Micro-perforated Panel Absorber Research Articles

Design of an adjustable sound-absorbing structure with low frequency advantages

The thickness of microperforated panel absorbers with low-frequency sound absorption ability is often large and the absorption bandwidth is limited. To address this issue, a adjustable microperforated panel structure with low-frequency advantage (ASWLF) was designed. Compared to conventional structures, the introduction of inclined chambers allows the structure to absorb lower-frequency noise at the same thickness, enhancing spatial utilization efficiency. The addition of a rotating structure design broadens the frequency range over which the structure can operate effectively. The addition of a rotating structure design broadens the frequency range of the structure's action. The acoustic absorption coefficient of the model can be calculated using acoustic-electric analogy, which is used to predict the acoustic absorption coefficient of the ASWLF structure. The experimental results show that, with the same acoustic absorption performance, the thickness of the structure is reduced by 36.7%. The multi-stage combination allows the structure to achieve better acoustic absorption performance in a wider frequency band. By introducing the PSO optimization algorithm, the optimized ASWLF structure further improves the acoustic absorption frequency band by 32.9%. This study provides an effective method for solving noise environments under different conditions in industrial practical applications.

Random incidence sound absorption of micro-perforated panel absorbers backed with parallel-arranged cavities of different depths

This study investigates sound absorption performance of a kind of wideband micro-perforated panel (MPP) absorbers with parallel-arranged backing cavities at different depths in a diffuse field. An analytical prediction model is presented for evaluating the random incidence absorption coefficient of this type of absorbers, and it exhibits excellent agreement with finite element model and experimental measurements. Then, numerical cases were established using this model to investigate the absorption characteristics of this type of absorbers under random incidence, considering the influence of cavity depths and MPP parameters. Results show that the cavity depth and MPP perforation diameter in this absorber are primary influencing parameters controlling the overall sound absorption performance. Cavities at different depths introduce multiple resonant absorption peaks, while the perforation diameter determines matching conditions between the MPP and air impedance, which simultaneously broaden the absorption bandwidth of this MPP absorber. Although individually adjusting the panel thickness and perforation rate has a smaller impact compared to the perforation diameter, the interaction between these two parameters has significant influences across the entire absorption frequency band.

Investigation on the band narrowing and shifting effects of micro-perforated panel absorbers.

Micro-perforated panel (MPP) absorbers exhibit multiple resonance bands with increased bandwidth narrowing and shifting in higher frequencies, limiting their effectiveness. This study investigates the effects of narrowing and shifting in higher-order resonance bands of MPP absorbers. First, an acoustic impedance model for MPP absorbers is introduced, and the narrowing and shifting coefficients are defined and modeled to quantify these effects. It is observed that a larger ratio of acoustic resistance to acoustic mass is favorable for reducing the narrowing and shifting effects. Subsequently, the theoretical model is validated using a numerical model, and a parametric study is conducted to explore the influence of geometric parameters on the narrowing and shifting effects. The study reveals that decreasing aperture and panel thickness, while increasing perforation ratio and cavity depth, reduces the narrowing and shifting coefficients. Remarkably, ultra-micro-perforated panels (UMPPs) with an aperture below 0.1 mm and perforation constant below 0.0046, having relatively larger acoustic resistance and smaller acoustic mass, demonstrate near-zero band narrowing and shifting. Finally, UMPPs are fabricated using micro-electro-mechanical systems (MEMS) technology, and their normal absorption coefficients are measured. Results align with theoretical predictions, confirming UMPPs' ability to achieve zero narrowing and shifting compared to ordinary MPPs and verifying the study's findings.

The sound absorption properties of a double-leaf micro perforated panel (DL-MPP) made from a double-layered polypropylene material.

Micro-perforated panel sound absorbers are recognized as an alternative method for noise control, offering advantages over conventional porous absorbers. However, the sound absorption of a single micro-perforated panel (MPP) is limited in its ability to cover a broad range of frequencies, especially in the low-frequency region. Therefore, the primary focus of this study is to investigate the sound absorption performance of a double-leaf micro-perforated panel (DL-MPP) absorber, which aims to achieve high sound absorption across a wide frequency range. This study examines the impact of varying parameters, including perforation ratio, diameter of perforated holes, and air gap thickness, on the sound absorption characteristics of a polypropylene-based DL-MPP absorber. Polypropylene was used as the sound absorption material for this investigation. The DL-MPP absorber consists of two perforated panels installed parallel to a rigid back wall, with an air gap between them. To assess the sound absorption performance, a prototype of the DL-MPP absorber was fabricated and tested using a two-microphone impedance tube method following ASTM E1050 standard. The results demonstrate that the optimal sound absorption performance was achieved by designing the MPPs with a 1% perforation ratio, 1 mm diameter of perforated holes, and a 1 mm sample thickness.

Sound absorption properties of the metamaterial curved microperforated panel

This paper proposes a metamaterial that utilizes resonators to enhance the sound absorption capacity of the curved microperforated panel (CMPP), and reliable theoretical analyses and experimental verification are comprehensively given. A metamaterial curved microperforated panel (MCMPP) absorber is formed by attaching local resonators on the CMPP. The displacements of the midplane of the curved panel are expanded into the improved Fourier series based on Spectral-Geometry Method (SGM), and the vibration equations of the MCMPP are derived by the Rayleigh-Ritz method. The sound absorption characteristics of the MCMPP are solved by the effective medium method and the acoustic-electric analogy method. Both a structure-acoustic coupling simulation model and an impedance tube test system are established to validate the accuracy of the present theoretical model. The essential parametric analyses are carried out in conjunction with mode splitting and acoustic-vibrational coupling, and the results show that the local resonance absorption peak within 1000 Hz increases from 1 to 5 with the absorption coefficients greater than 0.8 at these peaks under the installation options of the periodic gradient array and multiple resonances, realizing the effective absorption in multiple frequency bands. This paper provides theoretical support and experimental protocols for the design and application of the metamaterial curved microperforated panel.

Acoustic properties of natural fiber reinforced composite micro-perforated panel (NFRC-MPP) made from cork fiber and polylactic acid (PLA) using 3D printing

The present study investigated the acoustic performance of biodegradable MPP absorbers made of natural fiber-reinforced composites (NFRC) using 3D printing. The novelty of this current research lies in the recent development of a methodology that aids industry professionals in optimizing the production of MPP (Micro-Perforated Panel) at a competitive cost. This is achieved by addressing and eliminating various issues commonly faced in traditional manufacturing processes, such as manual preparation and pressing. The FDM technique was used to fabricate test samples utilizing the PLA/corkwood composite. Using an impedance tube device with two microphones, the acoustic absorption coefficients of MPPs with different perforation diameters, thicknesses, and perforation rates were measured. Maa's analytical model was used to predict the acoustic absorption performance. Moreover, considering the average sound absorption and total cost of fabricating the samples, RSM-CCD was employed to optimize these samples. In the end, the parallel arrangement of MPP double layer and the combination of MPP with kenaf porous material were tested in order to improve the sound absorption performance. The results showed that the average sound absorption coefficient of the NFRC-MPP sound absorber is 25 % more than that of conventional MPP sound absorbers. The sample with a perforation diameter of 0.70 mm, a panel thickness of 0.90 mm, and an 8 mm distance between the perforations was selected as the optimal sound absorber. The measurement and model data for NFRC-MPP panels do not correspond well. The parallel arrangement of two layers of MPP and the addition of an optimized kenaf layer behind the MPP significantly improved the sound absorption performance in the intended frequency range. The findings of this study, coupled with data available in the literature for other types of biocomposite Micro-Perforated Panel (MPP), strongly indicate that Cork fiber-based MPP exhibits substantial promise for application, either independently or in conjunction with Kenaf materials, as a material for acoustic conditioning. Implementing smart manufacturing techniques for acoustic panels not only enhances engineering noise control efforts but also amplifies the overall effectiveness of Hearing Conservation Programs.

Low-Frequency Wideband Sound Absorption Properties of Composite Layer Micro-perforated Panel Absorber

Low-Frequency Wideband Sound Absorption Properties of Composite Layer Micro-perforated Panel Absorber

Ultra-broadband sound absorption in a compact multi-chamber micro-perforated panel absorber with varying depths

This study presents an enhanced multi-chamber micro-perforated panel absorber (MC-MPPA) with varying sub-chamber depths, offering ultra-broadband low-frequency sound absorption. Traditional micro-perforated panel absorbers are constrained by a limited bandwidth, necessitating impossibly small perforations for optimal low-frequency absorption. Our innovative design addresses these constraints with a lightweight, compact panel structure that uses varied chamber depths and unique porosities. Using the two-point impedance method from graph theory, an MC-MPPA was modeled and optimized. Notably, our MC-MPPA test pieces achieved impressive sound absorption coefficients experimentally of over 0.8 in the whole frequency ranges of [397–1000] and [698–1895] Hz. The absorber’s thickness is a mere 47 mm, equivalent to 1/18.2 and 1/10.5 of the sound wavelength at the minimum operational frequency, respectively. Theoretically, with a maximum sub-chamber depth of just 20 mm, average absorption coefficient values of 0.6780 and 0.6483 were observed in [200–3000] and [200–4000] Hz ranges, respectively. Our optimization algorithm permits the definition of practical geometric parameters, promising substantial industrial benefits. The results have been validated theoretically, numerically, and experimentally.

Sound radiation control of an unbaffled long enclosure using wavy micro-perforated panel absorbers

The sound radiation control of an unbaffled long enclosure using wavy micro-perforated panel absorbers (WMPPAs) was numerically and experimentally investigated. First, a hybrid model based on the finite element method (FEM) and the Wiener-Hopf (W-H) technique was established to calculate the sound pressure level (SPL) radiated from an unbaffled long enclosure lined with MPPAs. Then, the hybrid model was validated, and the sound suppression performance of a single WMPPA was evaluated. The results demonstrate that the WMPPA exhibits better sound absorption properties than the flat and corrugated micro-perforated panel absorber (FMPPA and CMPPA), especially in the middle- to high-frequency range. To further enhance the sound attenuation performance, an array of WMPPAs was subsequently proposed to absorb higher-order acoustic modes inside the long enclosure so that broadband-radiated noise could be suppressed. The comparison results demonstrate that the WMPPAs can reduce the radiated SPL within a wide frequency range which is favourable for the suppression of environmental noise. In addition, the WMPPA array shifts the sound reduction curve towards a low-frequency range by approximately 500 Hz compared with that of a single WMPPA, which is promising for the reduction of broadband noise. After that, a mechanistic study was conducted to explain the sound reduction performance of WMPPAs from the perspective of modal contributions. In addition, the diffraction effect of sound waves at the sharp edge of a long enclosure was illustrated. Besides, the contribution of the wavy structure's scattering effect to the sound reduction performance of WMPPAs was analysed. Finally, the sound absorption abilities of the WMPPAs were examined using a quasi-two-dimensional (quasi-2D) experiment. The results show that the WMPPAs exhibit good sound reduction performance in the middle- to high-frequency range, which verifies the feasibility of applying WMPPAs to suppress the broadband noise radiated from long enclosures.

A parallel piezoelectric micro-perforated panel absorber with flexible acoustic characteristics

In this paper, the sound absorption characteristics of a parallel micro-perforated panel absorber (MPPA) fabricated from polyvinylidene fluoride (PVDF) piezoelectric film are studied under an alternating voltage excitation. The simulation and experimental results show that when a certain frequency of AC voltage is applied to the parallel micro-perforated panel, the sound absorption characteristics of the MPPA at the excitation frequency can be improved due to the electrically induced vibration. With an increase in the alternating voltage amplitude, the improvement in sound absorption characteristics is more obvious. Therefore, the sound absorption coefficient of parallel PVDF-MPPA in the target frequency band can be improved by adjusting the parameters of excitation voltage reasonably. Based on the convenience of voltage regulation, this method is very suitable for suppressing the noise of the main frequency fluctuation without changing the structural parameters of the absorber. More importantly, the parallel PVDF-MPPA can apply voltage excitations of different parameters simultaneously, which is beneficial to improve the sound absorption effect in a wider frequency band. This study may provide a reference for the design of intelligent absorbers, especially for noise reduction structures in narrow spaces.

Optimization of micro-perforated panel absorber backed with parallel-arranged cavities of different depths

Micro-perforated panel absorber (MPA) is one of the most effective materials in noise control. For the MPA backed with parallel-arranged cavities of different depths (PCD-MPA), the panel thickness, perforation diameter, perforation rate, and cavity depth are essential parameters for absorption performance. In this study, a Genetic Algorithm (GA)-based optimization design method for PCD-MPA is proposed to achieve specific performance objectives. Firstly, a prediction model for absorption performance of PCD-MPA is proposed. Secondly, a GA optimization method suitable for PCD-MPA is established, taking into account interactive parameters and generating multi-objective and high-quality optimal solutions through iterative adjustments. Finally, an acoustic standard sample was fabricated for experimental validation and compared with analytical results. The findings demonstrate that the proposed optimization method can achieve nearly perfect absorption with a well-selected combination of parameters that meet specific absorption requirements.

Uncertainties and inaccuracies of micro-perforated and micro-slit panel absorbers in multiple layers

Multilayer micro-perforated/slit panels (MPP/MSP) are used to achieve broadband absorption in Architectural Acoustics. Both micro-perforated and micro-slit panels can achieve a high absorption coefficient, but the frequency bandwidth is usually narrow for single-layer panels. For this reason, the multilayer design is proposed to achieve a broadband high absorption coefficient. This work applies Bayesian inference to the design of multilayered micro-perforated/-slit panels for a given design scheme. A higher level of inference, Bayesian model selection, estimates the parsimonious number of layers needed to achieve the design scheme. The lower level of inference estimates MPP/MSP parameters once the structure of the multilayer design is chosen. Once the exact parameters and the model are given, the theoretical absorption coefficient is compared with the measurement results. The causal inference is further applied to analyze the possible reasons for the deviations between the model and the experimental data. Applying the Bayesian framework and causal inference minimizes the uncertainties and inaccuracies during the manufacture and leads to a satisfactory design.

Acoustics performance of compact micro-perforated panel absorber with grazing flow

This paper presents numerical and experimental investigations into the acoustic behaviour of a micro-perforated panel (MPP) backed by a shallow cavity under a fully developed grazing flow. A three-dimensional time-domain computational fluid dynamics approach is applied alongside hybrid evaluation methods to accurately capture the acoustic behaviour variations of an MPP orifice. Meanwhile, a hybrid Rayleigh's end correction model for T-shaped resonators is utilized to calculate the acoustic reactance of a very shallow cavity and an acoustics impedance model of an orifice backing with shallow cavity at grazing flow is established. The proposed acoustic impedance model of orifice has been validated by experimental study. The practical significance of this study is that it elucidates the influence of cavities upon the acoustic behaviour of MPPs in the presence of grazing flows; furthermore, it proposes an impedance model that is more suitable for predicting the acoustic performance of booming compact meta-structures.

Improvement of micro-perforated panel absorbers sound performance using resistive screens and sensitivity analysis of the composite absorber models

The sound attenuation performance of a composite absorber constituted by a micro- perforated panel, a resistive screen that is inserted into the air cavity and a rigid back plate, is studied. Using a nonlinear acoustic impedance model, the surface impedance of the absorber is determined by transfer matrix method, and the models are validated by comparison with experimental measurements performed at different sound pressure levels up to 150 dB. The contribution of the resistive screens in improving the acoustic performances of micro-perforated panel absorbers is presented. It is shown that the resistive screen increases the resistance of the absorber resulting in broadening of the absorption frequency band and improvement of the absorption coefficient when an appropriate perforation ratio of the panel is used. Sensitivity analysis is performed at higher pressure excitations, and the dimensionless input parameters are the perforation ratio of the panel, the ratio of the perforation diameter by the thickness of the panel, the ratio of the cavity depth by the thickness, the orifice Mach number and the resistance per unit area of the screen. The analysis shows the impact of the input parameters on the normalized surface impedance and the sound absorption coefficient of the absorber. It is demonstrated that the resistance per unit area of the screen influences strongly the acoustic properties of the absorber in the linear regime, while the perforation ratio of the panel and the orifice Mach number are the dominant parameters which control the acoustic behavior of the absorber in the nonlinear regime.

Sound absorption characteristics of a compact meta-structure under grazing flow

This study proposes a compact meta-structure for broadband and low-frequency sound absorption under grazing flow. It comprises a microperforated panel (MPP) and resonators with embedded tunable orifices and C-shaped cavities. The sound absorption characteristics of the proposed absorber are investigated via theoretical model (acoustic impedance circuit) and numerical model (finite element method). Consistent predicted results from both models manifest that the proposed meta-structure possesses good absorption capacity under a deep-subwavelength scale, compared to the conventional MPP absorber. To simultaneously achieve better low-frequency and broadband noise damping, a parallel configuration of the meta-structure and an MPP absorber is studied numerically and experimentally. Besides, notable variation on the acoustic performance of the proposed absorber is observed in case of grazing flow, which leads to the decline of the absorption peak and makes the peak wider than that without flow. The practical significance of this study is that it proposes a promising meta-structure for wide frequency sound attenuation applications; furthermore, it elucidates the flow effects on the acoustic performance of the meta-structure, providing a route to reduce flow-related noise in a broad frequency range.

From Micro-Perforates to Micro-Capillary Absorbers: Analysis of Their Broadband Absorption Performance through Modeling and Experiments

A challenging issue is currently the design of non-fibrous ultra-thin acoustic absorbers that are able to provide broadband performance in demanding environments. The objective of this study is to compare using simulations and measurements the broadband absorption performance of highly porous micro-capillary plates (MCPs) to that of micro-perforated panels (MPPs) under normal incidence while considering unbacked or backed configurations. MCPs are unusual materials used for sound absorption with micron-sized channels and a high perforation ratio. Impedance-based modeling and Kundt tube experiments show that MCPs with suitable channel diameters have a pure constant resistance that outperforms the acoustic efficiency of MPP absorbers. Unbacked MCPs exhibit a controllable amount of high absorption that can exceed 0.8 over more than five octaves starting from 80 Hz, thereby achieving a highly sub-wavelength absorber. MCPs still provide broadband high absorption when backed by a rigid cavity. Their bandwidth-to-thickness ratio increases toward its causal limit when the cavity depth decreases. A parallel MCP resonant absorber partly backed by closed and open cavities is proposed. Such MCP-based absorbers could serve as short anechoic terminations for the characterization of acoustic materials at low frequencies.

Acoustic behaviour of micro-perforated panel backed by shallow cavity under fully developed grazing flow

This paper presents numerical and experimental investigations into the acoustic behaviour of a micro-perforated panel (MPP) backed by a shallow cavity under a fully developed grazing flow. A three-dimensional time-domain computational fluid dynamics approach is applied alongside hybrid evaluation methods to accurately capture the acoustic behaviour variations of an MPP orifice, considering the interaction of flow dynamics and the acoustics in the orifice and cavity. Meanwhile, a hybrid Rayleigh's end correction model for T-shaped resonators is utilized to calculate the acoustic reactance of a very shallow cavity. The numerical results show that the vertical shear layer is observed alongside the hole and cavity and impinges the bottom of shallow cavity. Subsequently it alters acoustic and flow interaction significantly that leads to variation of acoustic behaviour of orifice. In this regard, an acoustic impedance model of an orifice backing with shallow cavity at grazing flow is established. The proposed acoustic impedance model of orifice has been validated by experimental study. The transmission loss predictions for a compact MPP absorber with single or double cavities agree well with the experimental results for the grazing flow condition. The practical significance of this study is that it elucidates the influence of cavities upon the acoustic behaviour of MPPs in the presence of grazing flow; furthermore, it proposes an impedance model that is more suitable for predicting the acoustic performance of booming compact meta-structures.

On Woven Fabric Sound Absorption Prediction

For building applications, woven fabrics have been widely used as finishing elements of room interior but not in particular aimed for sound absorbers. Considering the micro perforation of the woven fabrics, they should have potential to be used as micro-perforated panel (MPP) absorbers; some measurement results indicated such absorption ability. Hence, it is of importance to have a sound absorption model of the woven fabrics to enable us predicting their sound absorption characteristic that is beneficial in engineering design phase. Treating the woven fabric as a rigid frame, a fluid equivalent model is employed based on the formulation of Johnson-Champoux-Allard (JCA). The model obtained is then validated by measurement results where three kinds of commercially available woven fabrics are evaluated by considering their perforation properties. It is found that the model can reasonably predict their sound absorption coefficients. However, the presence of perturbations in pores give rise to inaccuracy of resistive component of the predicted surface impedance. The use of measured static flow resistive and corrected viscous length in the calculations are useful to cope with such a situation. Otherwise, the use of an optimized simple model as a function of flow resistivity is also applicable for this case.

Investigation on low frequency acoustic characteristics of parallel-arranged microperforated panel with aerogel-filled back cavities

Parallel-arranged microperforated panel (PMPP) absorber has an improved sound absorption performance compared with the single microperforated panel (MPP) absorber. However, its low frequency sound absorption depends on the use of some bulky structures. Therefore, we present a PMPP absorber that can achieve above 80% continuous sound absorption within 430–800 Hz with a thickness of only 3.3 cm. The PMPP absorber is composed of five different MPP members, each of which contains a perforated panel and a back cavity partially filled with different aerogel (AG). The resonance frequency of each MPP member shifts to lower frequencies because of the increased effective cavity depths, which can be adjusted by changing the acoustic impedance of the AG. The analytical model of the PMPP absorber is also proposed, and good agreement is observed between the predicted results and the experimental results. This finding and proposed method may provide new design guidance for the practical application of the PMPP absorber.

Investigation on Minute Holes of Woven Fabrics for Wide-Band Micro-Perforated Sound Absorbers

Woven fabric perforation is helpful to be adopted to meet the microstructure requirement of a micro-perforated panel (MPP) absorber. Unlike conventional MPP, the woven fabric micro-perforations are formed by yarn in the x (weft) and y (warp)-directions. Hence, minute holes of the MPP with a diameter of 0.1–0.3 mm or a high perforation ratio are expected to be easily fabricated, while such a specification is difficult to realize on a solid surface, as found in some studies. The study presented here focuses on the use of minute holes in MPP absorbers by woven fabrics and discusses the minute hole properties of woven fabrics and their associated absorption characteristics. Theoretical results by Maa’s model are also used to validate resulting characteristics found from the experimental investigations. It is found that minute holes with around 0.10–0.20 mm diameter have been successfully fabricated by controlling weft yarn density. The woven fabrics are capable of producing half-absorption bandwidth of up to 5000 Hz (>3 octaves), while the peak of the absorption coefficient can be more than 0.8. In addition, varying hole diameter with the order of 10−2 mm can change the absorption behavior for both peak absorption and absorption bandwidth. Such behavior is confirmed by comparing the results with the theoretical model. This study also indicates that Maa’s model is still applicable for predicting absorption of MPP developed based on woven fabric material.