Micro-perforated Panel Research Articles

Acoustic modelling and analyses of geometrically complex systems with Micro-perforated panels

Modeling of vibro/acoustic systems with embedded acoustic components of complex geometries is a challenging task. In particular, the presence of irregular-shaped acoustic modules, which are usually treated by finite element (FE) method, increases system complexities and makes the use of existing sub-structuring modeling techniques cumbersome. To tackle the problem, an efficient three-dimensional sub-structuring modeling method is proposed in this paper. As an important sub-structural module, a dedicated coordinate transformation technique is established to cope with polygon acoustic components. The embodiment of the technique into the existing sub-structuring framework avoids the use of conventional FE modules, thus increasing the flexibility and the efficiency of the simulation, conducive to system optimization. As an example, noise reduction in a duct, comprising a Micro-Perforated Panel (MPP) liner and a trapezoidal expansion chamber, is examined. The accuracy of the proposed model is firstly validated against both FE simulations and experiments. Numerical results uncover a dual hybrid sound reduction process, namely sound absorption of the MPP and wave reflection due to the geometry changes of the duct. Optimizations based on the proposed sub-structuring technique allow one to balance the hybrid reflective-absorptive effects through proper parameter tuning to maximize the sound attenuation within a prescribed frequency range.

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.

Effect of Thickness and Perforation Size on the Acoustic Absorption Performance of a Micro-Perforated Panel

In current times, noise pollution is especially apparent in urban areas due to rapid development in transportation, industrialization, and urbanization. The worsening noise pollution is detrimental to human health and behaviour as it can contribute to disorders and psychological disturbance. Thus, noise regulation is crucial and must be addressed with immediate effect. Micro-perforated panels (MPP) can be a potential solution to mitigate noise on a commercial scale. Researchers have addressed the mechanics behind the enhancement of acoustic absorption through micro-perforation and some suggestions have been made, such as the effect of structural variation on sound absorption performance. Hence, this research aims at optimizing the sound absorption performance of an MPP by determining the connection between thickness and perforation size with sound absorption coefficient. Three cases were considered: (i) varying perforation size, (ii) varying thickness, and (iii) varying perforation size and thickness simultaneously. Based on the Maa prediction model, the sound absorption performance for all three cases have been simulated through the MATLAB software. Results show that the increase in both thickness and perforation size together increases the peak value of sound absorption coefficient (SAC). It also shifts the peak towards the higher frequency region and narrows the bandwidth. The findings of this study indicate the potential of thick MPPs as commercial sound absorbers by adjusting the size of perforations. Thicker and sturdier MPPs with optimal acoustic resistance and reactance can act as reliable sound absorbers for sound insulation purposes.

Effect of Plastocene Embedment in the Microperforated Plate on Its Acoustic Performance

Noise pollution is one of the contemporary environmental pollution, which seriously damages people’s green and healthy life. In order to further improve the low frequency sound absorption performance of microperforated panel (MPP), a new plastocene coupled microperforated plate (PCMPP) is proposed. The acoustic properties of PCMPP with different apertures and perforation ratio were measured by transfer function method and compared with that of conventional MPPs. It is found that when the aperture was 0.8 mm, the peak value of sound absorption coefficient of PCMPP decreased by 150 Hz compared with MPP. In a certain range, PCMPP with larger apertures showed a greater influence on sound absorption property in low frequency. In addition, higher perforation ratio led to a greater PCMPP bandwidth of sound absorption. On the other hand, the effect of PCMPP with aperture of 0.2 mm on the performance of MPP was reduced, which could be compensated by increasing the perforation ratio. Furthermore, we found that the effect of aperture, perforation ratio and cavity on the sound absorption performance of PCMPP was consistent with that of ideal rigid MPP. The step cooling curve showed that the plastocene began to soften at about 50°C, representing a great potential for a non-high temperature work environment.

Acoustic Properties of Micro-Perforated Panels Made from Oil Palm Empty Fruit Bunch Fiber Reinforced Polylactic Acid

This paper presents the development and performance of micro-perforated panels (MPP) from natural fiber reinforced composites. The MPP is made of Polylactic Acid (PLA) reinforced with Oil Palm Empty Fruit Bunch Fiber (OPEFBF). The investigation was made by varying the fiber density, air gap, and perforation ratio to observe the effect on the Sound Absorption Coefficient (SAC) through the experiment in an impedance tube. It is found that the peak level of SAC is not affected, but the peak frequency shifts to lower frequency when the fiber density is increased. This phenomenon might be due to the presence of porosity in the inner wall of the holes. Increasing or decreasing the air gap and perforation ratio shifts the peaks of acoustic absorption either way.

Application of transparent microperforated panels to acrylic partitions for desktop use: A case study by prototyping.

There are various measures currently in place to prevent the spread of coronavirus (COVID-19); however, in some cases, these can have an adverse effect on the acoustic environment in buildings. For example, transparent acrylic partitions are often used in eating establishments, meeting rooms, offices, etc., to prevent droplet infection. However, acrylic partitions are acoustically reflective; therefore, reflected sounds may cause acoustic problems such as difficulties in conversation or the leakage of conversation. In this study, we performed a prototyping of transparent acrylic partitions to which a microperforated panel (MPP) was applied for sound absorption while maintaining transparency. The proposed partition is a triple-leaf acrylic partition with a single acrylic sheet without holes between two MPP sheets, as including a hole-free panel is important to prevent possible droplet penetration. The sound absorption characteristics were investigated by measuring the sound absorption in a reverberation room. As the original prototype showed sound absorption characteristics with a gentle peak and low values due to the openings on the periphery, it was modified by closing the openings on the top and sides. The sound absorption performance was improved to some extent when the top and sides were closed, although there remains the possibility of further improvement. For this study, only the sound absorption characteristics were examined in the prototype experiments. The effects during actual use will be the subject of future study.

A review on latest acoustic noise mitigation materials

In this evolving technology, we move towards a more comfortable and swifter life, which comes with certain drawbacks. Among them, noise finds one of the prominent places due to modern urbanization and industrial developments. So, it becomes necessary to control the noise level effectively in all the fields of engineering. The purpose of this work is to highlight the most promising acoustic materials ranging from eco-friendly natural materials to artificial structures. This review primarily focuses on the current trends of acoustic materials that include natural fibers and recycled materials, metamaterials, acoustic black holes, micro-perforated panels, and advanced foams. All these acoustic materials have gained much attention in the present and also promote future directions. Furthermore, the leverage of additive technology also enhances the development of the customized design of acoustic structures is also reviewed.

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.

Acoustic behaviour of 3D printed bio-degradable micro-perforated panels with varying perforation cross-sections

Influence of perforations having arbitrarily varying cross-sections on the acoustic behaviour of 3D printed bio-degradable panels made of Poly Lactic Acid (PLA) is presented. Circular perforations having six different types of cross-sectional variations namely convergent-divergent (CD), divergent-convergent (DC), convergent (C), divergent (D) with two different perforation diameters are realized using Fused Filament Fabrication (FFF) based 3D printing. Sound absorption and transmission loss characteristics of these perforated panels are estimated through impedance tube technique. Results revealed that sound absorption of perforated panels with varying cross-section is better than uniform cross-sectional perforation for the given frequency range. Among, the different cross-sectional variations explored, comparable and lower transmission losses are exhibited by DC and D perforation pattern with respect to constant diameter 1 mm panel. The sound transmission results of all other five specimens were significantly higher than constant diameter 8 mm panel and observed to be increasing with frequency. Geometrical perforation variations are noted to be a very crucial factor in designing soundproof panels as presented in this work. The experimental results are compared with the numerical results and found to be in good agreement. Such numerical analysis paves the guidelines for designing optimum perforation geometries prior to the on-field testing of the functional prototypes.

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

A Machine Learning Based Prediction Model for the Sound Absorption Coefficient of Micro-Expanded Metal Mesh (MEMM)

Recently, micro-perforated panels (MPP) have become a popular sound absorbing material in the field of architectural acoustics. However, the cost of MPP is still high for the commercial market in Taiwan, and MPP is still not very popular compared to other sound absorbing materials and devices. The objective of this study is to develop a prediction model for MEMM via a machine learning approach. An experiment including 14 types of MEMM was first carried out in a reverberation room based on ISO 354. To predict the sound absorption coefficient of the MEMM, the capability of three conventional models and three machine learning (ML) models of the supervised learning method were studied for the development of the prediction model. The results showed that in most conventional models, the sound absorption coefficient of using an equivalent perimeter had the best agreement compared with other parameters, and the root mean square error (RMSE) between prediction models and experimental data were around 0.2~0.3. However, the RMSE of all ML models was less than 0.1, and the RMSE of the gradient boost model was 0.033 in the training sets and 0.062 in the testing sets, which showed the best agreement with the experiment data.

Experimental characterization of aero-acoustic performance of micro-perforated materials in a wind tunnel

Mitigating the propagation of low frequency noise in ducted flows represents a challenging task since wall treatments have often limited dimensions. Micro-perforated panels have been successfully used backed by a cavity constituting a bulk-reacting resonator or in combination with honeycomb for a locally reacting system. In this work, a cost-efficient methodology for the study and characterization of the aero-acoustic properties of flush-mounted micro-perforated resonators has been developed. Although Laser Doppler Velocimetry is a non-intrusive technique, it requires delicate instrumentation to obtain an estimation of the acoustic velocity and pressure fluctuations at several points over a wall liner. Here, the attenuation has been estimated from sound pressure level measurements performed with two nosecone microphones positioned along a vertical line centered on the resonator axis. Experiments have been performed in a wind tunnel in presence of a low-speed turbulent boundary layer of air fully developed over different samples made up of micro-perforated sheets or porous materials with surface roughness. The micro-perforates have been flush-mounted over a cylindrical cavity of depth 30 mm situated on the floor of the test section. The ability of bulk-reacting resonators for reducing the acoustic or flow-induced noise has been assessed in comparison with locally reacting treatments.

Sound Absorption of Microperforated Panel Made from Coconut Fiber and Polylactic Acid Composite

ABSTRACT This paper highlights the sound absorption performance of microperforated panel (MPP) made from coconut fiber and polylactic acid (PLA) composite named as biocomposite microperforated panel (BMPP). Coconut fiber was used to produce BMPP specimen and PLA was used as matrix. Impedance tube method was employed to identify the sound absorption performance of BMPP specimen while a porosity tester was used to determine the porosity of BMPP specimen. Comparison on the sound absorption performance between BMPP and steel MPP will be discussed in this paper. It has been found that BMPP with different composition percentage of coconut fiber and PLA possessed different sound absorption performance mainly due to the existence of pores and tortuous structure within the specimen itself. Scanning electron microscope (SEM) scan was performed to further analyze the structure of BMPP specimen.

Low-frequency sound absorption of a metamaterial with symmetrical-coiled-up spaces

A theoretical, numerical and experimental investigation of a low-frequency acoustic absorber (100–600 Hz) is reported herein. The acoustic metamaterial is based on a micro-perforated panel coupled to a multi-cavity of coiled-up spaces that is similar to a symmetrical labyrinth. On considering the visco-thermal losses, the effect of increasing the number of symmetrically coiled-up spaces that determines the peak position of the sound absorption and allows a greater sound energy absorption is discussed theoretically and proven through numerical analysis (FEM). Prototypes were manufactured using 3D printing technology and evaluated in an impedance tube. The results for the sound absorption coefficient acquired in the desired frequency range were greater than 91% with relative bandwidth above 35%, in agreement with the analytical model. Therefore, it is demonstrated that the proposed absorber presents a scale of deep-subwavelength since its total thickness is 0.033λ.

The Effect of Deviation Due to the Manufacturing Accuracy in the Parameters of an MPP on Its Acoustic Properties: Trial Production of MPPs of Different Hole Shapes Using 3D Printing

In this study, we discuss the effect of the manufacturing accuracy of a microperforated panel (MPP) produced by 3D printers on acoustic properties through measured and calculated results as a pilot study. The manufacturing costs of MPPs have long been one of their shortcomings; however, with recent developments in the manufacturing process, low-cost MPPs are now available. In a further attempt at reducing the cost, 3D printing techniques have recently been considered. Cases of trial production of MPPs manufactured by 3D printing have been reported. When introducing such new techniques, despite the conventional microdrill procedure, manufacturing accuracy can often become an issue. However, there are few studies reporting the effect of manufacturing accuracy on the acoustic properties in the case of 3D-printed MPPs. Considering this situation, in this pilot study, we attempted to produce MPPs with circular and rectangular perforations using a consumer 3D printer of the additive manufacturing type. The hole sizes of the specimens were measured, and the accuracy was evaluated. The normal incidence absorption coefficient and specific impedance were measured using an impedance tube. The measured results were compared with the theoretical values using Guo’s model. Through these basic studies, the MPPs produced by an additive manufacturing 3D printer demonstrated good sound absorption performance; however, due to the large deviations of parameters, the agreement with the theoretical values was not good, which suggests that it is difficult to predict the acoustic properties of MPPs made by a consumer-grade additive manufacturing 3D printer.

Additional length model for the impedance end correction of microperforated panels.

An additional length model is usually used to describe the reactive part of the impedance end correction of microperforated panels, which is extended to describe the resistive part. The cross-sectional impedance is computed along the axis of one perforation cell with a circular hole. Except for the obvious jumps in the narrow regions at the inlet and outlet of the perforation, the impedance varies linearly along the axis following exactly that of the viscous wave in the circular hole. The additional length for the impedance end correction is obtained by extrapolating the linearly varying impedance inside the hole. Empirical models for the resistive and reactive additional lengths are obtained based on the thermoviscous acoustic simulation with 96 test cases. Within an error of about 10%, a unified additional length model is presented for both the resistive and reactive parts of the impedance end correction. Comparison with other existing models shows the accuracy of the proposed model.

Experimental Research on Noise Reduction for Continuous Transonic Wind Tunnel Loop

The noise level of wind tunnel test section is respected as one of the most important performance specifications to represent the flow field quality, especially for large scale wind tunnel. According to the acoustic experimental research conducted in the 0.6 m continuous transonic wind tunnel of CARDC, main noise sources in the tunnel loop included the compressor, the high-speed diffuser and the test section. To reduce the noise in the test section, it is necessary to prevent the test section from the compressor noise propagated both forward and backward. In 0.6 m wind tunnel loop, acoustic treatments were installed on both the compressor rear cone and the fourth corner to prevent the noise emitted from the compressor from propagating forward. The vanes in the forth corner were filled with glass fibers and covered with perforated panels. And the compressor rear cone was covered with three layers of micro-perforated panels. With acoustic treatment in the tunnel loop and the second throat throttling, the fluctuation pressure coefficient (ΔCp) is lower than 0.8%, which is close to the international advanced level.