Sound absorption of microperforated panel mounted with helmholtz resonators

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, MPP mounted with Helmholtz resonators (MPPHR) is introduced. Based on the MPP, Helmholtz resonators theory and electro-acoustical equivalent circuit principle, sound absorption properties of MPPHR are studied. Simulation and experimental results show that MPPHR have two peak frequencies and one anti-resonant frequency. The low-frequency peak is dependent on the Helmholtz resonators, while the high frequency peak is close to the peak of the single-layer MPP. The low-frequency sound absorption peaks move to low frequency with the neck length and the volume of Helmholtz resonators increasing. The high-frequency sound absorption peaks move to high frequency with the volume of Helmholtz resonators cavity increasing. Multiple Helmholtz resonator parallel MPP structure can provide more sound absorption than single MPPHR at low frequency range due to the introduction of more additional sound absorption peaks.     

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.     

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.     

Sound absorption characteristics of a double-leaf structure with an MPP and a permeable membrane

As for the sound absorbing system using an MPP (microperforated panel), a double-leaf MPP sound absorber has been studied so far. However, this structure uses two MPPs, which are still expensive, and is disadvantageous when its cost is concerned. Therefore, it is considered that it can be advantageous if one of the leaves can be replaced with a less expensive material keeping high sound absorption performance. In this study, the possibility of producing a useful sound absorbing structure with an MPP and a permeable membrane as an alternative less expensive material is examined. The acoustic properties of this MPP and permeable membrane combination absorber are analysed theoretically with a Helmholtz integral formulation. The absorption performance and mechanism are discussed through the numerical examples. Also, the effect of a honeycomb in the air cavity, which is to be used for reinforcing the structure, is also discussed through a theoretical analysis.     

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.     

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.     

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.     

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。     

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。     

Enhancing the low frequency sound absorption of a perforated panel by parallel-arranged extended tubes

This paper is concerned with the use of a perforated panel with extended tubes (PPET) to improve the sound absorption confined to low frequencies. In comparison with a micro-perforated panel (MPP), the sound absorption can be significantly improved by using the PPET at the expense of the bandwidth of the sound absorption. A particular configuration combining four parallel-arranged PPETs with different cavities is introduced to achieve a wider bandwidth of the sound absorption at low frequencies. The analysis is extended to the combination of three parallel-arranged PPETs and a MPP to further increase the bandwidth of the sound absorption. A theoretical model is described to predict the sound absorption coefficient and the simulated annealing method is introduced to the proposed absorbers, allowing optimization of the overall performance. The theory with experimental validations demonstrates that the proposed configurations offer a potential improvement of more than one octave in the bandwidth of the sound absorption at low frequencies.     

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。      

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.     

耦合声阻抗在扩散吸声体设计中的应用研究

为解决小房间的音质设计问题,需要设计不同的扩散吸声体。利用共振吸声的边缘效应,通过不同共振频率的共振器耦合共振时的非线性声阻抗变化组合,形成既能高效吸声,又能均匀散射的声学界面。数值分析及实验结果表明,新型的扩散吸声体内部没有任何传统吸声材料的情况下,单位面积吸声量在中低频段可达1.3 m2,在高频段由于非线性声阻抗与共振器的辐射阻抗不匹配影响,相应吸声量降低到0.7 m2左右。耦合声阻抗的运用使得新型扩散吸声体吸声的效率高,频带宽,免去传统吸声材料的使用,在小房间的声学应用中具有突出的优势。     

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.     

Acoustic properties of micro-perforated panel absorber having arbitrary cross-sectional perforations

Micro-perforated panel absorber is used in many noise control applications as a next-generation absorbing material. Perforation shapes of micro-perforated panel studied are usually circular in the past. However, in practice, the perforations are often non-circular or irregular shape due to manufacturing techniques. Sound absorption coefficient and absorption bandwidth of the micro-perforated panel absorber may be further improved, when the perforations in shape are changed. In view of the existing exact solutions of sound propagation in tubes, the simple formulas of specific acoustic impedances of the tubes for triangle and square cross-sectional perforations are derived. Mass reactance end correction of the micro-perforated panel is obtained based on the sound radiation of a shaped piston. The specific acoustic impedance ratio of the micro-perforated panel absorber is calculated and analyzed, which can predict its sound absorption bandwidth. Finally, for closed perforations, the influences of the perforations in shape (including triangle, circle, square and irregular circle) on sound absorption of the MPP absorber are discussed in collaboration with FE simulations.     

Oblique incidence sound absorption of parallel arrangement of multiple micro-perforated panel absorbers in a periodic pattern

The sound absorption performance of a micro-perforated panel (MPP) absorber array at oblique incidence and in diffuse field is investigated both numerically and experimentally. The basic module of the MPP absorber array consists of four parallel-arranged MPP absorbers with different cavity depths, and the whole MPP absorber array is created by arranging the basic modules in a periodically repeating pattern. Results show that the influence of incidence angle mainly lies in two aspects. First, the parallel absorption mechanism breaks down at lower frequencies at oblique incidence than at normal incidence due to the non-compactness of the resonating MPP absorber, which becomes non-compact if the time delay of incident wave across it is comparable to or larger than π/2. Second, the equivalent acoustic impedance of the MPP varies with respect to incidence angle which in turn changes the sound absorption performance of the MPP absorber array. Influence of the azimuthal angle is insignificant. Because of mutual influence among the member MPP absorbers, the normal incidence sound absorption of the MPP absorber array can be noticeably different from that of the basic module tested in impedance tube. The measured sound absorption coefficients of a prototype specimen in reverberation room compare well with the numerical predictions. The extra sound absorption due to diffraction of sound at the free edges of test specimen is the most efficient around 500 Hz.     

Evaluation of the acoustic and non-acoustic properties of sound absorbing materials using a three-microphone impedance tube

This paper presents a straightforward application of an indirect method based on a three-microphone impedance tube setup to determine the non-acoustic properties of a sound absorbing porous material. First, a three-microphone impedance tube technique is used to measure some acoustic properties of the material (i.e., sound absorption coefficient, sound transmission loss, effective density and effective bulk modulus) regarded here as an equivalent fluid. Second, an indirect characterization allows one to extract its non-acoustic properties (i.e., static airflow resistivity, tortuosity, viscous and thermal characteristic lengths) from the measured effective properties and the material open porosity. The procedure is applied to four different sound absorbing materials and results of the characterization are compared with existing direct and inverse methods. Predictions of the acoustic behavior using an equivalent fluid model and the found non-acoustic properties are in good agreement with impedance tube measurements.     

利用含多孔介质微穿孔周期结构增强声吸收

为获得理想吸声性能,提出了一种由多孔材料,微穿孔板及空气层构成的周期复合结构,并利用微穿孔板理论和等效流体多孔材料模型,结合等效电路法进行了分析。结果表明,复合结构显著增强了微穿孔板结构的中低频吸声性能,但其高频性能较单独多孔材料差;采用合适填充比例并联布置多种多孔材料,可适当调节复合结构的吸声性能。此外,周期复合结构的堆叠层数N≥1时,相对单层复合结构,中低频吸声带宽提升至少40%(≥380 Hz);相对多层微穿孔板结构,增大N对相应中低频吸声带宽提升不低于30%(≥300 Hz)。总体上,文中周期复合结构可显著增强传统微穿孔结构的中低频性能,是一种简单高效的中低频宽频降噪方案。