Noise Control In Duct Research Articles

Modified-filtered-u LMS algorithm for active noise control and its application to a short acoustic duct

This paper presents a new adaptive algorithm for active noise control (ANC) that can be effectively applicable to a short acoustic duct, such as the intake system of an automobile engine, where the stability and fast convergence of the ANC system is particularly important. The new algorithm, called the modified-filtered-u LMS algorithm (MFU-LMS), is developed based on the recursive filtered-u LMS algorithm (FU-LMS) incorporating the simple hyper-stable adaptive recursive filter (SHARF) to ensure the control stability and the variable step size to enhance the convergence rate. The MFU-LMS algorithm is implemented by purely experimental ways, and is applied to active control of noise in a short acoustic duct, and is validated using two experimental cases of which the primary noise sources are a sinusoidal signal embedded in white noise and a chirp signal. The experimental results demonstrate that the proposed MFU-LMS algorithm gives a considerably better performance than other conventional algorithms, such as the filtered-x LMS (FX-LMS) and the FU-LMS algorithms.

하이브리드 제어 알고리즘을 이용한 덕트내 능동소음제어

This study presents the active noise control of duct noise. The duct was excited by a steady-state harmonic and white noise force and the control was performed by one control speaker attached to surface of the duct. An adaptive controller based on filtered x LMS(FXLMS) algorithm was used and controller was defined by minimizing the square of the response of the error microphone. The assemble controller, which is called a hybrid ANC(active noise control) system, was combined with feedforward and feedback controller. The feedforward ANC attenuates primary noise that is correlated with the reference signal, while the feedback ANC cancels the narrowband components of the primary noise that are not observed by the reference sensor. Furthermore, in many ANC applications, the periodic components of noise are the most intense and the feedback ANC system has the effect of reducing the spectral peaks of the primary noise, thus easing the burden of the feedforward ANC filter.

Effects of Cross-Sectional Partitioning on Active Noise Control in Round Ducts

Active noise control (ANC) is particularly useful in hard-walled ducts where plane waves propagate. Higher order mode waves are much more difficult to control. Basic acoustic principles dictate that the cut-on frequency at which higher order modes will first begin to eclipse simple plane waves in a duct will be determined by the cross-sectional diameter of the duct. The lowest frequency for higher order modes will increase as duct diameter decreases. Therefore, the range of frequencies where plane waves dominate will be greater, and effective control using ANC will be better as duct diameter decreases. The result is that somewhat higher frequencies can be controlled with ANC for smaller diameters. If smaller diameters have broader frequency ranges that can be controlled with ANC, perhaps one could extend the frequency range for a large cross section by partitioning it into smaller cross sections using axial vane splitters. This hypothesis was tested by two methods of cross-sectional partitioning. Partitioning was achieved in one design by inserting a smaller duct inside a large duct. In a second design, a cross-shaped splitter was inserted inside the large duct. Summed ANC insertion loss (IL) at low frequencies (≤250 Hz) was at least 16 dB and at least 14 dB at middle frequencies (≥315 Hz). ANC IL results were 1.7 to 2 dB better for the large duct partitioned by a smaller inner duct than the large duct alone (p = 0.0146 for low frequency and p = 0.0333 for middle frequency). ANC insertion loss was 5.6 dB better for the large duct partitioned by a cross-shaped splitter at high frequencies than the large duct alone (p = 0.0003). However, the cross-shaped partition system was 5.8 dB less effective at low frequencies than the large duct ANC IL alone (p < 0.0001).

Effects of Cross-Sectional Dimensions on Active Noise Control in Rectangular and Round Ducts

Active noise control (ANC) works best to reduce low frequency noise. Because many industrial noise sources are broadband, ANC may be used more if it can be successfully applied to higher frequency ranges. This study explored one method to increase ANC effectiveness at higher frequencies. ANC is particularly useful in hard-walled ducts where plane waves propagate. Higher order mode waves are much more difficult to control. Basic acoustic principles dictate that the cut-on frequency at which higher order modes will first begin to eclipse simple plane waves in a duct will be determined by the cross-sectional geometry of the duct. The lowest frequency for higher order modes increases as duct diameter decreases; therefore the range of frequencies where plane waves dominate will be greater and effective control using ANC will be better as duct diameter decreases. The result is that somewhat higher frequencies can be controlled with ANC for smaller diameters. Below the first higher order mode cut-on frequency for the largest size studied, there should be little difference in ANC effectiveness between the duct sizes. To test those suppositions, a commercially available ANC system was used to reduce random noise in rectangular and round ducts having different diameters. Results showed that insertion loss (IL) ranged from 5 dB to 29 dB in frequencies ranging from 40–1000 Hz and varied inversely with cross-sectional size as expected. There was no difference in IL below 280 Hz (p = 0.7751) between the different diameter ducts. There was a significant difference between duct diameters above 280 Hz (p < 0.0001). The same tests were conducted on a rectangular duct with one cross-sectional dimension fixed and one varied at seven different sizes. Results showed similar IL from 5 dB to 29 dB that varied inversely with size.

Modelling and control of noise in a flexible flow duct

Stability and control of noise in a flow duct are discussed in this paper. The flow is assumed to be uniform for the steady state. But an unstationary interaction between acoustic waves and a flexible part of the wall can induce wave amplification up to an instability. This phenomenon is similar to flutter encountered in aeroelasticity of airfoils [H. Dowell, C. Curtiss Jr., R.H. Scanlan, F. Sisto, A Modern Course in Aeroelasticity, Kluwer Academic Publishers, 1989; Y.C. Fung, An Introduction to the Theory of Aeroelasticity, J. Wiley, New York, 1955]. In fact, one can suggest that the flexible wall can act as a trap for acoustic energy and using a smart device located on this structure, one can design an efficient control method for noise. The goal of this paper is to describe a simple model and few numerical tests which partially justify these considerations.

Optimal virtual sensing for active noise control in a rigid-walled acoustic duct

The performance of local active noise control systems is generally limited by the small sizes of the zones of quiet created at the error sensors. This is often exacerbated by the fact that the error sensors cannot always be located close to an observer’s ears. Virtual sensing is a method that can move the zone of quiet away from the physical location of the transducers to a desired location, such as an observer’s ear. In this article, analytical expressions are derived for optimal virtual sensing in a rigid-walled acoustic duct with arbitrary termination conditions. The expressions are derived for tonal excitations, and are obtained by employing a traveling wave model of a rigid-walled acoustic duct. It is shown that the optimal solution for the virtual sensing microphone weights is independent of the source location and microphone locations. It is also shown that, theoretically, it is possible to obtain infinite reductions at the virtual location. The analytical expressions are compared with forward difference prediction techniques. The results demonstrate that the maximum attenuation, that theoretically can be obtained at the virtual location using forward difference prediction techniques, is expected to decrease for higher excitation frequencies and larger virtual distances.

Multi-Channel Adaptive Feedforward Control of Noise in an Acoustic Duct

The single-reference/multiple-output active noise control (ANC) of the accurate physical model of an acoustic duct system, developed earlier by the authors [1] is investigated. An adaptive feedforward algorithm is developed that minimizes a generic cost function consisting of the p-norm of the error vector. A computer model of a multi-channel ANC system, with the tonal and sweep sine input signals, is established, and the results for different single-input/single-output (SISO) configurations (error microphone and secondary source locations) of the ANC system are compared. The dynamic response of the single-reference/multiple-output ANC systems, using the Minimax and MEFXLMS algorithms, is studied and the effect of the acoustical feedback neutralization in the multi-channel ANC system is examined. Simulation results demonstrate that the multi-channel adaptive feedforward ANC system with the Minimax algorithm has a superior performance to that of the same system with the MEFXLMS algorithm. However, the total noise reduction of the Minimax algorithm, compared with that of the MEFXLMS, is much dependent on the frequency content of the input signal.

Multivariable Control of Noise in an Acoustic Duct

The paper presents a methodology for designing a multi-input, multi-output active noise control system for an acoustic duct. The design technique proposed in the paper begins with measured frequency response data for the duct from which a state space model is constructed using the subspace approach to system identification. Errors between the measured data and the model are taken into account in constructing an uncertain system model for the duct. Then the minimax linear quadratic Gaussian (LQG) control approach is used to synthesize a controller which minimizes the duct noise level at a specified location in the duct. This controller is robust against uncertainties introduced by neglecting the higher order modes of the duct.

Discussion on: “Multivariable Control of Noise in an Acoustic Duct”

Discussion on: “Multivariable Control of Noise in an Acoustic Duct”

Improving robustness of active noise control in ducts.

A robust active noise controller (ANC) is proposed here for finite ducts. While the H(infinity) control theory provides theoretical ground and numerical algorithms to design robust controllers, it is important for an engineer to design and formulate a robust controller so that the objective is more achievable and the H(infinity) constraints less restrictive without sacrificing robustness. A new robust ANC is designed this way with an extra actuator to improve achievable performance and introduce more degrees of freedom to controller parameters. The new strategy relaxes H(infinity) constraints without sacrificing robustness and enables the ANC to tolerate a wide variety of errors and uncertainties including truncation errors between a finite model and an infinite field. Theoretical analysis, numerical examples, and experimental results are presented to demonstrate the improved performance of the proposed ANC when subject to a certain level of uncertainties in a duct.

Stiff light composite panels for duct noise reduction

Low-frequency duct noise reduction using stiff light composite panels is developed and tested. Since these composite panels are made of lightweight and stiff materials, this actuation strategy will enable the creation of composite panels for duct noise control without using traditional heavy structural mass. The results suggest that the mass-spring resonance absorption in the case of a comparatively stiff thick panel with a thin flexible plate is more efficient with minimum weight, verifying that when subjected to low-frequency (<500 Hz). The efficiency of the panel absorber depends on the mass of the thin flexible plate and the stiffness of the panel.

Application of Damped-Oscillation Control Signals for Wide-Band Feedback Active Noise Control in Acoustic Ducts.

Multi-damped-oscillation control signals are applied in this study for active noise control (ANC) in an acoustic duct. Feedback active noise controllers that can generate damped-oscillation control signals are designed by using an internal model-based linear quadratic Gaussian (LQG) design technique. The low and upper bounds containing a desired ANC bandwidth is specified by two frequencies of the control signals. To acquire desired ANC performance, sound pressure attenuation ratios within the bandwidth are tuned by control signals of other frequencies and damping ratios. Computer simulation and experimental results show that sound pressure attenuation ratio of ≤ 0.3 and ≤ 0.5, respectively, can be achieved in a wide-band of 100Hz by using 3 damped-oscillation control signals. Experimental results also show effective noise reduction for time varying disturbance and band-limited white noise within the ANC bandwidth. These results support the feasibility of the designed controllers with damped-oscillation control signals for wide-band feedback active noise control in acoustic ducts.

Active control of low-frequency broadband air-conditioning duct noise

Active control of low-frequency broadband air-conditioning duct noise

Developing active noise control systems for noise attenuation in ducts

The present work describes some of the research effort on Active Noise Control (ANC) being jointly developed by the Catholic University of Minas Gerais (PUC-MINAS) and the Federal University of Minas Gerais (UFMG). Considerations about the implementation of Digital Signal Processing for noise control in ducts has been presented. The objective is to establish a study on Active Noise Control in ducts combining geometry and acoustic parameters modification together with adaptive digital filtering implementation. Both algorithm and digital signal processing details are also discussed. The main results for a typical application where real attenuation has been obtained are presented and considered according to their use in developing real applications. The authors also believe that the present text should provide an interesting overview for both designers and students concerned about Active Noise Control in ducts. (To be presented in Portuguese.)

ROBUST CONTROL OF A 2D ACOUSTIC DUCT

This paper reports the experimental results in the application of feedback control of acoustic noise in a 2D duct. It is shown that the feedback control of 2D ducts poses peculiar problems compared to simple 1D ducts. The chief amongst them are the need for explicit budgeting of uncertainties and dealing with high model orders. This paper demostrates a practical way to design controllers for such systems using minimax LQG methods. It is shown that an important step in the controller design is the proper choice of a weighting function. The results presented in this paper are impressive and they can be further improved by proper choice of actuator and sensor placements.

Zero/pole placement control of periodic duct noise

A zero/pole placement controller (ZPPC) is presented here for active control of periodic duct noise. It uses sound pressure feedback instead of modal feedback. The controller is able to place closed-loop zeros in addition to closed-loop poles. A new adaptation law is proposed to help the controller to track parameter drifting and maintain good performance. The controller can combine with any feedforward scheme to form a hybrid (feedforward/feedback) active noise controller. It has an advantage of on-line identification of the secondary path without persistent excitations.

A theoretical study of passive control of duct noise using panels of varying compliance

It is theoretically demonstrated that, in a duct, a substantial amount of sound energy can be transferred to flexural waves on a finite wall panel when the upstream portion of the panel is made to couple strongly with sound. The flexural wave then loses its energy either through radiating reflection sound waves or by internal friction. The effectiveness of the energy transfer and damping is greatly enhanced if the panel has a gradually decreasing in vacuo wave speed, which, in this study, is achieved by using a tapered membrane under tension. A high noise attenuation rate is possible with the usual viscoelastic materials such as rubber. The transmission loss has a broadband spectrum, and it offers an alternative to conventional duct lining where a smooth air passage is desired and nonacoustical considerations, such as chemical contamination or cost of operation maintenance, are important. Another advantage of the tapered panel is that, at very low frequencies, typically 5% of the first cut-on frequency of the duct, sound reflection occurs over the entire panel length. This supplements the inevitable drop in sound absorption coefficient, and a high transmission loss may still be obtained at very low frequencies.

A THEORY OF REACTIVE CONTROL OF LOW-FREQUENCY DUCT NOISE

A theoretical prediction is undertaken for low-frequency duct noise control by a compact reactive muffler. The muffler achieves a pressure-release condition over a broad frequency band (from near zero to the first cut-on frequency of the duct) by four flush-mounted pistons, one on each duct wall, with a structure to fluid mass ratio of the order of unity. The concept of a magnetic device is introduced to allow the stiffness of the system to be made sufficiently small, leading to a high transmission loss near the DC frequency. Interactions among the pistons yield negative virtual mass which neutralizes the structural inertia. The inertia neutralization is nearly perfect over a wide frequency band, so that the high transmission loss extends to the first cut-on frequency of the duct. The near-field analysis reveals that the negative virtual mass derives from the plane wave mode of sound radiation due to the mirror effect of the opposite duct wall, an effect which can be maximized by the pairing of pistons on the two opposing walls. For relatively heavy pistons a minimum transmission loss of 14 dB may be obtained for plane waves, while that of relatively compact and light pistons could exceed 30 dB. The effects of residual system stiffness required by the system stability, and of structural damping are shown to be insignificant except for a narrow band near the DC frequency.

Modeling the normal modes and acoustics of a jet engine

Fan noise is a major contributor to the noise problem of an airplane jet engine. An understanding of the acoustics of the jet engine is necessary in order to successfully control the fan noise of the engine. A numerical boundary element method model has been developed using Sysnoise to simulate active noise control of fan noise in the engine duct. Control of multiple radial modes is targeted through the use of a modal controller using Newton’s algorithm. Results from the model are compared with measurements made previously using the NASA Glenn Research Center test rig facility. Currently work is progressing on the numerical implementation of control on a more realistic geometry.

Active noise control in ducts in presence of standing waves. Its influence on feedback effect

Much of the theoretical and experimental work which has been carried out in the area of active noise control in ducts has neglected the fact that a standing wave is formed in them. A feedforward strategy of control is the most widely used in ducts. Nevertheless, it presents a feedback problem, which can make the system unstable by corrupting the reference signal. Although there are some different solutions to this effect, most of them need special arrangements or algorithms. A model, which demonstates that the acoustic field in the duct is formed by standing waves before and after the active control, is developed and from it, the optimal location of the secondary source to avoid feedback is derived. This result is experimentally validated for a pure tone noise in a duct, and then applied to attenuate fan noise propagating in the same duct. In addition, experimental work has provided some extra information about the location of the error microphone and the usefulness of its signal to evaluate the active noise control system performance.