Flexible Micro-perforated Panel Research Articles

Bandwidth widening for sound absorption of a flexible microperforated panel backed by a multi-frequency panel-type resonator

A microperforated panel (MPP) often suffers from a limited absorption bandwidth and poor sound absorption in the low frequency range. The present study aimed to propose an MPP-panel-type resonator (PR) compound structure that can simultaneously widen the half-absorption bandwidth and improve the poor absorption at the low frequency range. A vibroacoustic model is developed and compared to finite element simulations to demonstrate its accuracy. The vibration characteristics of the compound structure and the surface impedance are analyzed to reveal the mechanism forming the absorption characteristics of an MPP-PR structure. It is found that backing an MPP with a panel-type resonator (PR) creates a low frequency absorption peak but followed by an absorption valley that decreases the absorption bandwidth. The analysis showed that the phase difference between velocity of air particles in perforation and the PR panel velocity dictates the absorption characteristics associated with the PR resonance. A multi-resonators panel-type resonator (MPR) is found that the absorption valley associated with the host panel resonance splits into multiple smaller amplitude absorption dips improving absorption performance. The proposed MPP-MPR compound structure demonstrated a relatively wider half-absorption bandwidth with improved low frequency absorption.

Acoustic absorption of a metamaterial panel: Mechanism, boundary effect and experimental demonstration

The present paper investigates absorption of a metamaterial panel composed of a flexible micro-perforated panel attached periodically with local resonators. It gains good agreement between the results of the theoretical prediction and finite element simulation for the metamaterial panel while considering the vibroacoustic coupling between the sound excitation and the plate vibration. Both theoretical and finite element simulation results show that the local resonator can tune the acoustic surface impedance at desired frequency. The boundary effect of the metamaterial panel under the simply supported or clamped condition at all four edges is investigated. It shows that new absorption peak can be acquired differently under various boundary conditions. The absorption mechanism is further revealed by the displacement pattern and the surface impedance. Finally, a sample of four-edge clamped metamaterial panel is designed and fabricated. The experiment and simulation results validate the theoretical prediction.

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.

Sound transmission characteristics of underwater flexible micro-perforated panels

It is unlikely to ignore the coupling effect between the structure of the micro-perforated panel (MPP) and the water medium during the application of MPP in the water medium. Theoretical and experimental studies on the sound transmission characteristics of multi-layer flexible micro-perforated panels (FMPPs) under normal incidence conditions were performed. The transfer impedance of finite-sized FMPPs was calculated with the modal superposition method, and the transmission and energy dissipation factor of the FMPPs was derived using the transfer matrix method. Besides, the studies on the effect of panel elastic properties and the arrangement order on the sound insulation performance were carried out. The sound transmission loss (STL) curve remains unchanged when the arrangement order was transposed symmetrically, and it was beneficial to improve the energy dissipation factor when the perforation rate decreased from the sound source side. Theoretical results were verified on a water-filled impedance tube using the two source-location methods.

Flexible micro-perforated panels prepared by CNC machining

With the advantages of light weight, thin thickness and environment friendly, the micro-perforated panel (MPP) has been widely studied for noise reduction. Maa’s model pointed out that the MPP could obtain higher sound absorption over broader frequency band when the perforations were reduced to less than 100 μm. However, it is challenging to manufacture MPPs with the aperture of approximate 100 μm, thus its potential application has been restricted. In this study, we used a computer numerical control (CNC) milling machine to process MPP. Four different kinds of raw materials including paperboard, polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polycarbonate (PC) were taken to prepare micro-perforated panels (MPPs). It has been indicated that MPPs with good sound absorption properties were successfully prepared by this facile method.

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.

Improvement of the sound absorption of flexible micro-perforated panels by local resonances

Abstract To further improve the sound absorption enhancement of flexible micro-perforated panels (FMPPs), a new sort of perforated sound absorbers – metamaterial-based micro-perforated panels (MMPPs) – is proposed by combining a micro-perforated host panel and local resonators (LRs) attached on a sub-wavelength scale, targeting the flexural waves. Theoretical and numerical models show that MMPPs are able to further enhance sound absorption in a wide frequency range. The theoretical model is developed based on the effective medium method as the structural wavelength in the host panel is much larger than the distance between the LRs, and the full simulation model, including visco-thermal effects, is conducted by utilizing multi-physical coupling integrated in COMSOL. Besides, a structural finite element unit cell method is used to evaluate the stop band behavior of the MMPP. Good agreement is achieved between the theoretically predicted acoustical properties and the simulation results for both conventional FMPPs and the proposed MMPPs, validating the numerical and theoretical models. Both models reveal that the sound absorption enhancement of the MMPP stems from the resulting acoustic surface impedance improvement, caused by the sub-wavelength attached local resonances. The effect of key properties of the LRs (i.e. mass, damping and multiple resonances) on the sound absorption performance of MMPPs is then analyzed by applying the theoretical model and effective frequency-adjustability of the absorption enhancement performance is found. The proposed MMPP shows great potential for the noise reduction industry.

Investigation on the Acoustic Absorption of Flexible Micro-Perforated Panel with Ultra-Micro Perforations

Flexible micro-perforated panel has unique advantages in noise reduction due to its good flexibility compared with traditional rigid micro-perforated panel. In this paper, flexible micro-perforated panel was prepared by computer numerical control (CNC) milling machine. Three kinds of plastics including polyvinylchloride (PVC), polyethylene terephthalate (PET), and polyimide (PI) were taken as the matrix materials to prepare flexible micro-perforated panel. It has been found that flexible micro-perforated panel made of PET possessing good porosity and proper density, elastic modulus and poisson ratio exhibited the best acoustic absorption properties. The effects of various structural parameters including perforation diameter, perforation ratio, thickness and air gap have also been investigated, which would be helpful to the optimization of acoustic absorption properties.

A parametric study of flexible micro-perforated panels with a patch-impedance numerical model

The absorption characteristics of a single flexible micro-perforated panel with a back cavity are compared for different combinations of physical parameters. Provided that the panel is made of a rigid plate, the acoustic properties of a micro-perforated panel are determined by the plate thickness, the distribution and diameter of the perforations, the porosity, the edge profile and back cavity depth. Nevertheless, some applications can involve a material choice or plate thickness values invalidating the rigidity assumption. In such cases, additional absorption peaks are observed in the absorption frequency spectrum, which cannot be explained by the classical micro-perforated plate theory. The vibro-acoustic coupling of the flexible plate and the acoustic medium has to be taken into account. This is done by a thin shell model employing a patch-impedance approach, modeling each perforation separately. Using this model, absorption coefficient values are calculated for various combinations of plate thickness, perforation diameter and perforation distribution. The results of this study provide a better understanding of the influence of the design parameters of flexible micro-perforated panels on the sound absorption. The method is numerically efficient and can be used for optimizing the acoustic absorption of such panels.

A note on the fabrication methods of flexible ultra micro-perforated panels

Micro-perforated panel (MPP) absorber has been studied and developed for decades and is becoming more and more popular in an environment protective society. Although the traditional MPPs are rigid, the research results in recent years have indicated that flexible MPPs with ultra-micro perforations of diameter less than 100μm (ultra-MPPs) may have great potential to become a wideband sound absorber in high frequency range. However, it is a great challenge for both traditional mechanical methods and micro-fabrication technology to fabricate so small holes directly on flexible materials, which greatly limits the development of flexible ultra-MPPs. Therefore a new technical solution is strongly desired. To meet this demand, a new process, based on mold manufacturing technology and casting technology, is developed, and the key and specific process steps are presented in this paper.

Optimization of double-layered micro-perforated panels with vibro-acoustic effect

A vibro-acoustic effect, in which the classical plate equation is coupled to the acoustic wave equation, was utilised to optimally tune a double-layered flexible micro-perforated panel (MPP), to achieve higher sound absorption coefficient and wider frequency band. The first nine modes considered in the present modelling study were obtained by using the finite element analysis. Results of the vibro-acoustic model, applied to a single layer MPP, were found to be in a good agreement with other published results; thus, the model was used to develop the equivalent electro-acoustical circuit of a double-layered MPP sound absorber. The simulated annealing optimization technique was used to obtain the optimal design parameters of the MPP sound absorber that were needed to achieve the maximal averaged sound absorption coefficient for the frequency band 200–1,000 Hz. The optimized double-layered MPP sound absorber improved the sound absorption coefficient by 8.88–12.52 % compared with the model without the vibro-acoustic effect. Mode (1, 1) was found to dominate the sound absorption performance compared to the other nine mode shapes that were considered in the present study. The effects of configuration, thickness, damping ratio, and boundary condition of the MPP on the sound absorption coefficient were also investigated.

Sound absorption of a flexible micro-perforated panel absorber based on PVDF piezoelectric film

Micro-perforated panel (MPP) has been widely used in the noise control field. However, the performance at low frequencies is relatively poor due to the restricted space limitations in actual applications. Hence in this paper a flexible micro-perforated panel absorber (PVDF-MPPA) based on Polyvinylidene fluoride (PVDF) piezoelectric film is proposed, which is composed of a perforated aluminum-electrode PVDF piezoelectric film and a small air cavity. Its sound absorption at low frequency can be improved by employing panel vibration effect and shunting the electrodes of piezoelectric film with some electrical impedance. The corresponding structure of the PVDF-MPPA was designed and fabricated. Experiments were performed under different shunting cases such as open circuit, short circuit, and resonant circuit in an impedance tube with the testing frequency range from 100Hz to 1600Hz. The results demonstrated that the PVDF-MPPA with shunt circuits had a wider absorption bandwidth compared with a rigid MPP absorber with the same structural parameters. When compared with the open circuit and short circuit cases, the PVDF-MPPA with resonant circuit definitely has a better low frequency absorption performance if the shunting parameters have been carefully tuned. By using shunt-damping technology, the PVDF-MPPA has a potential to be widely used as an efficient sound absorber in practical situations because of its simple structure and good absorption performance at low frequencies.

Research about Flexible PDMS Micro-Perforated Panel Based on MEMS Silicon Mold

Micro-perforated panel (MPP) is known for its remarkable noise absorption capability and has been studied and developed for decades. Although the panel and cavity walls of traditional MPP are rigid, some previous works have shown that the absorption bandwidth of flexible MPP can be improved by the vibration of the panel. But this result is only tested at low frequencies. In this paper, the flexible MPP aiming at high frequency range is studied under a control experiment. But the perforations of high frequency MPP are in the range of micron and difficult to be fabricated in traditional mechanical methods. A new process, based on MEMS silicon mold technology and PDMS casting method, is developed to meet this challenge. Experimental results show that the noise absorption bandwidth of flexible MPP does get widened conspicuously only at a little sacrifice of absorption peak.

A Compact Hybrid Sound Absorber Using Flexible Micro-Perforated Panel and Piezoelectric Actuator

This paper presents the concept of a compact hybrid sound absorber, based on a combined approach for sound absorption. A flexible micro-perforated panel (MPP) is used as the passive sound absorber for mid and high frequencies and a piezoelectric patch as the active control actuator for low frequency. The volume of this new absorber is highly reduced compare to conventional hybrid systems which employ porous layer as passive part and loudspeaker as active part. The vibration effect of the MPP in the hybrid system is also considered. Theoretical and experimental results show that the flexible MPP has the potential to dissipate more energy and can be utilized to improve absorption performance of the hybrid system by appropriately selecting its parameters.

Sound absorption and transmission through flexible micro-perforated panels backed by an air layer and a thin plate

This paper describes theoretical and experimental investigations into the sound absorption and transmission properties of micro-perforated panels (MPP) backed by an air cavity and a thin plate. A fully coupled modal approach is proposed to calculate the absorption coefficient and the transmission loss of finite-sized micro-perforated panels-cavity-panel (MPPCP) partitions with conservative boundary conditions. It is validated against infinite partition models and experimental data. A practical methodology is proposed using collocated pressure-velocity sensors to evaluate in an anechoic environment the transmission and absorption properties of conventional MPPCPs. Results show under which conditions edge scattering effects should be accounted for at low frequencies. Coupled mode analysis is also performed and analytical approximations are derived from the resonance frequencies and mode shapes of a flexible MPPCP. It is found that the Helmholtz-type resonance frequency is deduced from the one associated to the rigidly backed MPPCP absorber shifted up by the mass-air mass resonance of the flexible non-perforated double-panel. Moreover, it is shown analytically and experimentally that the absorption mechanisms at the resonances are governed by a large air-frame relative velocity over the MPP surface, with either in-phase or out-of-phase relationships, depending on the MPPCP parameters.

Sound absorption and transmission through flexible micro-perforated panels backed by an air layer and a thin plate

Sound absorption and transmission through flexible micro-perforated panels backed by an air layer and a thin plate

Sound Absorption of Hybrid Passive-Active System Using Finite Flexible Micro-Perforated Panels

Although the vibration effect of micro-perforated panels (MPPs) in passive sound absorption systems has been considered in some studies, research into active systems remains to be accomplished. In this paper, theoretical and experimental studies of hybrid systems using finite flexible MPPs are presented. The impedance of the flexible MPP is derived using the modal analysis solution of the classical plate equation. Then absorption coefficients are predicted based on an electro-acoustic equivalent circuit model. A study of the absorption mechanism of the hybrid system shows that panel vibration can degrade the Helmholtz-resonance absorption performance and bring out panel-type absorption, especially increasing the cavity absorption in systems using impedance-matching conditions. Theoretical results show good agreement with the experiments. It can be seen that flexible the MPP has the potential to dissipate more energy and can be utilized to improve the absorption performance of the hybrid system by appropriately selecting parameters.

Sound insulation characteristics of multi-layer structures with a microperforated panel

Multi-layer structures have issues with sound insulation at low and mid-frequencies due to mass-air-mass resonance. The purpose of this study is to investigate improvements to the sound insulation performance of multi-layer structures using a microperforated panel (MPP), which can absorb well over a wide frequency range. Although MPPs have been investigated over the last several decades, almost all studies have been conducted in terms of sound absorption. Herein the sound transmission loss of multi-layer structures with flexible MPPs of infinite extent is theoretically investigated. The calculation is based on the wave equation and the equation of panel vibration including the effect of perforation of the panel. Additionally to consider a more realistic sound insulation performance, the effect of the directional distribution of the incident energy in a reverberation chamber is taken into account. Experiments are conducted using an acoustic tube to validate the calculated results and the reverberation chamber method to verify the actual sound insulation characteristics. Both experiments agree well with the theoretically calculated perforation effects. Consequently, MMPs are confirmed to improve the deterioration of sound insulation performance due to mass-air-mass resonance of multi-layer structures.

Relationship between Helmholtz-resonance absorption and panel-type absorption in finite flexible microperforated-panel absorbers

Microperforated panels (MPPs) can provide wide-band absorption without fibrous and porous materials and are recognized as next-generation absorption materials. Although the fundamental absorbing mechanism of an MPP absorber is Helmholtz-resonance absorption, sound-induced vibration of an MPP itself can affects the absorption characteristics. There have been some studies considering the effects of the sound-induced vibration and there even is a proposal to widen the absorption bandwidth by positively utilizing the vibration of an MPP itself. On the other hand, in a previous study, the relationship between MPP absorbers and panel-type absorbers was investigated with infinite theory. However, the relationship between Helmholtz-resonance absorption and panel-type absorption in finite flexible MPP absorbers has not been clarified. Herein, from the viewpoint of an absorption-characteristics transition with the perforation ratio, the relationship between Helmholtz-resonance absorption and panel-type absorption including the effects of eigen-mode vibrations of the panel is theoretically and experimentally investigated. The analytical model considers a finite flexible MPP supported in a circular duct, and the predicted data for the absorption coefficient under normal incidence is validated by an experiment using an acoustic tube. From this investigation, it is found that panel-type absorption due to eigen-mode vibrations of the panel occurs independently from Helmholtz-resonance absorption, while panel-type absorption due to a mass–spring resonance of a panel and a back cavity has a trade-off relationship with Helmholtz-resonance absorption with respect to the perforation ratio.

Widening the sound absorption bandwidths of flexible micro-perforated curved absorbers using structural and acoustic resonances

In this paper, the sound absorptions of flexible curved micro-perforated panels that are backed by an air cavity are studied in detail. A theoretical model that is based on the classical plate equation coupled with an acoustic wave equation is developed for the prediction of sound absorption. This model considers both symmetrical and antisymmetrical structural acoustic responses. Using the electro-acoustic analogy approach, another model is developed that only considers single structural and acoustic modes. It is proposed to make use of panel and cavity resonances to widen the absorption bandwidth of a single/double perforated absorber. The absorption of a flexible micro-perforated panel can be further enhanced by adjusting its curvature to bring the resonant frequencies closer together. The effects of various parameters such as boundary condition, incidence angle, and curvature on the absorption are studied. Predicted results for the single and double layer absorbers show good agreement with measurements.