Micro-perforated Plate Research Articles

Enhanced Low-Frequency Sound Absorption of a Porous Layer Mosaicked with Perforated Resonator.

A composite structure composed of a porous-material layer mosaicked with a perforated resonator is proposed to improve the low-frequency sound absorption of the porous layer. This structure is investigated in the form of a porous-material matrix (PM) and a perforated resonator (PR), and the PR is a thin perforated plate filled with porous material in its back cavity. Theoretical and numerical models are established to predict the acoustic impedance and sound absorption coefficient of the proposed structure, and two samples made of polyurethane and melamine, respectively, are tested in an impedance tube. The predicted results are consistent with that of the measured. Compared with a single porous layer with the same thickness, the results show that the designed structure provides an additional sound absorption peak at low frequencies. The proposed structure is compact and has an effective absorption bandwidth of more than two octaves especially below the frequency corresponding to 1/4 wavelength. A comparison is also made between the sound absorption coefficients of the proposed structure and a classical micro-perforated plate (MPP), and the results reveal equivalent acoustic performance, suggesting that it can be used as an alternative to the MPP for low–mid frequency sound absorption. Moreover, the influences of the main parameters on the sound absorption coefficient of PPCS are also analyzed, such as the hole diameter, area ratio, flow resistance, and porous-material thickness in the PR. The mechanism of sound absorption is discussed through the surface acoustic impedance and the distributions of particle velocity and sound pressure at several specific frequencies. This work provides a new idea for the applications of the thin porous layer in low- and medium-frequency sound absorption.

Optimization of Sound Absorption and Insulation Performances of a Dual-Cavity Resonant Micro-Perforated Plate

This study investigates a dual-cavity resonant composite sound-absorbing structure based on a micro-perforated plate. Using the COMSOL impedance tube model, the effects of various structural parameters on sound absorption and sound insulation performances are analyzed. Results show that the aperture of the micro-perforated plate has the greatest influence on the sound absorption coefficient; the smaller the aperture, the greater is this coefficient. The thickness of the resonance plate has the most significant influence on the sound insulation and resonance frequency; the greater the thickness, the wider the frequency domain in which sound insulation is obtained. In addition, the effect of filling the structural cavity with porous foam ceramics has been studied, and it has been found that the porosity and thickness of the porous material have a significant effect on the sound absorption coefficient and sound insulation, while the pore size exhibits a limited influence.

Proof-of-Concept Design for MPP Acoustic Absorbers with Elements of Art

A micro-perforated plate or panel (MPP) is a device used to absorb sound. It consists of a thin flat plate made from several different materials with small holes and a back cavity. Several reported modifications and enhancements to the original design of the MPP acoustic absorber were modified by the holes or the back-cavity shape and sizes following the original idea. The present study attempts to artistically beautify the MPP acoustic absorbers by incorporating dotted arts into the design of MPP. The perforation for micro-perforated panels could be dotted arts with a perforation size smaller than 1 mm for enhanced acoustic absorption performance in the form of various artistic designs. Small LED lights could be placed inside the acoustic chamber to create the color lights emanating from the perforations instead of dots with different colors. Several MPP incorporated artistic designs of dotted patterns were presented and their acoustic absorption performance was analyzed using impedance tube in this paper.

Optimization design and analysis of honeycomb micro-perforated plate broadband sound absorber

The sound absorption performance of micro-perforated plate (MPP) is mainly restricted by four parameters: cavity depth, aperture, plate thickness and perforation rate. The concept of change rate of sound absorption coefficient is introduced, which is taken as the measuring standard to judge the influence of changing these parameters on the sound absorption performance of MPP. Generally, the parameter sensitivity of MPP sound absorber from large to small is as follows: cavity depth, aperture, perforation rate, plate thickness. According to the characteristic that the resonant frequency of MPP will shift when the cavity depth changes, combined with the acoustoelectric analogy principle, two kinds of single-layer honeycomb micro-perforated plate (HMPP) structures with different cavity depth are designed by using particle swarm optimization algorithm. Simulation and experimental results show that the structures can use their own different depth of honeycomb cores to achieve the purpose of broadband sound absorption. The feasibility of using particle swarm optimization algorithm to design broadband sound absorber is verified. In addition, through the contrastive analysis of the two structures, compared to the four regions HMPP, the experimental results of the seven regions HMPP are closer to the theoretical and simulation results, which is consistent with the characteristics of particle swarm optimization (PSO): the more optimization parameters are, the better the performance is.

Experimental Oxygen Mass Transfer Study of Micro-Perforated Diffusers

We studied new micro-perforated diffuser concepts for the aeration process in wastewater treatment plants and evaluated their aeration efficiency. These are micro-perforated plate diffusers with orifice diameters of 30 µm, 50 µm and 70 µm and a micro-perforated tube diffuser with an orifice diameter of 50 µm. The oxygen transfer of the diffuser concepts is tested in clean water, and it is compared with commercial aerators from the literature. The micro-perforated tube diffuser and micro-perforated plate diffusers outperform the commercial membrane diffusers by up to 44% and 20%, respectively, with regard to the oxygen transfer efficiency. The most relevant reason for the improved oxygen transfer is the fine bubble aeration with bubble sizes as small as 1.8 mm. Furthermore, the more homogenous cross-sectional bubble distribution of the micro-perforated tube diffuser has a beneficial effect on the gas mass transfer due to less bubble coalescence. However, the pressure drop of micro-perforated diffusers seems to be the limiting factor for their standard aeration efficiencies due to the size and the number of orifices. Nevertheless, this study shows the potential for better aeration efficiency through the studied conceptual micro-perforated diffusers.

Acoustic characteristics of microperforated plate with variable cross-sectional holes.

Broadband sound absorption performance and high mechanical strength are major concerns for designing a microperforated plate (MPP) absorber, which is generally difficult to achieve for a single-layer MPP with straight holes. A MPP was proposed with variable cross-sectional holes, including large dents on one side, grooves on the other side, and intermediate micro-slits. The beneficial effects of the special geometrical features are thoroughly discussed and experimentally validated. Models to calculate the acoustic impedance are established, and acoustic behaviors within holes are simulated. It was found that the micro-slits could provide necessary acoustic resistance for impedance matching and very low reactance to expand sound absorption bandwidth. The large dents or grooves could provide the necessary thickness with negligible acoustic impedance to enhance plate mechanical strength. This research helps to implement a single-layer MPP design with good acoustical and mechanical performance.

An acoustic impedance structure consisting of perforated panel resonator and porous material for low-to-mid frequency sound absorption

In this paper, an acoustic impedance compound structure composed of a perforated panel resonator (PPR) and porous sound absorbing material (PSAM) is designed to improve the sound absorption for low-to-mid frequency. The PPR component is an array cell of multiple Helmholtz resonators based on perforated panel arranged in parallel. By properly introducing a PSAM component that consists of porous material layers and rigid wall into the PPR component, a matched acoustic impedance appears at the coupling of the PPR and PSAM. Thus, the designed composite structure PPR-PSAM achieves an effective wide-band sound absorption through the effect of resonance dissipation by PPR and trapped energy by PSAM. An analytical model based on acoustic-electrical analogy is established to predict the sound absorption, and a finite element model is further proposed to demonstrate the sound absorption mechanism for the PPR-PSAM. Moreover, the influences of structural parameters on the sound absorption of the PPR-PSAM are discussed, and the sound absorption of the PPR-PSAM and the MPP (micro-perforated plate), MPP-PSAM are compared. The normal sound absorption coefficient measured by impedance tube is well agreed with the predicted results. In a limited space of 80 mm, the PPR-PSAM with low perforation rate has an average sound absorption coefficient of more than 0.89 from 200–1600 Hz.

Improving the Sound Absorption of Natural Waste Material-based Sound Absorbers Using Micro-perforated Plates

ABSTRACT Both natural and synthetic fibers do not perform well at low frequencies and are also subjected to erosion, moisture, fire, etc., when used as an interior finish of room walls. To overcome these problems, micro-perforated plate with different thickness is used in a multilayer sound absorber configuration to improve its sound absorption. Firstly, the acoustic properties of five natural wastes including sheep wool, goat wool, camel wool as well as pith and fiber bundles of sugarcane bagasse were determined by using an impedance tube following ISO 10534–2. Secondly, the effect of Panel thickness was investigated. The maximum sound absorption coefficient (SAC) of natural waste materials is at middle to high-frequency range and this shifted to lower frequency as the porous layer thickness increases. The sound absorption performance depends on the thickness of perforated plate and porous layer in a compound sound absorber. It is observed that the using perforated plate in the front of low thickness porous materials significantly improve the SAC and absorption bandwidth. However, it should be noted that this reduces the SAC at high frequencies. Accordingly, the thick micro-perforated panels backed by porous layer are not recommended to control sounds with frequency higher than 3000 Hz.

Dynamics of liquid drop on a vibrating micro-perforated plate

Abstract The present work presents the behavior of a liquid drop placed onto a surface of the perforated plate experimentally. Micro holes were created on the brass plate by a laser marking method. The actuator made of brass and piezoelectric discs driven by a sine input voltage, and it bonded rigidly to the perforated plate. This study aimed to reveal the behavior of the drop placed on the perforated plate and the effects of the initial acceleration value on the rate of change in the drop projected area onto the surface. For this purpose, a frequency-response function of the dry micro-perforated plate was measured by a laser vibrometer, and the acceleration distribution was determined on the perforated plate. Further, the drop was excited with five different initial acceleration values, and the drop projected area onto the surface was recorded by a camera during the atomization process. It was observed that droplets were atomized at the air–liquid interface on two sides of the perforated plate. The time-varying function of the drop projected area onto the surface for each initial acceleration value was presented graphically. The polynomial function was fitted to the experimental data points for each acceleration value, and thus the effects of the initial acceleration on the varying drop size were analyzed. The authors emphasized that the drop projected area onto the surface first increased and then decreased during the atomization process. The rate of change in the drop projected area onto the surface increased as the initial acceleration increased. The drop was atomized both by ejecting from the surface and sucking through the holes.

Experimental investigation of sound absorption in a composite absorber

A composite absorber made of a polyurethane sponge and multi-layer micro-perforated plates is presented in this study. Results from an acoustic impedance tube test show that the polyurethane sponge can exhibits higher low-frequency sound absorption in front of the micro-perforated plate, while sound absorption at medium and high-frequencies remains low. The physical mechanism behind this is that the micro-perforated plate increases the denpth cavity. If the polyurethane sponge is placed behind the micro-perforated plate, the amplitude of the original absorption peak will remain constant, but the absorption peaks will shift to lower frequencies. The reason for this phenomenon is that porous materials with low flow resistance can be approximately equivalent to fluid, which not only does not affect the resonance absorption coefficient of micro-perforated plate, but also makes the peaks move to low frequency. This study has the potential applications in the sound absorption design of composite structure.

Numerical simulations of perforated plate liners: Analysis of the visco-thermal dissipation mechanisms.

In the linear regime and in the absence of mean flow, the impedance of perforated liners is driven by visco-thermal effects. In this paper, two numerical models are employed for predicting these visco-thermal losses. The first model is the linearized compressible Navier-Stokes equations (LNSE), solved in the frequency domain. The second model is the Helmholtz equation with a visco-thermal boundary condition, accounting for the influence of the acoustic boundary layers. These models are compared and validated against measurements. The quantitative analysis of the dissipation rate due to viscosity, computed from the LNSE solutions of four perforated plates, highlights significant differences between the edge effects of a macro- and a micro-perforated plate. In the latter case, a jet is present at the entrances of the perforation. In contrast, the proposed numerical method to calculate the impedance of perforated liners, based on the Helmholtz equation and a visco-thermal boundary condition, is found to be computationally cheaper and to provide reliable predictions.

Passive Suppression of Self-Excited Combustion Instabilities in Liquid Spray Flame Using Microperforated Plate

Abstract This paper presents an experimental investigation of using a microperforated plate (MPP) backed by an adjustable cavity to mitigate the combustion instabilities of liquid fuel swirl flame. The acoustic properties of MPPs with different porosities and aperture diameters were first tested in an impedance tube. At low bias flow rates, the sound reflection coefficients of MPPs with large holes commensurate well with the Luong model, and at high bias flow rates, the results of MPPs with small holes agree well with the predictions. The maximum sound absorption coefficient of each panel at the target frequency exceeded 95%. The perforated panels were then selected and integrated into a spray combustor individually. It was shown that the maximum reduction of pressure and heat release fluctuations inside the chamber was 14.91 dB and 13.40 dB, respectively. After noise elimination, the main frequencies of pressure and CH* signals were slightly shifted toward low frequencies. When the combustion conditions change, the MPPs operating near the optimal bias flow rates still have good sound absorption characteristics. After noise suppression, the synchronization between pressure and heat release signals was reduced, and the flame shapes were relatively stable. More generally, this study can promote the application of MPPs under bias flow in stabilizing the liquid spray combustion.

SeMSA: a compact super absorber optimised for broadband, low-frequency noise attenuation

The attenuation of low-frequency broadband noise in a light, small form-factor is an intractable challenge. In this paper, a new technology is presented which employs the highly efficient visco-thermal loss mechanism of a micro-perforated plate (MPP) and successfully lowers its frequency response by combining it with decorated membrane resonators (DMR). Absorption comes from the membranes but primarily from the MPP, as the motion of the two membranes causes a pressure differential across the MPP creating airflow through the perforations. This combination of DMR and MPP has led to the Segmented Membrane Sound Absorber (SeMSA) design, which is extremely effective at low-frequency broadband sound absorption and which can achieve this at deep sub-wavelength thicknesses. The technology is compared to other absorbers to be found in the literature and the SeMSA outperforms them all in either the 20–1000 Hz or 20–1200 Hz range for depths of up to 120 mm. This was verified through analytical, finite element and experimental analyses.

A Novel Approach for Optimization of Vehicle Noise Insulators

Development of effective sound insulators for interior noise reduction of vehicle cabin is an essential phase of the process of vehicle manufacturing. One of the main issues in this process is the space limitations for packaging the firewall insulators. In this paper, a new approach in sound insulator optimization will be used, in which the space limitation has been considered. The insulator consists of a Micro-Perforated Plate (MPP) and a limp porous layer. MPP layer is suitable for use in vehicles due to low weight and flexibility in design. The main goal of this study was to reach better performance and less weight and volume in a specific thickness. So, a mathematical model of this sound package has been developed and validated by experimental data and SEA software. The simulation results illustrate the high performance of the optimized sound package in practical application, particularly at higher frequencies. On the other hand, the weight and thickness of the optimized sound package are less than the conventional sound packages. These qualities provide an optimized design that has better compatibility with the automotive industry.

Improvement of sound absorption and insulation using a double-layer metamaterial

Improvement of sound absorption and insulation using a double-layer metamaterial (DM) in the acoustic free field is proposed in the present paper. The front layer of the DM is composed of a flexible micro-perforated plate with periodic local resonators, the backing layer is a flexible plate attached with periodic local resonators too, and an air gap exists between the double plates. Good agreement is gained between the results of the theoretical prediction and finite element simulation for the DM and the original double-layer plate (DP) while considering the vibroacoustic coupling between the sound excitation and the plates. Both theoretical and simulation results verify that the local resonators can improve the sound absorption and insulation of the DP. The underlying mechanism of the DM is investigated using the acoustic impedance and displacement pattern. Filling the gap with the porous material can further improve the sound absorption and insulation of the DM. Finally, the influences of the number and additional mass ratio of local resonators on the acoustic performance of the DM are investigated, and the practical realization of the DM is verified. The present design shows great potential for practical noise reduction in the free field.

On-demand optimize design of sound-absorbing porous material based on multi-population genetic algorithm

Abstract Porous material (PM) shows good sound absorption performance, however, the sound absorbing property of PM with different parameters are greatly different. In order to match the most suitable absorbing materials with the most satisfactory sound-absorbing performance according to the noise spectrum in different practical applications, multi-population genetic algorithm is used in this paper to optimize the parameters of porous sound absorbing structures that are commonly used according to the actual demand of noise reduction and experimental verification. The results shows that the optimization results of multi-population genetic algorithm are obviously better than the standard genetic algorithm in terms of sound absorption performance and sound absorption bandwidth. The average acoustic absorption coefficient of PM can reach above 0.6 in the range of medium frequency, and over 0.8 in the range of high frequency through optimization design. At a mid-to-high frequency environment, the PM has a better sound absorption effect and a wider frequency band than that of micro-perforated plate. However, it has a poor sound absorption effect at low frequency. So it is necessary to select suitable sound absorption material according to the actual noise spectrum.

Design of subwavelength broadband hybrid sound absorption structure based on micro-perforated plate and coiled channels

In this paper, we propose a hybrid subwavelength broadband sound absorber based on micro perforated plate and multiple coiled channels. And the mechanism of low frequency broadband sound absorption of the hybrid sound absorber is analyzed in detail. Based on this, the theoretical analysis model and the finite element numerical analysis model are established, and the mutual verification of theoretical and numerical solutions is completed. The structure can theoretically achieve the low-frequency and high-efficiency sound absorption with an average absorption coefficient of 0.8 in a frequency band of 200–500 Hz when the overall thickness of the sound absorbing structure is 60 mm. At the same time when the overall thickness is 90 mm, quasi-perfect sound absorption with peaks up to 0.95 in a frequency range of 180–350 Hz is realized theoretically. The composite sound absorption structure has a certain application prospect in engineering low frequency noise in future.

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.

An Integration Strategy for Acoustic Metamaterials to Achieve Absorption by Design

As much of metamaterials’ properties originate from resonances, the novel characteristics displayed by acoustic metamaterials are a narrow bandwidth and high dispersive in nature. However, for practical applications, broadband is often a necessity. Furthermore, it would even be better if acoustic metamaterials can display tunable bandwidth characteristics, e.g., with an absorption spectrum that is tailored to fit the noise spectrum. In this article we present a designed integration strategy for acoustic metamaterials that not only overcomes the narrow-band Achilles’ heel for acoustic absorption but also achieves such effect with the minimum sample thickness as dictated by the law of nature. The three elements of the design strategy comprise: (a) the causality constraint, (b) the determination of resonant mode density in accordance with the input target impedance, and (c) the accounting of absorption by evanescent waves. Here, the causality constraint relates the absorption spectrum to a minimum sample thickness, derived from the causal nature of the acoustic response. We have successfully implemented the design strategy by realizing three structures of which one acoustic metamaterial structure, comprising 16 Fabry-Perot resonators, is shown to exhibit near-perfect flat absorption spectrum starting at 400 Hz. The sample has a thickness of 10.86 cm, whereas the minimum thickness as dictated by the causality constraint is 10.55 cm in this particular case. A second structure demonstrates the flexible tunability of the design strategy by opening a reflection notch in the absorption spectrum, extending from 600 to 1000 Hz, with a sample thickness that is only 3 mm above the causality minimum. We compare the designed absorption structure with conventional absorption materials/structures, such as the acoustic sponge and micro-perforated plate, with equal thicknesses as the metamaterial structure. In both cases the designed metamaterial structure displays superior absorption performance in the target frequency range.

Numerical optimization for acoustic performance of the micro-perforated plate and its application in high-speed trains

Currently, researches on the acoustic radiation efficiency and sound insulation performance of the micro-perforated plate mainly focused on the experimental test, and the numerical simulation is rarely reported. In this paper, firstly, the discrete element method was applied to test the acoustic radiation efficiency of the micro-perforated plate. It was shown that the experimental result was reliable, which made up for the deficiency of the traditional vibro-acoustic test method. Secondly, the pulse decay method was used to test the damping loss factor of the perforated plate. Then, the transfer admittance on both sides of the micro-perforated plate was computed to simulate the properties of the hole. Finally, the damping loss factor and admittance were imported into the boundary element model (BEM) of the micro-perforated plate, in order to compute the transmission loss, and it was compared with that of the experimental test. As shown from the compared result, the sound insulation performance of the micro-perforated plate can be predicted effectively using this method. Based on the verified simulation model, the impact of the hole diameter, plate thickness and perforation rate on sound insulation performance of the perforated plate was studied. However, each structural parameter of the perforated plate can't be optimal through the above analysis. As a result, an optimization design was conducted on it based on the improved genetic algorithm. Finally, the perforated plate which had the optimal structural parameter and performance was obtained. Finally, the optimized perforated plate was applied on the high-speed train, and the experimental results showed that the interior noise in the high-speed train was reduced obviously.