of the ANSI S12.60-2002 standard for classroom acoustics was a milestone event. Noise in cities and the understanding of our soundscapes are subjects of ongoing significance. The development of standards and regulations is a continuing process, with urban community noise regulations, aviation noise, and the preservation of natural quiet in national parks being of current concern. New methods to reduce noise are under development and include passive and active methods of noise control, techniques for modeling the performance of noise barriers, and approaches for designing product sound quality.

Active noise control of flexible linkage mechanism with piezoelectric actuators

An active noise control method for flexible linkage mechanism systems with piezoelectric actuators and strain gauge sensors is studied. By employing a set of wave number transformations on the original equations of motion of the flexible linkage mechanisms, a new set of equations of motion is obtained, in which the unknown variables can describe the structural acoustic radiation level of the mechanism system directly. On the basis of the new equations, the active noise control of the flexible linkage mechanism system with piezoelectric actuators is discussed. Firstly, the optimal control forces are determined based on the optimal control theory. Secondly, the controller of the system that consists of the output feedback and the disturbance feed-forward control laws is presented. Simulations show that the method presented in this paper is valid.

환형 스마트 폼을 이용한 덕트 내부의 능동 소음 제어 및 상쇄 경로 최적화

This paper presents a method for active noise control (ANC) in a duct by using a ring-tyPe smart foam. The ring-type smart foam consists of an elastic porous material of lining shape and a PVDF film embedded In the material. The PVDF element acts as a secondary sound source to reduce the noise. Active noise control using a ring-type smart foam is only effective locally because of the way to excite radially. To enlarge the quiet zone, the duct Is lined with additional acoustic foam between the smart foam and the error microphone. When cancellation path ks optimized by the LMS/RLS algorithm, the computation power is reduced while control performance Is maintained. The filtered-x LMS algorithm is used to minimize the error signal.

Hybrid active and passive control of fan noise

This paper deals with the global reduction of axial flow fan noise in ducts in a building using a hybrid passive–active noise control method. The effectiveness of using an infra-red device as a reference signal source is also investigated. It is shown that using such a hybrid noise control system over an axial-flow fan reduces the overall sound pressure level by 5 dB(A) in the surrounding environment and global control of the blade passing frequency can also be achieved. This paper also shows that using an infra-red device as a reference signal source produces marginally better control as compared with using a microphone reference sensor. Moreover, long term stability is guaranteed and the possibility of acoustic feedback is eliminated.

Causal impedance matching for broadband hybrid noise absorption.

The complementary strengths and weaknesses of passive and active noise control (ANC) methods have motivated many researchers to develop hybrid noise absorbers that integrate both control strategies. The impedance matching technique (IMT) is the most effective for such a purpose. An unsolved problem with available IMT schemes is the a priori reference signal that limits IMT applications. This study proposes the use of the forward wave, available by the two-microphone method, as the reference signal. Due to inevitable errors in wave separation and inlet reflection of the control signal, the absorber becomes a feedback system. A simple and stable ANC is developed for impedance matching without the a priori reference signal. The proposed absorber has an absorption coefficient of 0.9 or above in a frequency range of 60-850 Hz. It is stable in the presence of sensor mismatch and robust with respect to significant variation of inlet boundary conditions.

New active muffler system utilizing destructive interference by difference of transmission paths

A new active muffler system has been developed and its superior performance on both noise reduction and engine torque increase is demonstrated with experiment. Main characteristic of the proposed muffler system is the use of destructive interference by transmission path difference of divided exhaust pipes to reduce major exhaust noise components thereby overcoming problems of other active exhaust noise control methods. The exhaust pipe is divided into two sections and joined again downstream. One divided pipe has a sliding mechanism to vary its length, which is controlled to make half wavelength transmission path difference for the major engine rpm frequency. In this system one divided pipe is used to control major rpm frequency and its Harmonics and another pipe is used to control noise component double the frequency of rpm. An after-market tuning muffler, which has very simple internal structure and minimal back pressure, is also installed to remove remaining wideband noise. To make the system to be small enough to be practical, conventional muffler is also installed and used in low rpm range and active muffler is only employed in high rpm range. Noise reduction of the proposed system is comparable to conventional passive muffler. The engine dynamo test has proved the proposed system can recover almost all the torque lost by conventional muffler.

Methods to Control EMI Noises Produced in Power Converter Systems

A new method to control EMI noises produced in power converters (rectifier and inverter) composed of IPMs (Intelligent Power Modules) is studied especially focusing on differential mode noises. The differential mode noises are occurred due to switching operations of the PWM control. As they are diffused into the ground through stray capacitors distributed between the ground and the power transmission lines and machine frames, differential mode noises should be confined and suppressed within the smallest area where power converters are laid out. It is impossible to control differential mode noises easily occurring diffusion by the conventional methods like filtering techniques. So, a new EMI noise control method using a multi-power circuit technique is proposed. The proposed method of the effectiveness has been verified through simulations and experiments.

ACTIVE CONTROL OF ENVIRONMENTAL NOISE, VII: PERFORMANCE OF MULTI-FREQUENCY, MULTI-CHANNEL, FREEFIELD SOUND CANCELLING SYSTEMS

The paper considers the performance of multi-frequency, multi-channel, free field sound cancelling systems for the reduction of discrete frequency and periodic noise. The approach uses a method of directional active noise control. Large sound reductions from these systems have been made possible through: (a) synthetically generating the cancelling sound and synchronizing with the primary source; (b) automatic alignment of all stability regions of the control system and; (c) avoiding instability produced by these multi-channel systems.

Recent advances in active control of aircraft cabin noise

Active noise control techniques can provide significant reductions in aircraft interior noise levels without the structural modifications or weight penalties usually associated with passive techniques, particularly for low frequency noise. Our main objective in this presentation is to give a review of active control methods and their applications to aircraft cabin noise reduction with an emphasis on recent advances and challenges facing the noise control engineer in the practical application of these techniques. The active noise control method using secondary acoustic sources, e.g., loudspeakers, as control sources for tonal noise reduction is first discussed with results from an active noise control flight test demonstration. An innovative approach of applying control forces directly to the fuselage structure using piezoelectric actuators, known as active structural acoustic control (ASAC), to control cabin noise is then presented. Experimental results from laboratory ASAC tests conducted on a full-scale fuselage and from flight tests on a helicopter will be discussed. Finally, a hybrid active/passive noise control approach for achieving significant broadband noise reduction will be discussed. Experimental results of control of broadband noise transmission through an aircraft structure will be presented.

A Tentative Noise Control Method for Buildings above Railway Tunnels

In this paper, a design method to reduce the transmission of structure-borne sound in buildings to be built above railway tunnels is studied. The case studied is similar to the one studied previously by the author in the sense that the building is founded on rock. However, in the present case, the load-bearing columns on the first floor are not placed above other load-bearing columns or walls. Analytical models for the propagation of structure-borne sound upward in the buildings are derived. The models are based on the assumption that the initial vibrational energy that propagates from the foundation is in the form of quasi-longitudinal waves. The propagation of bending and quasi-longitudinal waves in the load-bearing columns is considered. A finite ground impedance is taken into account as well as the wave reflection from the roof. A parametric survey is conducted to investigate the influence of certain important geometrical properties of the first floor on the sound transmission. Finally, the results are compared with previous results for a “normal” building and it is shown that the relocation of the columns can considerably reduce the sound transmission upward in the building.

전달경로의 차이를 이용한 새로운 차량용 능동 머플러의 개발

A new active muffler system has been developed and its superior performance on both noise reduction and engine torque increase is demonstrated with experiment. Main characteristic of the proposed muffler system is the use of destructive interference by transmission path difference of divided exhaust pipes to reduce major exhaust noise components thereby overcoming problems of other active exhaust noise control methods. The exhaust pipe is divided into two sections and joined again downstream. One divided pipe has a sliding mechanism to vary its length, which is controlled to make half wavelength transmission path difference for the major engine rpm frequency. In this system one divided pipe is used to control major rpm frequency and its Harmonics and another pipe is used to control noise component double the frequency of rpm. An after-market tuning muffler, which has very simple internal structure and minimal back pressure, is also installed to remove remaining wideband noise. To make the system to be small enough to be practical, conventional muffler is also installed and used in low rpm range and active muffler is only employed in high rpm range. Noise reduction of the proposed system is comparable to conventional passive muffler. The engine dynamo test has proved the proposed system can recover almost all the torque lost by conventional muffler.

Silencer consisting of two-stage Helmholtz resonator with auto-tuning control

A Helmholtz resonator has advantages over other noise control methods in that it does not require energy to function and it can be applied to high-frequency noise. In this article, a noise reduction method based on a Helmholtz resonator is discussed in reference to application to a blower. However, as the frequency of noise generated by a blower, although high, varies over time, the Helmholtz resonator is ineffectual if applied directly. In order to reduce varying high-frequency noise, a new type of silencer, comprised of a two-stage rotary auto-tuning resonator, is proposed. A fast Fourier transform (FFT) analysis of noise obtained by a noise meter is used for control. The frequencies of the peak intensities obtained by the FFT vary according to the variations in source frequency. Two significant peaks in the frequency domain are considered, the cost functions of which are taken, allowing for frequency variation. The silencer is tuned by minimizing the cost function. The system and control algorithm are presented in this report, along with the results of experimental tests that were conducted on a typical blower in order to verify the effectiveness of the system and method.

Active Noise Hybrid Feedforward/Feedback Control Using Neural Network Compensation*

The nonlinear active noise control (ANC) is studied. The nonlinear ANC system is approximated by an equivalent model composed of a simple linear sub-model plus a nonlinear sub-model. Feedforward neural networks are selected to approximate the nonlinear sub-model. An adaptive active nonlinear noise control approach using a neural network enhancement is derived, and a simplified neural network control approach is proposed. The feedforward compensation and output error feedback technology are utilized in the controller designing. The on-line learning algorithm based on the error gradient descent method is proposed, and local stability of closed loop system is proved based on the discrete Lyapunov function. A nonlinear simulation example shows that the adaptive active noise control method based on neural network compensation is very effective to the nonlinear noise control, and the convergence of the NNEH control is superior to that of the NN control.

Full-Scale Demonstration of Higher Harmonic Control for Noise and Vibration Reduction on the XV-15 Rotor

Abstract : A higher harmonic control (HHC) investigation was conducted on a full-scale, isolated XV-15 Rotor in helicopter mode in the NASA Ames 80- by 120-Foot Wind Tunnel to independently control noise, vibration, and trim. The higher harmonic blade root pitch was generated using swashplate oscillations. The radiated blade-vortex interaction (BVI) noise footprint was measured on a plane beneath the rotor with eight microphones mounted on an acoustic traverse. Test results showed that HHC is highly effective in reducing BVI noise, achieving a 12 dB reduction in peak noise level within the noise footprint. Blade pressure feedback was demonstrated to be a viable method for closed-loop noise control. Perturbations in trim parameters and test conditions had small to moderate effects on noise reduction with HHC. Some noise reduction was achieved with no increase in vibratory hub loads. Increases in control loads due to HHC generally limited further noise reduction. The vibration controller achieved about 50 percent reduction in vibratory hub loads with control loads limiting the HHC amplitude. An automatic trim controller was demonstrated to be robust, reaching all target thrust and flapping schedules under all conditions tested.