Micro-perforated Panel Absorber Research Articles

Application of a micro-perforated panel absorber to reduce the curve squeal noise of railways

A micro-perforated panel absorber (MPPA) can reduce noise without the use of sound-absorbing materials. It is made of various materials and is poten- tially applicable in a variety of fields. However, the application of MPPAs is limited owing to the narrow absorption band. In a previous study, to enhance the sound- absorption performance of MPPAs, we proposed a parallel arrangement of multi- layer MPPAs. Based on the results of this previous study, we carried out the present study to apply an MPPA to the curve squeal noise of railways. Squeal noise, which occurs at approximately 500 to 5000 Hz, has high-level peaks. We designed and fabricated an MPPAwith the proposed structure according to the characteris- tics of the squeal noise. The noise reduction was analyzed after an MPPA barrier was installed in the field with a curve radius of 120 m. The reduction in noise level was approximately 17 dB(A) on the first floor and approximately 7 dB(A) on the fourth floor of the building. At 630 to 5000 Hz, the noise reduction level was at least approximately 5 dB(A). To analyze the noise absorption effect of the MPPA bar- rier, a simulation was carried out and subsequently verified by comparing the measurement results with the simulation results. In the simulation, the difference in noise reduction level between the MPPA and reflective barrier was analyzed. The noise reduction level of the MPPA barrier was approximately 7.5 dB(A) higher than that of the reflective barrier on the second and third floors of the build- ing at 630 to 5000 Hz. These results support the viability of MPPA application in re- ducing noise from curve squeal noise.

Active control of low frequency sound absorption of large sized micro-perforated panel absorber by using point source

This paper presents a theoretical investigation on actively controlling the low frequency sound absorption of large-sized micro-perforated panel absorber (MPPA) by using point source placed in the cavity. The large sized active MPPA is an easily implemented and cost-effective scheme for achieving superior low frequency sound absorption in large area. The modal analysis approach is used to establish the theoretical model of such active MPPA. The physical mechanism of active control is analyzed and some physical insights are summarized. Based on these conclusions, the error sensing strategy of the large sized active MPPA is also constructed. Finally, experimental tests are carried out to validate the theoretical modeling and findings. Results show that the sound absorption performance can be significantly improved after control in a wide low frequency range below the cutoff frequency. The optimal position of the point source is at the central of the cavity section, in which case the cutoff frequency is the largest and depends on the length of the short side of the active MPPA. The control mechanism of sound absorption improvement is to enhance the Helmholtz type resonant absorption by suppressing the cavity sound field. The high order cavity mode (except for (0,0,0) mode) has no contribution to sound absorption improvement due to the symmetrical property of their mode shape. The pressure release (PR) and impedance matching (IM) strategies are still applicable for the large sized active MPPA. Relative small sized cavity or low frequency excitation can guarantee the uniformity of the cavity sound field after control, which is conducive to achieve remarkable improvement of sound absorption.

Enhanced wideband low-frequency sound absorption of a single-layer multiple parallel-arranged inhomogeneous microperforated panel absorber

Enhanced wideband low-frequency sound absorption of a single-layer multiple parallel-arranged inhomogeneous microperforated panel absorber

Broadband sound absorbers of multilayered micro-slit panels using Bayesian probabilistic inference

Micro-perforated panel absorbers can typically achieve either visual transparency or broadband absorption, but not both. This paper assesses the potential of Multilayer Micro-Slit panels to maintain both of these characteristics simultaneously. Micro-slit panels are similar to micro-perforated panels, and can similarly achieve high absorption coefficients without fibrous backing materials. The arrangement of slits are better suited to visual transparency than perforated holes because it provides more unobstructed panel per perforated area. However, these types of absorbers are limited to a narrow frequency bandwidth of effective absorption. By combining several panels into a multilayer assembly, broadband absorption becomes possible. The inherent complexity stemming from optimizing the parameters for multiple layers to meet a given design criteria necessitates the use of the Bayesian framework. This probabilistic method rapidly hones in on the best parameters of each individual layer so that the overall composite meets the design goal. Furthermore, Bayesian inference implemented cyclically alongside panel fabrication and testing allows for corrections of fabrication tolerances while assessing visual transparency.

Sound absorption of a finite flexible micro-perforated panel absorber back by a rigid air cavity filled with a fibrous porous material

Back by a rigid cavity filled with a layer of porous layer, the sound absorption performance of a micro-perforated panel (MPP) can be enhanced in comparison with other resonance based sound absorbers. In this paper, a theoretical model of a finite flexible MPP back by a rigid air cavity filled with a fibrous porous material is developed to predict normal sound absorption coefficients. Displacements of MPP and sound pressure field in fibrous porous material and acoustic cavity are expressed using a series of modal functions, and the sound absorption coefficients of MPP system are obtained. Additionally, comparison of energy dissipation by MPP and fibrous material is performed to identify effects of a fibrous material on the sound absorption of a MPP. As expected, at anti-resonance frequency of an MPP, the fibrous material provide an alternative energy dissipation mechanism.

Error Sensing Strategy for Active Control of Low-Frequency Sound Absorption of Micro-Perforated Panel Absorber by Using a Point Force Controlled Thin Plate

This paper presents an analytical investigation on constructing an error sensing strategy of a new type of active MPPA. The proposed active MPPA is composed of MPP, air cavity, and point force-controlled backing panel, which can actively improve the low-frequency sound absorption of the MPPA. Constructing an appropriate error sensing strategy for obtaining an error signal that is highly correlated with the sound absorption coefficient of the active MPPA is a key problem encountered in practical implementation. The theoretical model of the active MPPA is firstly established using the modal analysis approach. Then, the active control performance and surface impedance characteristics in the controlled condition are analyzed in detail. Finally, the error sensing strategy of the active MPPA is constructed by measuring the surface average impedance ratio with an acoustic vector sensor (AVS). Simulation results show that, due to the antisymmetric property of the vibration of the backing panel on the resonant frequency, the surface impedance of the active MPPA after control also has symmetry or antisymmetry properties. Hence, the surface average impedance ratio of the active MPPA can be measured by using the limited number of acoustic vector sensors (sensing pressure and particle velocity). This variable is also highly correlated with the sound absorption coefficient of the active MPPA and thus can be used to construct the cost function (error signal). The active control result obtained by the proposed error sensing strategy is in good agreement with the theoretically optimal result, which validates the feasibility of this approach.

Distribution and Influence of Optimal Structural Parameters to Low-frequency Sound Absorption Property of the Microperforated Panel Absorber

Noise pollution is a global problem, no matter in developed or developing countries, which makes the noise reduction one research focus all over the world. microperforated panel (MPP) absorber is supposed as a potential and practical candidate for effective noise reduction, particularly in the urban area. Influence of the optimal structural parameters to low-frequency sound absorption performance of the MPP absorber are studied in this research. The theoretical sound absorption model provides the foundation for the further analysis, and the cuckoo search algorithm is treated as the technique support. Influences of the optimal structural parameters for thin/normal/thick MPP absorbers are studied by the combination of theoretical modeling and cuckoo search optimization with the total thickness of the MPP absorber ranging from 10 mm to 60 mm with the interval of 10 mm, from which it can be concluded that the major influent factor is total thickness of the MPP absorber. For a given total thickness of the MPP absorber, no matter what thickness of the panel is, it can always achieve appropriate structural parameters by optimization to obtain similar best sound absorption performance, which will be favorable to design and develop practical MPP absorbers.

Model and Optimization of Low-Frequency Sound Absorption Performance of Multilayer Microperforated Panel Absorber

Optimization of the low-frequency sound absorption performance of the multilayer microperforated panel (MPP) absorber is favourable to promote its practical application in the noise reduction. Theoretical model of the multilayer MPP absorber was constructed based on the Maa’s theory for the further optimization. For target frequency range of 100-500Hz, parameters of the multilayer MPP absorbers with layer number of 1 to 4 were optimized through the cuckoo search algorithm according to the certain optimization objectives and the constraint conditions. Judging from the achieved optimal structural parameters for the multilayer MPP absorber with the target frequency ranges and the corresponding average sound absorption coefficients, it could be concluded that the parameter optimization was not only effective, but also be quite necessary. Meanwhile, it could be seen that average sound absorption coefficients of the optimal multilayer MPP absorbers decreased gradually along with increase of the layer number from 1 to 4. Through the optimization of structural parameters of the multilayer MPP absorbers, not only the sound absorption performance could be improved, but also the fabrication cost could be reduced. The research results exhibited outstanding low-frequency absorption performance, and it would be favourable to promote their practical applications to reduce the industrial noise in urban area.

Parameter Optimization and Finite Element Simulation of the Inclined Microperforated Panel Absorber

Microperforated panel absorber was widely used in the fields of noise reduction and sound absorption, because it had the extraordinary advantages. As deformation of the standard microperforated panel, inclined microperforated panel absorber was proposed and investigated in this research. The theoretical sound absorption model of the inclined microperforated panel absorber was constructed on the basement of the Maa’s theory according to the electro-acoustic theory. Structural parameters of the inclined microperforated panel absorber were optimized on the basement of the constructed theoretical sound absorption model through the cuckoo search algorithm. The finite element simulation model of the inclined microperforated panel absorber with optimal parameters was built in the virtual acoustic laboratory, which could be considered as an effective method to measure and check the sound absorption performance of the inclined microperforated panel absorber. Through the theoretical modeling, parameter optimization, and finite element simulation, sound absorption performance of the inclined microperforated panel absorber was improved, which could be propitious to promote its practical applications in the fields of noise reduction and sound absorption.

Investigation of broadband sound absorption of smart micro-perforated panel (MPP) absorber

A smart micro-perforated panel (MPP) is investigated in order to enhance sound absorption performance of MPP absorbers, especially at low frequencies. The smart MPP consists of a conventional MPP and a surface-bonded piezoelectric ceramic (PZT) shunted with single- or multiple-resonance circuit. A three-dimensional (3D) finite element model is used to investigate the coupling effects between MPP and the shunted PZT. The numerical results demonstrate three energy-dissipation mechanisms: mechanical damping, electrical shunt damping, and Helmholtz resonance of the MPP. The effects of different multimode shunt design methods are explored so as to intensify the panel vibration and hence improve sound absorption. A low-frequency broadband sound absorber can then be constructed based on the strong coupling among the acoustical, electrical, and mechanical domains. Several absorption peaks induced by electromechanical coupling are found in the frequencies lower than the Helmholtz resonance frequency of MPP. The electrical and mechanical responses of the proposed absorber are investigated in detail. In the experiment, the transfer function of the shunt circuit of the proposed absorber is tested with a network analyser, and the sound absorption coefficients are measured in the impedance tube based on the two-microphone transfer function method. Fair agreement between experimental and numerical results is obtained.

Sound absorption behavior of woven fabrics as micro perforated panel absorber

Woven Fabrics are potentially developed as a sound absorber system because of the presence of micro perforation on their surface. By introducing air cavity backing the woven fabrics up, such system can undergo Helmholtz resonator mechanism as found in micro-perforated panel (MPP) absorber system. The micro perforations in a woven fabric are formed by yarn in x (weft) and y (warp) direction. The perforations can create a viscos-inertial effect when interact with sound fields that is useful as a basis for sound absorption mechanism. In this study, the works are focused on investigating the relationship of micro hole of woven fabric material to sound absorption characteristics. For this, micro holes are developed by controlling weft density. Meanwhile, the sound absorption coefficients are evaluated by measurement using impedance tube. From this study, it can be concluded that the sound absorption coefficients of woven fabric are affected by weft density variation by which the perforation parameter can be varied. Moreover, the performance of woven fabrics are promising as indicated by absorption coefficients that is ranging from 0.71-1.00 with absorption bandwidth of 3 kHz-5 kHz.

Theoretical investigation on the acoustic performance of Helmholtz resonator integrated microperforated panel absorber

Broadband sound absorption at low frequency with minimal space consumption and material cost is a trending research problem in engineering acoustics. Conventional sound absorbers like porous materials and microperforated panels (MPP) are not good enough to absorb sound in the low frequency regime. To enhance the acoustic performance, Helmholtz resonator with inserted neck (HRIN) is integrated with MPP in this investigation. The sound absorption characteristics of series and parallel arrangements of both MPP and multiple HRINs are theoretically modelled using electro-acoustic analogy and the results are compared with full field finite element simulations. Most importantly, it is observed that the parallel arrangement of MPP and multiple HRINs considerably reduced the thickness of the absorber. In addition, a single unit comprising four parallel Helmholtz resonators when arranged in conjunction with MPP absorbed more than 65% sound over a band width of 318 Hz to 880 Hz whereas the successive arrangement of the same unit with MPP is affected with high anti-resonance effect. It is demonstrated that, the proposed modelling strategy very well predicts the acoustic performance of HR integrated MPP absorbers on par with the computational intensive numerical simulations. Moreover, these configurations exhibited efficient low frequency sound absorption capability even with reduced absorber thickness.

Investigation on Terminal Correction in Theoretical Model of the Microperforated Panel Absorber

Microperforated panel (MPP) absorber is promising sound absorbing structure for the noise reduction. It is extremely especially to construct accurate theoretical sound absorbing model of the MPP structure for design and development of an excellent sound absorber. Thus, modification and validation of the terminal correction in theoretical sound absorption model of the MPP absorber is investigated in this research, which aimed to improve its accuracy and reasonability. Firstly, based on the Maa’s theory, classic sound absorption model of the MPP absorber was presented and treated as basic reference of the whole research. Secondly, through a set of verification experiments, necessity and importance of the research on reasonability of terminal correction was revealed. For the superposition of several monolayer MPP absorbers, when its sound absorption coefficient was derived as the multilayer MPP absorber through the transfer matrix method, the terminal corrections for the former n-1 monolayer MPPs should be removed, because length of these MPPs was 0. Thus, it was essential to take length of the rear cavity into account for the terminal corrections in the theoretical sound absorption model of the MPP absorber.

Microperforated Panel and deep subwavelength Archimedean-inspired spiral cavities for multi-tonal and broadband sound absorption

In recent years, metamaterial-structures and Microperforated panel (MPP) absorbers have been proposed as a valid alternative to porous materials for sound absorption in the low frequency range. However, high and broadband absorption cannot be achieved in the low frequency range as required in some engineering applications where large thicknesses of the absorbers are unsuitable. In this work, a deep subwavelength hybrid parallel-arranged MPP and Archimedean-inspired spiral (AIS) absorber is proposed to obtain broadband sound absorption at low frequency (i.e. 400–2000 Hz). The absorption properties are investigated using an equivalent electro-acoustic model and a parametric analysis is performed to optimise the geometric parameters of the device for the desired frequency range. Two parallel arranged MPPs and AIS structures were designed to achieve sound absorption in frequency ranges between 550–1650 Hz and 380–1250 Hz, with a total thickness less than 1/28 wavelength (24.3 mm). In addition, a parallel arrangement of an AIS and a double layered MPP were also considered to achieve absorption in a wider frequency range (i.e. 480–2800 Hz). The prototypes were then fabricated and tested with an impedance tube to evaluate the normal absorption coefficient via the Transfer Function method (TFM). Experimental results show a good correlation with the analytical models, with a absorption coefficient above 60% over the respective frequency ranges. Moreover, absorption peaks occur at the resonance frequencies and higher harmonics of the structures, with measured values above 95%. The low frequency broadband absorption shown by the proposed subwavelength hybrid structures makes the device suitable for many acoustic engineering applications.

Design of an in-duct micro-perforated panel absorber for axial fan noise attenuation

The reduction of fan noise in ducts is a challenging task for acoustic engineers. Usually, the confined space where an absorber can be integrated is small. In addition, one has to consider the influence of the absorber on the flow field and the attenuation of noise should be as great as possible. In this contribution, we investigate the application of a micro-perforated absorber (MPA) in the direct vicinity of a low-pressure axial fan operating at low Mach number conditions. The micro-perforated plates (MPP) are modeled using the Johnson–Champoux–Allard–Lafarge (JCAL) model for porous materials. The entire geometrical setup of duct, fan and MPA is then simulated with the Finite Element (FE) method; the pre-processing effort is reduced by using non-conforming grids to discretize the different regions. The influence of the cavity length and the positioning of the fan are analyzed. The results are then applied to the construction of a full-sized MPA duct component that takes the limited space into consideration. Simulation results and overall functionality are compared to experimental results obtained in an axial-fan test rig. The Finite Element framework proved to be robust in predicting overall sound pressure level reduction in the higher volume flow rates. It is also shown that the MPP increases sound reduction in the low-frequency regime and at two resonant frequencies of the MPA setup. However, its main benefit lies in maintaining the efficiency of the fan. The location of the fan downstream or within the MPA has a significant effect on both the simulated and measured sound reduction.

Experimental investigation on motor noise reduction of Unmanned Aerial Vehicles

Multi-rotor Unmanned Aerial Vehicles (UAVs) are developing rapidly recently in usage and popularity in civil applications. However, although the great benefits UAVs have brought, their noise pollution is one of the concerns with their proliferation. The two main noise sources of a UAV are the motors and propellers. And the propellers are often assumed to be the dominant noise source in the literature, hence noise reduction of propeller becomes research focus; however, recent studies suggest that the contribution from the motor may also be important. This paper explore a passive noise reduction approach based on micro-perforated panel (MPP) absorbers together with sound-proof materials. Experimental results show that the proposed noise reduction constructions can have a significant reduction to the motor's overall noise level.

Influence of a micro-perforated duct absorber on sound emission and performance of axial fans

In this article we investigate the influence of varying flow conditions on the acoustic behaviour of a micro-perforated panel absorber (MPA), applied in the vicinity of an axial fan. The predesign of the MPA was conducted by using the Finite Element Method (FE) in combination with porous material modelling approaches. Experimental results of emitted sound spectra and fan characteristic of different fan blade geometries, positioning of the fan to the absorber, as well as changes in turbulent intensity are presented. The studies showed that the investigated MPA duct reduces the overall sound pressure level emitted by the fan by up to 16 dB while maintaining the same efficiency and pressure build-up. From an acoustic and aerodynamic point of view, it was more efficient if the fan operate downstream of the MPA instead of within the MPA. This arrangement increased the efficiency and avoided high-frequency sound generation, which is caused by the flow through the MPA. The results show that the function of the MPA can be used for different fan geometries and that inlet turbulence improves the properties of the absorber.

Control of noise generated from centrifugal refrigeration compressor

Vibration and noise characteristic of two-stage centrifugal compressors used in refrigeration system is studied experimentally. Experimental results of vibration acceleration, sound pressure spectra and their coherence indicate that the compressor noise is dominated by tonal components at discrete frequencies and is mainly radiated from the vibration of the outlet pipe of the compressor. Therefore, a multi-layer micro-perforated panel absorber fixed at the outlet pipe is employed to attenuate the compressor noise, where acoustic transmission analysis and generic algorithm are combined to design optimal geometry parameters of the absorber. Experimental results confirm that the designed micro-perforated panel absorber can effectively reduce the noise of the centrifugal refrigeration compressor.

Wide absorption bandwidth of a light composite absorber based on micro-perforated sandwich panel

We present a light composite absorber consisting of a micro-perforated sandwich panel (MSP) filled with a thin porous materials, to achieve a wide absorption bandwidth. The MSP is of 1 mm thickness scale and possesses the excellent mechanic properties in contrast to a single homogeneous panel. The theoretical model of the light composite absorber is derived by the transfer matrix method, and is well agreed with the experimental results. The proposed absorber predicts a wider 0.5 relative absorption bandwidth and higher absorption coefficient, compared with the micro-perforated panel (MPP) absorber or the double-MPPs absorber. The absorption mechanism owing to the addition of the thin porous materials is conducted by analyzing the reflection coefficient, surface acoustic impedance, acoustic pressure and local velocity. The influences of the thickness, porosity, and hole diameter of the internal components on the absorption bandwidth and absorption peak are discussed. The proposed absorber has a wide scope of potential applications under multi-functional noise-controlled conditions

Sound absorption theory for micro-perforated panel with petal-shaped perforations.

Micro-perforated panel (MPP) absorbers with circular perforations are used in many noise control applications due to their attractive wide-brand sound absorption performance. Different from a common MPP with circular perforations, a unique type of MPP absorber with petal-shaped perforations is proposed. The sound absorption theory for the MPP with petal-shaped perforations is developed by accurately considering the fluid velocity in the petal-shaped perforation hole. This theory can account for the effect of altered perforation morphology (from circular to petal) on sound absorption. Finite element simulations are performed to validate the proposed theory, with good agreement achieved. The sound absorption of MPP with petal-shaped perforations is compared with that of the traditional MPP with the same porosity. It is demonstrated that the change in hole shape significantly modifies the fluid velocity field and the flow resistivity in/of the hole, and hence the sound absorption of the proposed MPP with petal-shaped perforations can outperform that of the traditional MPP in the considered case. This work proposes a general MPP theory that not only contains the classical Maa's theory for circular MPP, but also accounts for the MPP with petal-shaped perforations.