Micro-perforated Panel Research Articles

Sound absorbers with a micro-perforated panel backed by an array of parallel-arranged sub-cavities at different depths

This paper presents a type of wide band micro-perforated panel (MPP) absorber with an array of parallel arranged sub-cavities at different depths based on the quadratic residue sequence (QRS). An analytical prediction model is proposed for evaluating the normal incidence sound absorption coefficients of this type of MPP absorber in its design stage. Next, simulations with a finite element procedure and experimental measurements were conducted to provide cross-validations of the proposed analytical model and to facilitate the discussions on the sound absorption performance of this type of MPP absorber. The results show that the analytical model provides simple yet accurate prediction on the sound absorption performance of this MPP absorber; the absorption bandwidth of this type of MPP absorber can be significantly enhanced through the fine design on the sub-cavity depth sequence. It is also shown that the normal incidence absorption coefficient of this type of MPP absorber can remain beyond 0.5 in the frequency range of 450–3500 Hz, with the maximum value of 0.98.

Perforated panel absorbers with micro-perforated partitions

Some of the most extended approaches to widen the sound absorption bandwidth of perforated panel absorbers are the use of different cavity depths or multi-layer arrangements. Unfortunately, these solutions are rarely adopted in practice because of the technical installation difficulties or the large space requirements. This work explores an alternative compact solution consisting of a multi-size perforated panel absorber with micro-perforated partitioned cavities. While multi-size perforated panel absorbers with rigidly partitioned cavities show a wider sound absorption effective bandwidth when compared to those using single sized holes (i. e. multiple resonances instead of single resonance), the use of micro-perforated partitions was found to further broaden it and hence improve their sound absorption performance. A simplified approximate model for the prediction of the acoustic properties of such devices was derived using the Equivalent Circuit Method (ECM) and the Maa model for MPPs (Micro-Perforated Panels). Results in terms of the sound absorption coefficient under normal incidence showed a good agreement when compared to finite element simulations both for the cases with rigid and micro-perforated partitions. In addition, a parametric study was found very helpful to understand the influence of the geometrical features of the micro-perforated partitions on the acoustic behavior of the resonator, thus being useful for its design and analysis.

Modified theory of a microperforated panel with roughened perforations

Microperforated panels (MPPs) play important roles in sound absorbing systems. The classical Maa theory for the MPPs is modified to account for the effect of roughness on the surface of microperforations on sound absorption. Correspondingly, the relative acoustic resistance and relative acoustic mass of the system are determined theoretically. Full numerical simulations with the method of finite elements are performed on the roughened MPP to validate the modified theory, with good agreement achieved. It is demonstrated that surface roughness decreases resonant frequency and promotes viscous dissipation, thus enhancing the sound absorbing capability of the MPP. This work extendes Maa's theory for the sound absorption of MPP from smooth perforations to rough perforations. The modified theory for MPP with roughened perforations has a great significance in sound absorption field, since it is more applicable for the realistic situations.

Design and manufacture of metal membrane microperforated panel absorbers

Metal Membrane MPP (Microperforated Panel) is a kind of promising acoustic absorber due to its steady absorption performance and the convenience in process. In order to reveal the effects of pivotal parameters on absorption coefficient, an acoustic model of MPP is derived based on classic theory. The optimized parameters are found using genetic algorithm by the aid of this model. The MPP samples made of aluminum are manufactured by laser drilling and then the absorption coefficient is measured in impedance tube. The result shows that the theoretical model can predicted the absorption behavior of metal MPP relatively accurately. And the peak absorption coefficient, peak frequency, and the half-absorption band width are about 0.9, 2000Hz and 3200Hz respectively.

Prediction of microperforated panel absorbers using the finite-difference time-domain method

Microperforated panel (MPP) sound absorbers give selective but relatively wide and high sound absorption. In addition, they are environmentally friendly, hygienic, and visually attractive. As such, MPPs are very promising alternatives for next-generation sound absorbers. The effects of MPPs have been predicted by analytical approaches as well as numerical techniques such as the finite element method, the boundary element method, and computational fluid dynamics. However, the effects have yet to be predicted by the finite-difference time-domain (FDTD) method. In this paper, the FDTD formulation is proposed as an option to predict the acoustic performance of MPPs. For the time-domain calculations, the frequency dependence of the transfer impedance for a hole in an MPP is approximated based on regression analyses, which removes the frequency dependence. Furthermore, the stability conditions for the MPP boundary are derived by considering the state transition equations. Comparisons of numerical and analytical results verify the formulation.

Acoustic attenuation of a curved duct containing a curved axial microperforated panel.

This paper is an extension of the previous work by Yang [J. Acoust. Soc. Am. 143, 1102-1105 (2018)], which dealt with achieving acoustic absorption using a microperforated panel (MPP) in the absence of the backing cavity. It was shown experimentally that a MPP can attenuate the acoustic wave without necessarily being accompanied by a backing cavity, provided it is placed in an acoustic environment where the acoustic pressure on the two sides of the MPP is different in terms of the amplitude and/or phase. Such an environment was found in a curved duct with a curved MPP aligned along its axial direction. To further obtain the physics underlying the design, a model is developed in this work to study the property of the acoustic transmission through the duct bend and an eigenvalue analysis is made to study its influence on the decay rate of each mode. It is shown that the treatment results in attenuation for all duct modes including the fundamental mode. A geometrical acoustic approximation is made to assist in interpreting the acoustic absorption effect on each mode and a comparison with the finite element method result is used to validate the proposed model.

Acoustic Signal Separation and Noise Reduction of a Contra-Rotating Fan

How to reduce fan noise has always been a significant research topic in equipment ranging from small-scale devices, such as computer cooling fans and building ventilation fans, to large high bypass ratio turbo engines. In recent years, in order to produce higher pressure rise or larger flow rate with lower noise level in the fan stage, interest in the contra-rotating fan (CRF) configuration has been rekindled. The Vold-Kalman order tracking filter technique was employed to separate tonal and broadband noise components from CRF acoustic data to study the characteristics of the noise spectrum. Based on the spectrum peaks, noise reduction solution with casing treatment is proposed according to acoustic performance prediction by finite element simulation. In the simulation, the CRF is modelled at one end of a duct, which is surrounded by a semi-spherical space with perfect match layer to simulate the free field condition. The other end of the duct is modelled with non-reflecting boundary condition to simulate the anechoic termination. The simulated and experimental results show that casing treatment by micro-perforated panels can significantly reduce the CRF noise.

Noise control for a dipole sound source using micro-perforated panel housing integrated with a Herschel–Quincke tube

This study presents a passive noise control approach for directly suppressing sound radiation from an axial-flow fan, which involves micro-perforated panels (MPPs) backed by cavities and integrated with a hollow tube with the characteristics of a Herschel–Quincke (HQ) tube (MPPHQ housing). Noise suppression is mainly achieved by sound cancellation between the sound fields from the fan with a dipole nature and sound radiation from a vibrating panel via vibro-acoustic coupling. In addition, noise mitigation is supplemented by sound absorption in micro-perforations and acoustic interference at the hollow tube boundaries. A two-dimensional theoretical model is established to investigate the HQ segment housing a sound source. Results show that the HQ segment enhances the sound suppression performance in the passband region of the MPP housing device, and thus, a widened effective working band is obtained. Optimization is conducted to identify the optimal parameters for the MPPHQ housing. The proposed method has the potential to effectively control ducted-fan noise and to enhance the quality of products with a ducted-fan system.

Pilot study on compact wideband micro-perforated muffler with a serial-parallel coupling mode

Micro-perforated panel (MPP) as a new generation of sound absorbing material has great potential for machinery and vehicle applications. In this study, according to the intake noise behaviors of an automobile engine under accelerating working conditions, a compact MPP muffler with a serial-parallel coupling mode is proposed for silencing. The mathematical model of transmission loss for the serial-parallel coupled MPP muffler is established by utilizing the transfer matrix method, and then validated by utilizing the finite element method. On the basis of that, the multi-population genetic algorithm is adopted to optimize the muffler so that optimal combination of structure parameters can be obtained. In addition, the optimized muffler is manufactured by 3D printing technology for experimental investigation. The results show that the optimal design of the proposed serial-parallel coupled MPP muffler is practical and effective; and the wideband engine intake noise within the medium-high frequency range is obviously attenuated through adopting the optimized serial-parallel coupled MPP muffler, which may provide a good solution for vehicle and other noise controls.

Theoretical model of absorption coefficient of an inhomogeneous MPP absorber with multi-cavity depths

Micro-perforated panel (MPP) absorber has been known as an alternative absorber to classical porous material given its facile installation, long durability, environmental friendliness and attractive appearance. Extensive studies of MPP absorber proposing the improvement of its absorption frequency bandwidth have been published. This study presents a MPP absorber introduced with inhomogeneous perforations and with multi-cavity depths. The MPP is divided into two sub-area, where each area has different hole diameter, perforation ratio and a separated backed cavity depth. The acoustic impedance is modelled using electrical equivalent circuit and the absorption coefficient is calculated under normal-incidence of sound. It is found that the inhomogeneous MPP can have good bandwidth of absorption by designing the sub-MPP of smaller perforation ratio with large hole diameter and the one having the larger perforation ratio with smaller hole diameter. The absorption bandwidth can be conveniently controlled by adjusting the cavity depth of each sub-MPP. The results from the experimental work show good agreement with the theoretical model.

Optimal design and experimental validation of sound absorbing multilayer microperforated panel with constraint conditions

Sound absorption performance of the multilayer microperforated panel can be improved through optimal design of structural parameters. Theoretical model of sound absorbing coefficient of the multilayer microperforated panel with different layers was constructed according to Maa’s theory. Structural parameters of the multilayer microperforated panel with layer number from 1 to 8 were optimized through the cuckoo search algorithm with constraint conditions. Preliminary verifications of the achieved optimal parameters were conducted by the analog simulation according to the finite element method. The obtained optimal design of multilayer microperforated panel with no more than 4 layers was finally validated by testing experiments based on the standing wave method, and the optimal average sound absorbing coefficients in the frequency range of 100–6000 Hz were 57.21%, 66.29%, 68.33%, and 69.36%, respectively. Through theoretical modeling, parameter optimization, analog simulation, and experimental validation, an effective method for development of the desired sound absorber was proposed, which will be propitious to promote the applications of the multilayer microperforated panel products in the field of noise reduction.

The identification of minimum-weight sound packages

Generally, heavier noise control treatments are favored over lighter treatments since heavier acoustical materials generally insulate (block) noise sources more effectively than do lighter materials. In automotive applications, however, heavier materials cannot always be adopted because of concerns about the total weight of the vehicle. Thus, it would be useful to identify lightweight acoustical treatments that can mitigate vehicle interior noise. Automotive sound packages have both absorption and transmission characteristics, and there is inevitably a trade-off between these two. Therefore, it is important to study the exchange between the absorption and transmission of acoustical materials particularly as it pertains to weight. Here, we describe a procedure, based on plane wave analysis, that can be used to identify weight reduction opportunities by adjusting the acoustical properties of a generic sound package, consisting of a fibrous layer and a flexible microperforated panel surface treatment, so that it meets a target sound pressure level in a downstream interior space. It has been found for the configuration studied here that there are lightweight sound package configurations that can maintain acoustical performance equivalent to that of heavier noise treatments, and further, it has been found that the lightest treatments tend to favor barrier performance rather than absorption. Further, the impact of acoustical leaks is considered, and it has been found that even very small leaks can result in a very substantial weight penalty if a specified level of acoustical performance is to be ensured.

Sound Absorption Performance of Micro-Perforated Panel (MPP) Made from Biodegradable Composite (<i>Hibiscus Cannabinus</i>+PLA) Material

Micro-perforated panel (MPP) has been widely considered as a very promising alternative in absorbing sound by utilizing the concept and mechanism of Helmholtz resonator. Most of the existing MPP are made of metallic material such as aluminium or stainless steel. In this study, biodegradable composite micro-perforated panel (BC-MPP) made from kenaf fibre and polylactic acid (PLA) will be implemented. Impedance tube test shows that BC-MPP possessed excellent sound absorption properties and could rival with conventional MPP. The peak absorption of BC-MPP is also more significant compare to conventional MPP as the peak absorption almost reaches unity.

A multilayer microperforated panel prototype for broadband sound absorption at low frequencies

Microperforated panel (MPP) absorbers are one of the most promising alternatives to porous sound absorbing materials. However, these structures cannot achieve high and broadband absorption at low frequencies. To be effective, once defined the material properties the geometrical parameters of the absorber need to be optimized to match the prescribed absorption level. This paper presents a multiple layer MPP absorber with a high sound absorption coefficient and broadband absorption at low frequencies. An electro-acoustical equivalent circuit model was used for a parametric analysis to study the relationships between the absorption mechanism and the absorbers geometrical parameters in the proposed multilayer MPP. A prototype of this absorber was machined and tested in an impedance tube test ring and the experimental acoustical properties in terms of absorption coefficient were extracted using the transfer function method. It was demonstrated that the five-layer MPP absorber was capable of guaranteeing a high absorption (constantly over 90%) in a frequency range from 400 to 2000 Hz. The results indicate that the proposed multilayer MPP absorber provides a good alternative for sound absorption applications.

Sound insulation analysis of micro-perforated panel coupled with honeycomb panel considering air cavity for offshore plants

The wall panels used in offshore plants require sound insulation performance as well as fireproofing. A honeycomb panel made of metal is incombustible but unsatisfactory at the middle frequencies for sound transmission loss because the coincidence frequency occurs when the bending wavelength on the panel matches the wavelength of the incident wave. In this study, the application of a micro-perforated plate to the honeycomb panel was considered to supplement the sound transmission loss at the middle frequencies. The honeycomb core was assumed to overlap an orthotropic layer with an air layer, and face sheets were assumed to be isotropic. The kinetic and potential energy for the face sheets and the honeycomb core, the kinetic energy for the air layer located between the face sheets, and the sound absorption coefficient for the panel were derived. These were substituted into the Lagrange equation, and by solving the equation, the sound transmission loss was obtained. By comparing the experimental results with theoretically predicted results, it was found that the theory well reflected the measured surface density, elasticity, and absorption coefficient. Finally, simulations were performed for the micro-perforated plate perforation presence, micro-perforated plate perforation diameter, cell wall thickness, and cell size. These were analyzed with regard to the surface density, elasticity, and absorption coefficient.

Simplified Method of Simulating Double-Layer Micro-Perforated Panel Structure

The micro-perforated panel (MPP) structure has been widely used in various noise control applications, and thus its acoustic performance prediction has been receiving increasing attention. The acoustic performance of simple MPP structures, such as a MPP sound absorber, has been predicted using an analytical calculation method. However, this is not a suitable approach toward predicting the acoustic performance of complicated MPP structures, owing to the structural complexity of these structures. Moreover, the many perforations of submillimeter scale diameter render the MPP structures very difficult to analyze using numerical simulation. Thus, this study focused on two different simplified MPP simulation methods: the transfer admittance method and the equivalent fluid method, and their application on double-layer MPP structures. Based on the two simplified MPP simulation methods, the transmission loss value of the double-layer MPP mufflers with two sets of different structural parameters was calculated, respectively. The predicted results were compared with the impedance tube measurements. The results revealed that the two simplified MPP simulation methods could effectively predict the acoustic performance of double-layer MPP structures. Moreover, the prediction based on the transfer admittance method can outperform the two simplified simulation methods.

Additive Manufacturing of Silencers with Microperforates

A microperforated panel (MPP) is generally defined as a perforated plate, in which the impedance of below one millimetre perforations is dominated by viscous losses. Using MPPs in duct and silencer applications, target is to maximize transmission loss (TL) by choosing proper surface impedance parameters. Additive manufacturing (AM) has recently reduced conventional design limitations and enabled fast prototyping of complex shaped structures. MPP-based model scale silencers can be printed within reasonable time, price, and accuracy. In this paper, design and validation of AM silencers with MPPs are studied. First, the theoretical background of MPP acoustics is summarized. Second, feasible parameters for a MPP absorber for a certain tuning frequency are sought numerically using acoustic finite element method (FEM). Third, several test MPPs are prototyped and their acoustic properties are measured. Finally, MPP silencers are simulated using different approaches and the results are compared against experiments.

Composite honeycomb metasurface panel for broadband sound absorption.

Composite honeycomb sandwich panels have been adopted in a wide range of applications owing to their excellent mechanical properties. This paper demonstrates a design of a composite honeycomb metasurface panel that can achieve 90% sound absorption from 600 to 1000 Hz with a thickness less than 30 mm. The panel is comprised of periodically and horizontally arranged honeycomb "supercells" which consist of unit cells of different geometric parameters (pore size). Two different analytical models (Helmholtz resonator model and micro-perforated panel model) are used to calculate the sound absorption of the panel, and they are further validated by a numerical model. The relatively broadband sound absorption is found to be attributed to the coupling between unit cells, which is illustrated by both the complex frequency plane theory and the calculated sound intensity field.

The use of microperforations to attenuate the cavity pressure fluctuations induced by a low-speed flow

The paper describes experimental and numerical studies on the use of micro-perforated panels to reduce the flow cavity pressure fluctuations under a low-speed turbulent boundary layer. This passive strategy has been hardly studied in shallow cavities with length-to-depth ratios of the order of 10, for which flow reattachment might occur at the cavity floor. This implies the formation of a localized recirculation bubble upstream in the cavity, which is referred to as a closed cavity flow regime. An open flow regime takes place when the cavity is separated from the main flow by a shear layer over the full length. For a transitional case with a length-to-depth ratio of 10.6, micro-perforating the cavity floor reduces by up to 8 dB the dominant spectral peaks related to the bottom wall-pressure fluctuations in the first half of the cavity. For the closed regime with a length-to-depth ratio of 17.6, up to 6 dB reduction is found. The broadband fluctuations that are dominating in the downstream part of the cavity are not affected by the presence of the micro-perforations. Reduction of the peak pressure levels by the apertures is confirmed by two-dimensional Lattice Boltzmann simulations. Maximum dissipation occurs when outflow conditions are established within and at the inlet-outlet of the orifices. The impedance of the micro-perforated floor has been optimised and its effect on the bottom wall-pressures has been assessed.

Comparison Sound Absorption Methods of Multilayer Micro-perforated Panels

Micro-perforated panel (MPP) absorbers as the next-generation sound absorbers are getting more widely use in aviation, automobiles, construction and other fields. It’s a complex problem to predict the acoustic performance using surface impedance methods. The acoustic absorption performance of micro-perforated panels were compared in terms of Transfer matrix method and Equivalent electric circuit approach (EECA). The calculated results show that transfer matrix method is more convenient and effective than the equivalent circuit approach especially to multiply layer micro-perforated absorbers.