Micro-perforated Panel Research Articles

Multi-chamber micro-perforated panel absorbers optimised for high amplitude broadband absorption using a two-point impedance method

An optimised, multi-chamber, micro-perforated panel absorber (MC-MPPA) with micro-perforated adjoining panels is presented in this paper. An MPPA is a clean and efficient noise absorber with a simple structure making it an increasingly popular choice over porous material-based solutions. For the basic configuration, the bandwidth is typically limited by the perforation size and the frequency range by the backing cavity depth. In order to improve these parameters, innovative variations of the MPPA have been proposed in the literature, but the development of a solution which delivers both a broadband frequency response at low frequencies, with a shallow cavity has yet to appear. This paper proposes such a technological breakthrough by employing a graph-theory-based method to the MPPA problem. By using the two-point impedance method (TpIM) based on graph theory, a model for multi-chamber MPPAs which are coupled with each other in two dimensions through perforated side panels can be developed, an onerous task for standard equivalent circuit analysis due to the generation of non-planar circuits. The resulting model lends itself to optimisation and thus allows the differing geometric parameters of a tessellated network of chambers to be combined to maximise the sound absorption in a frequency range for a particular depth. An overall absorption coefficient of 0.83 can be achieved experimentally in the frequency range of 660 Hz to 2 kHz with an MC-MPPA of only 22 mm thick. In addition, when the air cavity depth is slightly increased to 50 mm, an overall absorption coefficient of greater than 0.82 can be achieved in the 281 Hz to 1 kHz frequency range. Experimentally, an absorption coefficient of approximately 0.82 is achieved at 340 Hz at an air cavity depth of 50 mm, which is a depth-to-wavelength ratio of 20, making the MC-MPPA a deeply subwavelength absorber. Results have been verified theoretically, numerically and experimentally.

Customizable Acoustic Metamaterial Barrier with Intelligent Sound Insulation

Conventional acoustic metamaterial barriers are usually used to realize sound insulation for a continuous broad bandwidth. In some practical scenes, however, the noise signal from complex noise sources, such as gearboxes and motors, are usually complicated spectra with discrete peaks or valleys, instead of perfect continuous ones. In this case, the broadband design is unnecessary and may increase the thickness of metamaterials if too many frequency components are included. We need only a customized spectrum to achieve sound insulation at desired discrete frequencies, while, at other ones, insulation can be unwanted to preserve an effective signal other than noise. Here, we propose a customizable acoustic metamaterial barrier (CAMB) with intelligent and selective sound insulation. The CAMB is a composite structure consisting of a Helmholtz resonator (HR) and a microperforated panel (MPP) destined to control sound insulation at low frequencies, within 200--1000 Hz, and high-frequencies, within 1000--3000 Hz, respectively. An inverse process is proposed for customizable design. We use analytical and numerical methods to obtain the design libraries for the parameters of the HRs and MPPs. We numerically and experimentally demonstrate an example of a CAMB customized for a practical complex noise source. The sample has a thickness of 48 mm, approximately \ensuremath{\lambda}/11 for the lowest targeted frequency. The proposed customizable concept may pave the way for acoustic insulating metamaterials for complex sound sources with different frequency components.

Evaluation of acoustic performance of cascaded cylindrical micro-perforated panel

A cylindrical micro-perforated panel (MPP) can be used to absorb the sound of a flow system in a circular duct of a vacuum cleaner. A cascaded cylindrical MPP is a special class type where two cylindrical MPPs are arranged in a series to improve sound attenuation. The manufacturing of MPP primarily involves the machining of micro-perforations, because small holes are not readily made using injection moulding due to the complexity of the die, flow control of the molten polymer through the small orifices and dimensional stability, making it unsuitable for mass production. This limitation can be overcome with the use of additive manufacturing (AM) technology, where the micro-perforations can be designed and manufactured, with relatively larger tolerances. Experimental validation ensures that the manufactured prototype in this study is performed according to design. Results show that the transmission loss of the model and the experimental outcomes agree. The cascaded arrangement of the cylindrical MPP results in a wider effective frequency range and an increased transmission loss. Parametric studies of the combined effects of the perforation diameter, perforation ratio and the depth of air cavity on the diameter of the duct and length ratio are conducted using a transfer matrix method. A case study is demonstrated here in the design. Moreover, an AM of cascaded cylindrical MPP is performed to attenuate peak noise at 1650 Hz, where the optimum parameters of the cascaded cylindrical MPP are obtained using a genetic algorithm. The manufactured cascaded cylindrical MPP is installed on a vacuum cleaner duct, and the measurement of sound power level shows a reduction of 4 dB(A).

Passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber on oblique incidence condition

This paper presents a comprehensive investigation on the passive and active low frequency sound absorption performance of the finite and large sized micro-perforated panel absorber (FLS-MPPA) for oblique incidence excitation (OIE), which is of great significance for the engineering implementation of this technic. The theoretical model of the FLS-MPPA is firstly established by applying the modal analysis approach. It is also validated by experimental test. Then, the passive sound absorption performance is explored and the underlying physical mechanisms of these peculiar findings are investigated thoroughly. Finally, the active control performance (applying point source to actively control the sound field) and physical mechanism of the sound absorption improvement of the FLS-MPPA are analyzed. Results obtained demonstrate some key findings. The high order cavity modes excited by OIE reflect most of the incident sound power and have little contribution to the sound absorption in the low frequency range owing to their strong mutual coupling effects and being greatly excited with high modal amplitudes. They also cannot produce additional sound absorption peaks due to their wide resonant frequency band. Weakening such mutual coupling effects can improve the sound absorption and produce additional absorption peaks. The low frequency sound absorption of the FLS-MPPA is significantly improved with control and the achievable maximum sound absorption coefficient is slightly greater than the difference between 1 and the sound power reflection coefficient of the MPP, which validates the feasibility of the active control method for OIE. The reflected sound power induced by the cavity modes is highly reduced and part of the reflected sound power produced by the MPP can even be absorbed by these modes when their amplitudes are reduced to the optimal value, which is the physical nature of the active control. Sensing and controlling the cavity sound field in the corner achieves almost equal sound absorption improvement as that of the optimal control state, based on which the error sensing strategy is constructed.

Hybrid fractal acoustic metamaterials for low-frequency sound absorber based on cross mixed micro-perforated panel mounted over the fractals structure cavity

The proposed work enumerates a hybrid thin, deep-subwavelength (2 cm) acoustic metamaterials acting as a completely new type of sound absorber, showing multiple broadband sound absorption effects. Based on the fractal distribution of Helmholtz resonator (HRs) structures, integrated with careful design and construct hybrid cross micro-perforated panel (CMPP) that demonstrate broad banding approximately one-octave low-frequency sound absorption behavior. To determine the sound absorption coefficient of this novel type of metamaterial, the equivalent impedance model for the fractal cavity and the micro-perforated Maa’s model for CMPP are both used. We validate these novel material designs through numerical, theoretical, and experimental data. It is demonstrated that the material design possesses superior sound absorption which is primarily due to the frictional losses of the structure imposed on acoustic wave energy. The peaks of different sound absorption phenomena show tunability by adjusting the geometric parameters of the fractal structures like cavity thickness ‘t’, cross perforation diameter of micro perforated panel, etc. The fractal structures and their perforation panel are optimized dimensionally for maximum broadband sound absorption which is estimated numerically. This new kind of fractals cavity integrated with CMPP acoustic metamaterial has many applications as in multiple functional materials with broad-band absorption behavior etc.

Sound absorption characteristics of thin parallel microperforated panel (MPP) for random incidence field

In this study, the absorption coefficient of a thin parallel MPP for random incidence field is investigated. A coiled backing air cavity realizes the thin structure, and the parallel configuration is applied to have a wider sound absorption bandwidth than a single MPP. The effects of the oblique sound incidence on the thin parallel MPP are investigated by numerical studies for both single oblique incidence and random incidence. These effects are significant from a practical standpoint, whereas the result of a coiled backing air cavity must be carefully evaluated. This paper focuses on developing, modeling, and evaluating parallel MPP with a coiled backing cavity. It is found that the oblique incidence can result in lower sound absorptions and shift in resonance absorptions compared to the normal incidence angle, especially for high incidence angles (>70°). In addition, the performance of the thin MPP at a normal incidence angle can be maintained up to the incident angle of 70°, where the sound absorption coefficients of 0.8 and above are present. Compared with a straight backing cavity, the use of coiled one does not introduce deviation in sound absorption characteristics except for the cavity modes slightly higher resonance frequency under the same axial length. Moreover, the proposed structure behaves as a locally reacting material as long as the cavity’s longest dimension is smaller than half an operating wavelength for the targeted frequency range. This allows the sound absorption characteristics of normal incidence and diffuse sound field excitation is insignificantly different. Hence, this characteristic can deal with two folded problems in terms of bulky absorber issue and sensitive performance on oblique incidence.

Extremely effective broadband low-frequency sound absorption with inhomogeneous micro-perforated panel (iMPP) backed with spider-web designed cavities

Low-frequency wideband noise reduction has posed a significant problem to the scientific and technical communities in recent years. A single layer of a parallel-arranged inhomogeneous micro-perforated panel (iMPP) coupled with spider-web designed cavities is offered as a composite acoustic structure in this paper. Three different spider-web shapes have been designed and studied, i.e. circular, octagonal, and square. By controlling the different inhomogeneous patterns, perforation ratio, the thickness of iMPP, and back cavity depths, a broader multipeak low-frequency sound-absorbing performance equivalent to different resonant frequencies can be achieved. To anticipate the sound absorption coefficient of the new design, both theoretical analysis and finite-element method (FEM) simulation are executed. The predicted and FEM simulation sound absorption results of the new composite structure are verified in the experimental investigation using a square-designed sound impedance tube. By a subwavelength thickness of just 100 mm, a highly effective low-frequency broadband sound-absorbing composite structure is successfully attained by integrating many inhomogeneous MPP unit cells supported with spider-web-designed cavities. The average sound absorption coefficient is over 90% ( α = 0.94) within the bandgap of 230 Hz to 470 Hz. Compared to traditional sound-absorbing materials, the composite structure comprises inhomogeneous MPP coupled with spider-web-designed cavities, which may provide good absorption performance while maintaining a modest and robust construction for active low-frequency noise suppression.

Case study: A type of low-noise power capacitor based on combined microperforated panels

Power capacitors of high-voltage alternating/direct current transmission systems have become one of the main community noise sources because of their large number and because they employ ultrahigh voltage and power. According to the broadband noise characteristics of power capacitors and based on the theory of combined microperforated panels, a low-noise capacitor with broadband sound absorption is designed and machined. Noise comparison tests were conducted between the modified and conventional capacitors in a semi-anechoic chamber. The results indicate that the proposed low-noise capacitor with combined microperforated panels provides a good alternative for low-noise design of power capacitors.

Flow effects of microperforated-panel casing treatments in a contra-rotating fan

The flow effects of microperforated-panel (MPP) casing treatments installed over the front rotor of a small contra-rotating (CR) fan are numerically studied, with emphasis put on the reshaped front-rotor tip leakage flow, the blade response to rotor-rotor interaction, and the unsteadiness of static pressure on blade surfaces. It is found that the secondary flow through the sub-millimeter holes of the MPP hinders the formation of the front-rotor tip leakage flow and reduces its circumferential stretching, resulting in an improved inflow to the rear rotor. Consequently, the unsteady characteristics of static pressure and lift force on the rear rotor are generally attenuated. An enlarged back cavity can enhance such attenuation, in which case a 14.5% decrease in pressure unsteadiness is observed. The first, second, and third harmonics of the unsteady lift force experience reductions of 15.6%, 37.4%, and 60.9%, respectively. However, the unsteady characteristics on the front rotor dominated by potential interaction are not significantly influenced by the MPP. Furthermore, analysis of blade-to-blade phase differences shows that, whether the CR fan is treated with the MPP or not, the phase differences are not precisely repeated but vary randomly and that the higher the order of the force harmonic, the more likely the phase relationship is disturbed by the treatments. The varying nature of the phase relationship would render the acoustic energy of an induced interaction tone leaked into shaft tones and broadband noise. Radiation efficiency analysis of force harmonics demonstrates that the front rotor is not an efficient contributor to interaction noise compared with the rear rotor, which radiates more interaction tones with much higher efficiency. The current work is expected to be useful not only for devising effective MPP casing treatments to control small CR fan noise but also for understanding their underlying flow effects on rotor-rotor interaction.

Analysis of the sound absorption characteristics of the curved microperforated panel with elastically restrained edges

This paper describes the analytical and experimental study on the sound absorption characteristics of the curved microperforated panel (CMPP) with elastically restrained edges backed by an air cavity. The arbitrary boundary conditions are realized by setting the restrained spring at the edges to zero or infinite. The acoustic impedance formula of the CMPP is derived based on the modal superposition method. The acoustic electrical analogy method is used to obtain the sound absorption coefficient of the CMPP, in which the effects of the structural parameters on the sound absorption coefficient such as aspect ratio, radius of curvature, perforation rates, micro holes diameters and panel thicknesses are also studied. To validate the accuracy of the present method, the impedance tube test system is designed to measure the sound absorption coefficient under different micro holes diameters and panel thicknesses. The results derived by the analytical method and the test one match well with each. This paper provides a new kind of sound absorption structures which could be used in the health environment.

Sound absorption and insulation performance of a finite cylindrical micro-perforated panel absorber.

This paper describes the theoretical prediction, simulation research, and experimental verification conducted to understand the potential of finite cylindrical micro-perforated panel absorbers (FC-MPPAs) in noise control. The simplified Rayleigh integral method is extended to predict the dissipation of sound energy in cylindrical micro-perforations with sub-millimeter aperture. The sound absorption and insulation performance of a FC-MPPA and the influence of its structural parameters on its acoustic performance were studied by model simulation and experiment, and the directional distribution of the sound field was revealed. The simulation and experimental results show that the FC-MPPA has unique acoustic characteristics different from the panel-type absorber, and its structural parameters are closely related to the acoustic-energy dissipation efficiency. Moreover, the absorption coefficient of FC-MPPA is a function of the incidence angle and frequency of the sound wave, and its acoustic properties are controlled by the coupling between the micro-perforated panel and the acoustic modes in the annular cavity domain. These studies can provide help in the design and application of FC-MPPA to achieve better noise reduction effect.

Acoustical performance of a wavy micro-perforated panel absorber

In this study, the sound absorption performance of a wavy micro-perforated panel absorber (WMPPA) is investigated. First, a numerical model is established to calculate the sound absorption coefficients of micro-perforated panel absorbers (MPPAs) under normal, oblique, and random incidences. Subsequently, the proposed model is validated using an existing theoretical formula. After that, the acoustical properties of a WMPPA are explored and then compared with those of flat and corrugated micro-perforated panel absorbers (FMPPAs, CMPPAs). It is observed that the normal sound absorption curve of the WMPPA shifts to a lower frequency range compared with those of CMPPA and FMPPA, which is promising for the control of low-frequency noise. Besides, the sound absorption performance of the WMPPA at the dips is considerably improved, which is favourable for the reduction of broadband noise. Modal analysis shows that, apart from the resonant mode, the adjacent non-resonating modes are also excited, which enhances the performance. Moreover, the WMPPA exhibits the properties of a multiple-layered MPPA, and extra sound absorption peaks are observed within the middle- to high-frequency range. In addition, a parametric study is conducted to explore the influence of the corrugation depth and offset distance on the sound absorption performance of the WMPPA. The calculated results demonstrate that the proposed WMPPA is suitable for low-frequency and broadband noise control. Furthermore, a compact WMPPA with a high random sound absorption coefficient is obtained following an optimization scheme. Finally, the sound absorption coefficients of the MPPAs under normal plane-wave incidence are obtained experimentally using an impedance tube. The measurement results agree well with the simulation results, validating the proposed model and verifying some of the numerical findings.

Graphite-oxide hybrid multi-degree of freedom resonator metamaterial for broadband sound absorption

Low frequency broadband sound absorption for thin structures is still a great challenge. A new concept of a stackable hybrid resonator metamaterial is proposed which exhibits super broadband low-frequency sound absorption. The proposed metamaterial is based on micrometric scale thickness Graphene Oxide (GO) embedded in a stacked structure or used as external skin in a designed honeycomb (HC) structure. The stackable nature of the proposed structure allows the GO-HC cores to be embedded within micro-perforated panels (MPP) providing enhanced stiffness/strength to the structure and high absorption characteristics. We demonstrate how the exploitation of the GO elastic and mass properties result in multiple hybrid structural–acoustic resonances. These resonances are tailored to occur in a frequency range of interest by the theoretical calculation of the sound absorption coefficient. The theoretical model combines the mutual interaction between the structural dynamic of the GO foil and acoustic higher modes of the HC core cell as well as stacked MPP-HC/GO-HC cores. The result is a multi-degree of freedom hybrid resonator which provides subwavelength scale broadband sound absorption in low frequency range between 300 and 2500 Hz.

Sound-absorption performance of a coupled square-neck embedded Helmholtz resonator

We propose a coupled square-neck embedded Helmholtz resonator (S-NEHR) with a deep-subwavelength thickness. By precisely designing each cell peak, the average sound–absorption coefficient of the nine coupled S-NEHR structure is greater than 0.9 at 300–410 Hz, and the thickness is only of the working wavelength. Adding a microperforated panel (MPP) in front of the coupled structure enables a smaller size and larger bandwidth (500–1000 Hz). The simulation and experimental results demonstrate that the S-NEHR structure has great application prospects in engineering noise control due to its extraordinary sound–absorption properties and compact structure.

Motor noise reduction of unmanned aerial vehicles

The noise of multi-rotor unmanned aerial vehicles (UAVs) mainly comes from high-speed rotating propellers and brushless direct current motors. With extensive research on the generation mechanism and suppression methods of propeller noise, fruitful results have been achieved on the noise reduction of propellers. Therefore, the noise radiated by UAV motors has received more attention in recent years. In this paper, the noise of the motor of a small quadrotor UAV is reduced by a composite sound-absorbing structure that couples a cylindrical micro-perforated panel absorber (MPPA) with a carbon fiber porous material. The cylindrical MPPA adopts an open design to balance its noise reduction performance and heat dissipation performance. To more realistically reproduce the actual working state of the motor, different shapes of motor loads were designed and manufactured to simulate the propeller. The noise reduction experiment of the UAV motor is carried out in a semi-anechoic chamber. The experimental results show that the composite sound-absorbing structure designed in this paper can effectively reduce the noise of UAV motors and has a good heat dissipation effect. In addition, a custom-made composite sound-absorbing structure was installed on a quad-rotor UAV, and field experiments were carried out. The results of field experiments show that the composite sound-absorbing structure designed in this paper has good practicability to absorb the noise of UAV motors, and this work can provide a reference for the design of silent UAVs in the future.

Broadband acoustic meta-liner with metal foam approaching causality-governed minimal thickness

Acoustic absorbers based on resonant cavities or porous materials have been extensively investigated for developing acoustic liners but still suffer from narrow working frequency bands or bulky sizes. Here, we present a meta-liner capable of high-efficiency and broadband sound attenuation via the causality-governed minimal thickness, which utilizes the coherent couplings among the resonant structures and porous materials to enhance the absorption efficiency. The meta-liner consists of a series of parallel-coupled neck-embedded Helmholtz resonators (NEHRs) and a micro-perforated panel (MPP) backed with a metal-foam-filled gap. In constructing the meta-liner, the metal foam plays an essential role in achieving the over-damped condition and the suppression of impedance oscillation, which therefore facilitates the realization of the minimum thickness and the consistently-high absorption avoiding absorption dips. Distinct from the design methods employed in previous studies that take deficient consideration of the effect of high-order acoustic waves, this work introduces the acoustic grating diffraction theory to comprehensively modulate the coupling effects of meta-liners’ components from fundamental and high-order waves, which enables more precise modulation of meta-liners and leads to improve absorption performance in practice. As a proof-of-concept demonstration, we theoretically designed and experimentally validated a meta-liner supporting unanimously high-efficiency sound absorption from 800 Hz to 3200 Hz with a thickness of only 40 mm. Our work enriches the design concepts of acoustic liners and provides an efficient pathway to construct broadband meta-liners against absorption dips via the causality-governed thinnest structures, which may benefit the applications in noise-control engineering and impedance engineering.

Tunable acoustic composite metasurface based porous material for broadband sound absorption

This work designs a tunable acoustic composite metasurface, with porous material and a modified microperforated panel (MPP) system. The broadband sound absorption performance of the composite metasurface is evaluated by the analytical theory, numerical simulation, and experiment in the frequency range between 200 Hz and 3500 Hz. The designed composite metasurface has a significant advantage in sound absorption performance compared with a uniform porous material, especially in the low-frequency range. The excellent sound absorption performance is achieved not only by the wavefront controlling with the phase gradient adjustment but also by the resonance dissipation of the modified MPP system. The high-order reflected waves have been converted into surface waves by the wave controlling ability of the designed metasurface for excellent sound absorption at the high frequency of interest. The modified MPP system is formed by a microperforated panel and a coiling-up space cavity to achieve excellent sound absorption in the low-frequency range. By adjusting the length of the coiling silt in the modified microperforated panel system, the absorption peak can move to the frequency of interest. The compact composite metasurface with 3 cm height has remarkably improved the sound absorption of porous material alone from low to high frequency, which could easily replace porous material for better sound absorption in industry.

An Empirical Model for Optimizing the Sound Absorption of Single Layer MPP Based on Response Surface Methodology

Micro-perforated panel (MPP) is a thin panel absorber capable of absorbing sound energy at a targeted frequency range by adjusting the MPP parameters. An analytical model is available, but it is not a direct, convenient method for practitioners to de

Design and fused filament fabrication of multilayered microchannels for subwavelength and broadband sound absorption

A comprehensive procedure to design and manufacture multilayered microchannels presenting competitive mechanical properties, effective subwavelength and near perfect broadband sound absorption in targeted frequency ranges is presented. The acoustic properties of microchannels are predicted with the Johnson-Champoux-Allard-Lafarge (JCAL) model. The JCAL parameters are calculated with the Two-scale Asymptotic Method (TAM) and the sound absorption coefficient of multilayered microchannels is simulated with the transfer matrix method (TMM). The simplex Nelder-Mead optimization method is used to find the size of the channels and the stacking sequence leading to effective acoustic absorption for three different frequency ranges (500–2400 Hz, 2400–6500 Hz and 500–6500 Hz). 30 mm-thick samples with up to 30-layers of unobstructed and non interconnected channels and microchannels were successfully produced via fused filament fabrication (FFF). The minimum channel size is ~ 100 µm which is very appropriate to produce micro-perforated panels or acoustic liners with optimal absorption and different degrees of freedom. Multilayered microchannels with absorption average up to 0.87 and noise reduction coefficient (NRC) up to 0.49 were produced. The multilayered microchannels offer a good compromise between effective acoustic properties and useful mechanical properties compared to other 3D printed acoustic structures and can be considered as viable candidates for applications where structural resistance is required. • Multilayered microchannels with high broadband sound absorption at three frequency ranges are presented. • Innovative FFF manufacturing technique allows production of 100 μm perforations and structure with up to 30-layer. • 30mm-thick multilayered microchannels present sound absorption average up to 0.87. • PLA 3D printed multilayered microchannels exhibit up to 468MPa compressive modulus and 9MPa compressive yield strength.

Fabrication of light-weighted acoustic absorbers made of natural fiber composites via additive manufacturing

Synthetic fiber is still considered the best sound absorptive material. However, due to the health concern of synthetic fiber usage, researchers are trying to find another viable alternative. A microperforated panel (MPP) is a promising alternative that relies on the concept of a Helmholtz resonator for sound absorption. MPP possessed excellent acoustic resistance and a considerable range of absorption bandwidth. In this paper, MPP made of natural fiber composite was fabricated and its acoustic absorption was measured using a two-microphone impedance tube method as per ISO 10534-2 standard. Later, the tensile strength of the fabricated acoustic absorbers was measured using an Instron Universal Testing Machine as per ASTM D638. The idea of employing additive manufacturing, better known as the 3D printing technique, is proposed to produce lightweight MPP. The 3D printing technique provides design freedom and is less tedious in creating complex and light structures. The 3D printing technique has various important parameters, and infill density is one of the parameters. It was found that the reduction of infill density leads to a decrease of the MPP’s mass and thus, slightly affects the resonance frequency of the MPP, still within the mid-frequency spectrum. It was also noted that the increment of air gap thickness leads to the shifting of MPP’s resonance frequency to a lower frequency range. The tensile strength of the 3D printed samples decreases with a decrease in infill density. A sample with an infill density of 100% has the highest tensile strength of 22 MPa, and a sample with an infill density of 20% has the lowest tensile strength of 12 MPa.