On the acoustic properties of parallel arrangement of multiple micro-perforated panel absorbers with different cavity depths

The acoustic properties of a compound micro-perforated panel (MPP) absorber array are investigated. The absorber array consists of three parallel-arranged MPP absorbers with different cavity depths. A finite element procedure is used to simulate its acoustic behaviors under normal incidence. Experimental studies are carried out to verify the numerical simulations. Due to different reactance matching conditions in the absorber array, strong local resonance occurs and the corresponding local resonance absorption dominates. Compared with single MPP absorber, the absorber array requires lower acoustic resistance for good absorption performance, and the resonance frequencies shift due to inter-resonator interactions. The different acoustic resistance requirement is explained by considering the reduced effective perforation rate of the MPP in the absorber array. The performance of the absorber array varies with the sizes and spatial arrangement of the component absorbers. When the distance between component absorbers is larger than a quarter-wavelength, the above-mentioned parallel absorption mechanism diminishes. In the experimental study, the normal incidence absorption coefficients of a prototype MPP absorber array are tested. The measured results compare well with the numerical predictions. The experimental study also shows that although other absorption mechanisms may exist, dissipation by the MPP is dominant in the MPP absorber array.     

Experimental study on sound absorption performance of microperforated panel with membrane cell

Microperforated panel (MPP) absorbers have been widely used in noise reduction and are regarded as a promising alternative to the traditional porous materials. However, the absorption bandwidth of a single-layer MPP is insufficient to compete with the porous materials. In order to improve the sound absorption ability of the single-layer MPP, a composite MPP sound absorber with membrane cells (MPPM) is introduced. Sound absorption properties of the MPPM are studied by the impedance tube experiment. Results show that the membranes have a significant influence on the sound impedance. The sound absorption performance of MPPM gradually increases with the increase of the membrane area. The single-layer MPP with some small area membrane cells may have the same effect and single large area membranes. By adjusting the size of the membrane cells, one can implement a sound absorber with a wider absorption bandwidth and higher absorption peaks than the single-layer MPP.     

A numerical study of double-leaf microperforated panel absorbers

Microperforated panel (MPP) absorbers are promising as a basis for the next-generation of sound absorbing materials. Typically, they are backed by an air-cavity in front of a rigid wall such as a ceiling or another interior surface of a room. Indeed, to be effective, MPP absorbers require the Helmholtz-type resonance formed with the backing cavity. Towards the creation of an efficient sound-absorbing structure with MPPs alone, the acoustical properties of a structure composed of two parallel MPPs with an air-cavity between them and no rigid backing is studied numerically. In this double-leaf MPP (DLMPP) structure, the rear leaf (i.e., the MPP remote from the incident sound) plays the role of the backing wall in the conventional setting and causes resonance-type absorption. Moreover, since a DLMPP can work efficiently as an absorber for sound incidence from both sides, it can be used efficiently as a space absorber, e.g., as a suspended absorber or as a sound absorbing panel. The sound absorption characteristics of the double-leaf MPP are analysed theoretically for a normally incident plane wave. The effects of various control parameters are discussed through a numerical parametric study. The absorption mechanisms and a possible design principle are discussed also. It is predicted that: (1) that a resonance absorption, similar to that in conventional type MPP absorbers, appears at medium-to-high frequencies and (2) that considerable “additional” absorption can be obtained at low frequencies. This low-frequency absorption is similar to that of a double-leaf permeable membrane and can be an advantage compared with the conventional type of MPP arrangement.     

阻尼耦合双层微穿孔板吸声体研究

提出并研究一种利用两板间微缝进行阻尼耦合的双层微穿孔板(DMPP)吸声体。该吸声体在两层微穿孔板(MPP)之间形成一个宽度小于1mm的微缝,因此其阻尼不仅可由板上的微穿孔提供,还可由两板之间形成的微缝提供。采用声电类比法建立了DMPP转移阻抗的理论模型,并进行实验验证,结果表明理论计算结果与实验吻合较好。然后利用建立的理论模型,对单层MPP和DMPP吸声体的吸声性能进行了对比研究,结果表明,相比于单层MPP,DMPP可以利用微缝提供的阻尼显著改善吸声性能,同时减少实际板厚。最后,对DMPP吸声体的吸声性能及其几何参数的关系进行研究,结果表明,当保持其它结构参数不变时,微缝宽度对DMPP吸声系数的提高存在一个最优值,超过或低于此值会导致吸声系数下降。     

Improvement of sound absorption characteristics under low frequency for micro-perforated panel absorbers using super-aligned carbon nanotube arrays

Super-aligned carbon nanotube (SACNT) arrays are grown on the surface of micro perforated panel (MPP) in the hope of improving the acoustic performance of MPP absorbers by virtue of their unique properties. Scanning electron microscopy reveals that SACNT arrays did not block the perforations of MPPs or changed the perforation diameter due to their “super-aligned” nature, although MPPs are thickened. The absorption effect of SACNT arrays which are of the same and different lengths with different incident side on MPP absorbers are investigated, and standing wave tube method is used to determine the normal sound absorption coefficient. Results show that both of the lengths of SACNT arrays and the incident side have effects on the sound absorption performance of MPP absorbers. And generally SACNT arrays help to improve the sound absorption capacity of MPP absorbers in low-frequency regions only when the SACNT arrays surface is the incident side. SACNT arrays decrease absorption performance of MPP absorbers when the MPP surface is used as the incident side. Moreover, SACNT arrays are found to increase the acoustic ability of MPP absorbers with the same structure parameters monotonically at lengths up to 600 μm in the condition that the SACNT arrays surface is used as the incident side.     

Enhancement of low-frequency sound absorption of microperforated panels by adding a mechanical impedance

In order to solve the bad low frequency sound absorption of the Micro-Perforated panel(MPP)absorber,mechanical impedance was introduced in the back of the MPP absorber to form a composite structure.According to the same particle vibration velocity on both sides of a plate,the mechanical impedance plate transfer matrix could be obtained.The units of the mechanical impedance,cavity and MPP were connected in series with the use of the transfer matrix method,thus creating the composite structure’s theoretical calculation model.The quality factor affecting absorption bandwidth was analyzed.Bandwidth is inversely proportional to the mechanical impedance plate mass.During the experiments,when at close to 400 Hz,the composite structure reached an absorption peak with a coefficient of above 0.8.Experimental results concurred with theoretical calculations.Mechanical resonance is added based on the traditional MPP resonance sound absorption mechanism.Through this,the performance of low frequency sound absorption can be improved without increasing the thickness of the structure.The frequency band can be broadened by reducing the mechanical impedance plate mass and controlling its boundary-damping coefficient.     

机械阻抗与声阻抗结合提高微穿孔板低频吸声性能

提出了在微穿孔板后部引入机械阻抗形成组合结构来解决微穿孔板低频吸声性能差的问题。由机械阻抗板两侧质点速度相同得出机械阻抗单元的传递矩阵,采用传递矩阵法将其与空腔、微穿孔板单元串接,建立组合结构理论计算模型;通过分析品质因子获得带宽与机械阻抗板质量成反比;试验得出组合结构在400 Hz附近有系数为0.8以上的吸声峰值,试验结果与理论计算吻合。在传统微穿孔板共振吸声机制的基础上加入机械共振,能够实现在不增加结构厚度的前提下提高低频吸声性能;降低机械阻抗板质量并且适当控制边界阻尼系数可以实现吸声频带的拓宽。      

A theoretical study on the effect of a permeable membrane in the air cavity of a double-leaf microperforated panel space sound absorber

A double-leaf microperforated panel absorber (DLMPP) is composed of a two microperforated panel (MPP) with a air cavity in-between, and without any backing structure. It shows a Helmholtz-type resonance peak absorption and additional low frequency absorption, therefore it can be used as a wideband space sound absorber. In this study, a theoretical study is made to examine the effect of a permeable membrane inside the air-cavity. Permeable membranes are studied in our previous studies and proved to be effective to improve the sound absorption performance of various type MPP sound absorbers. We investigate the absorption characteristics of a DLMPP with a permeable membrane in the cavity through numerical examples, and also studied the effect of honeycomb in the cavity of the same sound absorption structure.     

内置谐振器的轻薄宽频吸声器结构设计与分析*

微穿孔板吸声器的吸声频带相较于亥姆霍兹谐振器更宽,但其低频吸声的实现需要较大的空气背腔,这对结构尺寸有限制的场合存在一定局限性。本文设计了一种轻薄吸声降噪结构(内置亥姆霍兹谐振器的微穿孔板吸声器,简称MPPHR),将微穿孔板吸声器与亥姆霍兹谐振器进行了结合,提升吸声器的低频吸声性能的同时兼具了微穿孔板宽带吸声的优点。首先基于微穿孔板和亥姆霍兹谐振器理论建立了等效电路模型并计算了结构的声阻抗。然后通过有限元对MPPHR的吸声特性进行了参数研究。最后验证了MPPHR的声阻抗模型和有限元仿真的准确性。研究结果表明：MPPHR结构拥有更宽吸声频带,厚度仅为30mm的MPPHR的半吸收频带可达1294Hz,相较于同等厚度下的微穿孔板吸声器宽近500Hz。此外,MPPHR拥有更好的低频吸声效率。     

Numerical analyses of the sound absorption of three-dimensional MPP space sound absorbers

Because microperforated panels (MPPs) can provide wide-band sound absorption without fibrous and porous materials, they are recognized as next-generation absorption materials. The fundamental absorbing mechanism is Helmholtz-resonance absorption due to the perforations and air-back cavity. Consequently, MPPs are usually placed in front of rigid-back walls. However, one of the authors has proposed MPP space sound absorbers without backing structures. Among these space absorbers, cylindrical MPP space absorbers and rectangular MPP space absorbers are advantageous due to their design flexibility and easy-to-use properties. Although their performances have been investigated experimentally, it is necessary to predict their absorption characteristics to develop improved shapes and efficient designs. Herein their absorption characteristics are numerically predicted using the two-dimensional boundary element method, and the applicability of a numerical method as a design tool to sufficiently predict the performance of MPP space absorbers is discussed.     

Microperforated insertion units: An alternative strategy to design microperforated panels

Microperforated panels (MPPs) coupled to a rigid wall have been proposed recently as an alternative to porous absorbers in situations having concerns with bacterial contamination and small particles discharge, like food, pharmaceutical and microelectronic industries. There exists also an increasing interest for MPP absorbers in the transportation industry and civil engineering. In general, an optimally designed MPP with good broadband absorption requires many submillimetric holes distributed over a panel of also submillimetric thickness. Such thin plates or foils become so fragile that they need to be protected from mechanical damage. In this paper, an alternative strategy is investigated which allows the design of MPPs with panels of millimetric thickness while maintaining their acoustic performance. These absorbers, named microperforated insertion units (MIUs), avoid the structural problems of the classical MPPs. An assessment of the sound absorption properties of these structures is presented. Comparisons between calculations and measurements are also made under two experimental conditions: plane waves at normal incidence (impedance tube) and free field (anechoic room).     

Improving low-frequency sound absorption of micro-perforated panel absorbers by using mechanical impedance plate combined with Helmholtz resonators

The traditional Micro-perforated plate (MPP) is a kind of clean and non-polluting absorption structure in the middle and high frequency and has been widely used in the field of noise control. However, the sound absorption performance is dissatisfied at low frequencies when the air-cavity depth is restricted. In this paper, a mechanical impedance plate (MIP) is introduced into the traditional MPP structure and a Helmholtz resonator is attached to the MIP. Mechanical impedance plate (MIP) provides a good absorption at low frequency by using mechanism of mechanical resonance and the acoustic energy is dissipated in the form of heat with viscoelastic material. Helmholtz resonator can fill in the defect of the poor absorption effect between the Micro-perforated plate (MPP) and the mechanical impedance plate (MIP). The acoustic impedance of the proposed sound absorber is investigated by using acoustic electric analogy method and impedance transfer method. The influence of the tube’s length of Helmholtz resonator and the number of Helmholtz resonator on the sound absorption is studied. The corresponding results are in agreement with the theoretical calculation and prove that the composite structure has the characteristics of improving the low frequency sound absorption property.     

Effect of a honeycomb on the sound absorption characteristics of panel-type absorbers

Panel-type sound absorbers are commonly used to absorb low-frequency sounds. Recently, a new type of panel/membrane absorbers has been proposed as a next-generation sound absorber free from environmental problems. On the other hand, it is known that placing a honeycomb structure behind a porous layer can improve sound absorption performance and a similar effect can be obtained for microperforated-panel absorbers. Herein, the sound absorption characteristics of a panel sound absorber with a honeycomb in its back cavity are theoretically analyzed. The numerical results are used to discuss the variations in the sound absorption characteristics due to the honeycomb as well as the mechanism for sound absorption.     

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.     

Simulation and synthesis of elastic wave absorbers at the boundary of a rigid body: Incidence of longitudinal waves

Results of a computer simulation of an impedance absorber for longitudinal plane elastic waves incident on a free boundary of a rigid body are presented. The absorbing elements are mechanical resonators (of the elasticity-mass type) with two degrees of freedom and, hence, with two resonance frequencies, which correspond to the normal and tangential oscillations of the resonator. Formulas for calculating the absorber efficiency as a function of frequency and angle of incidence of longitudinal waves are derived with allowance for the absorption of both longitudinal and shear waves at their reflection from the absorbing surface. These formulas are used to solve the problem of synthesizing optimal absorbers that are characterized by the maximal average value of the absorption coefficient in preset ranges of frequency and angle of incidence. The possibility of increasing this average value by increasing the loss coefficient of the resonators or by using two types of resonators with different resonance frequencies is studied. The results of the calculations are presented in graphic form.     

Realization of wide-angle and wideband absorber using metallic and graphene-based metasurface for mid-infrared and low THz frequency

This article presents a study on mid-infrared and low-THz absorbers based on metallic and graphene metasurface. The absorber is constructed of a periodic array of patterned elements in patch form placed on a quarter-wavelength dielectric film terminated by a metallic reflector. A simple analytical circuit model equivalent to patch array is used for employing the matching impedance approach to realize the wideband absorber. This absorber is polarization independent for normal incident waves owing to its symmetric structure. Simulation and analytical circuit model results show that the graphene and metallic-based absorbers proposed in this paper can operate with an absorption value above 90% in a normalized bandwidth of 100% in the low terahertz (THz) and the mid-infrared regime, respectively. The proposed absorber is wide-angle for both TM and TE polarizations and polarization-insensitive for small incident angles.     

A pure dielectric metamaterial absorber with broadband and thin thickness based on a cross-hole array structure

A pure dielectric metamaterial absorber with broadband and thin thickness is proposed, whose structure is designed as a periodic cross-hole array. The pure dielectric metamaterial absorber with high permittivity is prepared by ceramic reinforced polymer composites. Compared with those with low permittivity, the absorber with high permittivity is more sensitive to structural parameters, which means that it is easier to optimize the equivalent electromagnetic parameters and achieve wide impedance matching by altering the size or shape of the unit cell. The optimized metamaterial absorber exhibits reflection loss below -10 dB in 7.93 GHz-35.76 GHz with a thickness of 3.5 mm, which shows favorable absorption properties under the oblique incidence of TE polarization (±45°). Whether it is a measured or simulated value, the strongest absorbing peak reaches below -45 dB, which exceeds that of most metamaterial absorbers. The distributions of power loss density and electric and magnetic fields are investigated to study the origin of their strong absorbing properties. Multiple resonance mechanisms are proposed to explain the phenomenon, including polarization relaxation of the dielectric and edge effects of the cross-hole array. This work overcomes the shortcomings of the narrow absorbing bandwidth of dielectrics. It demonstrates that the pure dielectric metamaterial absorber with high permittivity has great potential in the field of microwave absorption.     

穿孔率和穿孔直径耦合下微穿孔板吸声体整体吸声性能

微穿孔板几何参数的耦合性及其对整体吸声性能的影响,对于设计微穿孔板吸声体和优化其工作性能具有指导作用。根据微穿孔板吸声体基本理论,研究了穿孔率和穿孔直径双参数耦合作用下微穿孔板吸声体的整体吸声性能。穿孔率和穿孔直径之间的耦合性与其本身取值密切相关,而与板厚和板后腔深无明显关系;在穿孔率-穿孔直径参数域上,吸声体存在吸声系数为1.0的吸收峰,整体吸声性能随穿孔率或穿孔直径从小到大变化,呈现出先增强后减弱的变化趋势。该结论能够准确解释微穿孔板受粉尘污染后吸声性能的变化规律和演变路径。论文的工作为设计微穿孔板吸声体提供了一种新的理论依据和实施方法。      

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.     

A design of ultra-broadband metamaterial absorber

In this paper, an ultra-broadband, wide-angle polarization-insensitive metamaterial absorber is designed at microwave frequencies. The absorber consists of a periodic array of metallic strips with the lumped resistances fabricated on a dielectric substrate with a ground plane. The proposed absorber is single-layered and symmetric, thus leading to very thin and polarization-insensitive characteristics. According to the surface current distributions, the proposed absorber can operate three resonant modes including two dipole resonances and an LC resonance. By adjusting the geometry and lumped resistances of the structure, three resonant modes can be effectively connected together, and thus an ultra-broadband response can be realized. The simulated and experimental results demonstrate that the proposed absorber produces a good absorption above 80% at normal incidence and above 60% at oblique incidence with the incident angle smaller than 45° between 6 and 16.2 GHz.