Micro-perforated Panel Absorber Research Articles

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.

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.

Comparison Analysis and Optimization of Composite Micro-perforated Absorbers in Sound Absorption Bandwidth

Micro-perforated panel (MPP) absorber with a single uniform air cavity is regarded as a promising sound-absorbing structure; however, MPP absorption performance remains unfortunately limited due to the Helmholtz resonance mechanism, which results in more and more attention being placed on composite MPP absorbers. This study focuses on acoustic properties of different composite MPP absorbers, including three types of composite MPP absorbers which are coupled in serial, parallel and serial–parallel modes. Their mathematical models of the normal absorption coefficient are established by utilizing the acoustic electric analogy method. The single-cavity MPP absorber and three composite MPP absorbers are preliminarily designed to verify the equivalent circuit models and perform a pilot analysis of their sound absorption characteristics. Moreover, the particle swarm optimization algorithm is selected to optimize the absorbers so that optimal combination of structure parameters within a prescribed frequency range can be obtained. In addition, the absorbers are made based on the optimized parameters for experimental investigation. The results show that a wider absorption bandwidth may be achieved by composite MPP absorbers through introducing additional absorption peaks with reference to that for the conventional single-cavity MPP absorber, and there are more absorption peaks for the serial–parallel-coupled MPP absorber rather than the simply serial- or parallel-coupled MPP absorber so that better sound absorption effect may be achieved.

Engineering-oriented design strategy to obtain linear micro-perforated panel absorbers at high sound pressure environment

The nonlinear effect at high sound intensity not only degrades the absorption performance of a micro-perforated panel (MPP) absorber, but also makes its performance prediction and structure design become difficult and unreliable due to the absence of accurate nonlinear impedance models. Hence, it is desirable to develop a linear MPP absorber. Based on this, this paper presents an investigation on an engineering-oriented design method of a linear absorber according to practical needs for a specific high sound pressure level (SPL) application. The strategy actually involves a reverse design method and process and the output is a set of structure parameters that uniquely determine an MPP absorber. The input is the characteristic parameters of an MPP absorber which are associated with environment parameters, such as the most undesirable frequency and the maximum SPL. Firstly, the detailed process of design for a linear MPP absorber is expatiated in this study. Then a concrete design example based on the current approach is present and finally, the reliability of the design results is validated by experiment. This study provides a new thought and method for the effective application of MPP absorbers in high SPL environments.

Acoustical modeling of micro-perforated panel at high sound pressure levels using equivalent fluid approach

An acoustic impedance model to characterize micro-perforated panels at high sound pressure levels is proposed. The model uses a rigid frame porous medium, where the micro-perforated panel is modeled with an effective density, function of the frequency following the approach of Johnson-Allard and the equivalent fluid parameters such as the tortuosity and the flow resistivity are expressed as function of the incident sound pressure which is considered as a main variable. Unlike existing models which are limited to micro-perforated panels coupled to air cavity, the present model predicts correctly the acoustic response of micro-perforated panel backed by porous media which can be air cavity, porous material or resistive screen. Micro-perforated panel backed by porous media involves a distortion of the flow caused by the perforations through the porous media, thus the micro-perforated panel is modeled in this case using an equivalent tortuosity where a correction term is proposed to account for the flow distortion effect and it depends on the dynamic tortuosity of the porous layer and the incident sound pressure. The proposed impedance model is compared numerically with other existing nonlinear impedance models for different configurations of the micro-perforated panel. The results show a good agreement among each other for sound pressure levels up to 150 dB. In addition, experimental measurements were performed on several micro-perforated panels backed by air cavities or porous material using the classical impedance tube. A good correlation between theoretical and experimental results is obtained. Some validation and benchmarking results are illustrated and discussed in this paper. It is shown that the high sound pressure levels decrease the tortuosity and increase the flow resistivity of the micro-perforated panel. Furthermore, the acoustic energy dissipation by the micro-perforated panel absorber with a large perforation diameter which is low in the linear regime can become important and interesting at high sound pressure level.

Acoustic behaviors of the microperforated panel absorber array in nonlinear regime under moderate acoustic pressure excitation.

The acoustic performance of a microperforated panel (MPP) absorber array in the nonlinear regime is investigated both numerically and experimentally. The MPP absorber array is constructed by three parallel-arranged MPP absorbers with different cavity depths. A finite element model is used to simulate the acoustic response of the MPP absorber array by adopting the nonlinear impedance model. The results show that the absorption of the MPP absorber array is affected by the incident sound pressure when it is beyond around 100 dB. With appropriate structural and perforation property of MPP, the MPP absorber array in non-linear regime outperforms that in linear regime due to the improvement of equivalent acoustic impedance matching with ambient air over wide frequency range. However, when the sound pressure excitation is too high, the local resonance effect of the resonating component MPP absorber is diminished and the sound absorption is decreased. With the carefully chosen properties of MPP, the performance degradation induced by panel vibration can be avoided. An optimal set of MPP properties to avoid the performance degradation induced by panel vibration is determined. The measured normal absorption coefficients of a prototype MPP absorber array compare well with the numerical prediction in both linear and nonlinear regimes.

Application of Microperforated-Panel Absorber in Communication Products

Base on Prof. Maa Dah-You’s general theory of the microperforated-panel (MPP) absorber, We designed a noise reduction structure-sound attenuating cabinet (SAC). It can be applied to air-cooled communication products to reduce system noise. This article introduces engineering design method of SAC and laboratory test noise reduction results of application of SAC.

Tunable acoustic absorbers with periodical micro-perforations having varying pore shapes

Circular pores with sub-millimeter diameters have been widely used to construct micro-perforated panels (MPPs), the acoustical performance of which can be predicted well using the Maa theory (Maa D.-Y., J. Acoust. Soc. Am., 104 (1998) 2861). We present a tunable MPP absorber with periodically arranged cylindrical pores, with their cross-sectional shapes systematically altered around the circle while maintaining their cross-sectional areas unchanged. Numerical analyses based on the viscous-thermal coupled acoustical equations are utilized to investigate the tunable acoustic performance of the proposed absorbers and to reveal the underlying physical mechanisms. We demonstrate that pore morphology significantly affects the sound absorbption of MPPs by modifying the velocity field (and hence viscous dissipation) in the pores. Pore shapes featured as meso-scale circular pores accompanied with micro-scale bulges along the boundaries can lead to perfect sound absorption at relatively low frequencies. This work not only enriches the classical Maa theory on MPPs having circular perforations, but it also opens a new avenue for designing subwavelength acoustic metamaterials of superior sound absorption in target frequency ranges.

Enhancing low-frequency sound absorption of micro-perforated panel absorbers by combining parallel mechanical impedance

Micro-perforated panel (MPP) absorbers apply to sound absorption performance at the medium and high frequency, but is not ideal for low-frequency sound absorption performance. Meanwhile, multiple pieces of mechanical impedance and a MPP in series occupy too much volume, which wastes cost and resource. Based on these, a parallel mechanical impedance structure is arranged in the cavity of the micro-perforated panel to form a composite structure, the mass of every mechanical impedance is 6.4 g. A preliminary study on composite structure and a corresponding calculation model is established. Then the acoustic impedance of the whole structure is calculated by using the transfer matrix method and finally the sound absorption coefficient is measured by using standing wave tube. Experiment results show that three sound absorption peaks appear at the low frequency from 200 Hz to 600 Hz. The first peak at 0.85 or more, and the second and the third peaks are between 0.4 and 0.6. The calculation model is in agreement with the experimental results, thus proves the feasibility of the composite structure to improve low-frequency sound absorption performance.

A strategy for extending the effective application of micro-perforated panel absorbers to high sound intensity

Micro-perforated panel (MPP) absorbers are simple to construct, can be used in a variety of applications, show good absorption properties and allow relatively accurate prediction and design under linear conditions.However, when the sound intensity is high, the sound pressure level (SPL) under which the MPP is effective is limited due to the nonlinear effect. To solve this problem, we investigated the suppression of the nonlinear effect through design parameters. In this case, the linear response of an MPP with increasing SPL is enhanced; thus, the advantages of MPPs under linear conditions can be attained. First, the concept of a transition SPL is proposed to quantify the effect of the suppression. Then, the rules controlling the influence of the parameters on the transition SPL are investigated via a parameter study. Finally, the validity of the findings is confirmed using experimental data published in the literature. The obtained resultscan provide guidance for the design of MPP absorbers for high SPL applications.

A frequency domain finite element solver for acoustic simulations of 3D rooms with microperforated panel absorbers

Microperforated panel (MPP) absorbers, which provide broadband sound absorption without the use of fibrous materials, have favorable material properties that support recyclability, flexibility of design, hygiene demands, and cleaning. Many earlier studies have specifically examined the development of absorbers themselves. However, to use the absorption performance of MPP absorbers sufficiently in room acoustic applications, it is beneficial to develop accurate prediction methods of sound fields in rooms with MPP absorbers. Such methods are expected to be useful for room acoustics design and absorber design tools. This study constructs a frequency domain finite element (FE) solver for acoustic simulations of a practical sized room with MPP absorbers. Then the accuracy and effectiveness are evaluated. In the FE solver, spatial domains are discretized by fourth-order accurate FEs in terms of dispersion error, and MPP absorbers are modeled using first-order hexahedral limp MPP elements that can deal with sound propagation in the backing structure of absorbers. First, the accuracy of present FE solver is demonstrated using impedance tube problems in comparison with conventional second-order accurate FEs. Results show higher convergence of solutions for the present FE solver. Then, exploration of an iterative solver for efficient multi-frequency analyses reveals that the recently developed CSQMOR is a faster and more stable solver. Finally, comparison with a conventional surface impedance model based on a locally reacting assumption confirms the effectiveness of present FE solver by presenting the importance of dealing with the incident angle dependence of reactance of a rigid-backed air cavity in the modeling of single-leaf MPP absorbers.

Micro-perforated absorbers with incompletely partitioned cavities

Acoustic performance of a micro-perforated panel with an incompletely partitioned cavity (MPPIPC) is investigated. A MPPIPC is achieved by inserting separators, which are shorter than the depth of the cavity, periodically behind the micro-perforated panel (MPP). Due to these separators, the sound field in the cavity is significantly changed. This paper reveals these unique sound reflection modes among the separators, based on which a theoretical model is proposed to calculate the absorption coefficient of a MPPIPC in the diffusion field. Experiments verified the theoretical predictions. These results indicate that the appropriately arranged insertion can improve the performance of a MPP absorber effectively at low frequencies.

Techniques to improve the low frequency and broadband performance of microperforated panel absorbers

Microperforated panel absorbers can be considered as the combination of the perforated panel and backing airspace. It has been demonstrated that partitioning the airspace improves performance. Additional performance improvements can be obtained by varying the depth of the backing cavity, adding resonators behind the panel, or using double leaf panels. Each of these configurations is first considered for a single cell backing using plane wave simulation. Comparisons are made to impedance tube measurements with acceptable agreement. Following this, the diffuse field sound absorption for each configuration was measured in a small reverberation room. Each test sample consisted of a 0.6 m x 0.4 m panel with a backing cavity partitioned into 24 equal size cells. For the double leaf configuration, the cavity betwen panels was honeycomb partitioned but the backing cavity was unpartitioned.

Bayesian model selection for multilayer microperforated panel sound absorber design

Microperforated panel (MPP) sound absorbers are capable of providing high sound absorption coefficients without the use of fibrous materials; however, they typically function in narrow frequency ranges. By combining multiple MPPs into a multilayer absorber, the frequency bandwidth may be increased while maintaining a high absorption coefficient. Modeling the acoustic properties of an MPP absorber requires four physical parameters per MPP layer. Since each additional MPP layer in a multilayer absorber increases the complexity of the acoustic model, Bayesian model selection is well-suited to the task of designing a multilayer MPP absorber. In such a design, minimizing the number of layers used while still satisfying the design goals is desirable, in order to optimize material usage, cost, and space required by the absorber. In a full Bayesian design framework, model selection determines the number of MPP layers required, while parameter estimation determines the (physical) design parameters for each layer. In this work, an example design scheme is specified to satisfy a practical need for acoustic absorption. The Bayesian framework produces a three-layer MPP design which meets the target requirements. This absorber design is constructed, and impedance tube measurements are obtained to validate the acoustic absorption properties.

Experimental validation of Bayesian design for broadband multilayered microperforated panel absorbers

Single-layer microperforated panel (MPP) absorbers often exhibit limited absorbing bandwidth. A Bayesian inference framework (encompassing both model selection and parameter estimation) has been utilized to design broadband MPP absorbers. In this work, the broadband multilayered MPP absorbers are experimentally validated. In demonstrating ability to meet practical requirements on both high absorption and wide bandwidth, the current investigation uses a sample design scheme in the frequency range from 300 Hz to 2.4 kHz. A minimum requirement for three MPP layers and the relevant three-layer MPP parameters are derived by the two levels of Bayesian inference to meet the design scheme. MPP samples, based on the predicted design scheme, are fabricated and used to conduct normal-incidence sound absorption measurements in an impedance tube. In order to quantify the fabrication tolerance, the measured acoustic data are used to estimate the MPP parameters of the constructed absorber, using an inverse Bayesian inference method. This paper will discuss the initial MPP design, experimental validations of the designed absorption performance, as well as fabrication tolerance estimations of the MPP parameters.

Deep subwavelength acoustic metamaterial for low-frequency sound absorption

A novel class of low-frequency sound absorbers based on a honeycomb sandwich panel is theoretically designed and numerically demonstrated. The absorber with a remarkably small thickness (e.g., 1/131 of wavelength) is comprised of a perforated facesheet, a perforated honeycomb core and a non-perforated back panel. Built upon the classical microperforated panel absorber (MPPA), the idea of introducing a perforated honeycomb core which creates a double-layer perforated absorber (DLPA) without adding to the total thickness greatly enhances the low-frequency absorption performance. Theoretical predictions of the sound absorption coefficient are obtained and compared with numerical simulations obtained using the finite element method (FEM). A good agreement is achieved. The proposed sound absorber is promising for low-frequency noise absorption especially when limited space and high mechanical stiffness/strength are simultaneously demanded.

Preliminary Study of Acoustical Treatment in a Scaled Model Mosque

This paper presents the preliminary study of acoustical treatment in a scaled model of a mosque. The scaled model was constructed with similar use of reinforced concrete wall and dome without acoustical treatment as in the real mosque. This has caused poor acoustic quality especially with high reverberation times (RT). Two different acoustical treatments were applied on the scaled model, i.e. using kenaf fibres and micro perforated panel (MPP) to investigate their effects on the acoustics of the scaled mosque. Experimental measurement was conducted to measure the RT before and after the treatment. It was found that introducing the kenaf fibres and the MPP absorbers had successfully reduced the reverberation time mostly at frequencies above 1 kHz.

Pilot study on wideband sound absorber obtained by adopting a serial-parallel coupling manner

Micro-perforated panel (MPP) absorber has been widely used in noise control and is regarded as a promising alternative to the traditional porous materials. However, the absorption bandwidth of single MPP absorbers is always insufficient to compete with the porous materials. In this paper, aiming at obtaining high absorption over broad frequency band, a compound MPP absorber array adopting the series-parallel coupling manner is proposed and investigated. Firstly, a theoretical model for the normal absorption coefficient of the series-parallel coupled MPP absorber is established based on the electrical circuit analysis. And then measurements of the normal incidence absorption coefficients are carried out to verify the theoretical predictions. The experimental data are in agreement with the theoretical calculation and prove that the composite structure has the characteristics of broadening the absorption bandwidth through a combination of serial and parallel coupling, which may provide a new technique to improve the absorption performance of such type of MPP absorbers.

Acoustic properties of multilayer sound absorbers with a 3D printed micro-perforated panel

This paper investigates the acoustic absorption capability of a multilayer micro-perforated panel absorber (MPPA) whose front layer is produced by additive manufacturing. The MPPA layers are printed using a polymer material, where the different hole spacing is used to create different perforation ratios. The sound absorption coefficients are experimentally measured by using an impedance tube test, to investigate the effects of the perforation ratio and the depth of an airgap behind the MPPA. Also, a porous sound absorbing material layer is attached behind the MPPA layer in order to produce a multilayer acoustic absorber. The measurement results are compared to a theoretical approach. The comparisons show that the measured sound absorption coefficients agree fairly well with the theoretical model. The use of a porous sound absorbing material behind the MPPA broadens the frequency bandwith of the multilayer acoustic absorber. The frequency of the maximum sound absorption coefficient can be varied, by altering the perforation ratio of the MPPA and/or the depth of the airgap behind the panel.

Improved sound absorption performance of three-dimensional MPP space sound absorbers by filling with porous materials

Because microperforated panels (MPPs), which can be made from various materials, provide wide-band sound absorption, they are recognized as one of the next-generation absorption materials. Although MPPs are typically placed in front of rigid walls, MPP space sound absorbers without a backing structure, including three-dimensional cylindrical MPP space absorbers (CMSAs) and rectangular MPP space absorbers (RMSAs), are proposed to extend their design flexibility and easy-to-use properties. On the other hand, improving the absorption performance by filling the back cavity of typical MPP absorbers with porous materials has been shown theoretically, and three-dimensional MPP space absorbers should display similar improvements. Herein the effects of porous materials inserted into the cavities of CMSAs and RMSAs are experimentally investigated and a numerical prediction method using the two-dimensional boundary element method is proposed. Consequently, CMSAs and RMSAs with improved absorption performances are illustrated based on the experimental results, and the applicability of the proposed prediction method as a design tool is confirmed by comparing the experimental and numerical results.