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 note on the relationship between the sound absorption by microperforated panels and panel/membrane-type absorbers

The sound absorption mechanism of microperforated panel (MPP) absorbers and panel/membrane-type absorbers is both based on a certain resonance system and utilising its resonance effect. However, the relationship between the absorption mechanisms of MPPs and panel/membrane-type absorbers has not been discussed: it is not clarified whether they can occur simultaneously, or how they interfere each other. On the other hand, in a previous study there is an attempt to cause both absorption mechanisms simultaneously. In this paper, using an electro-acoustical equivalent circuit model, their sound absorption mechanisms and their relationship are discussed. In this study, three cases are considered: (1) the case in which only the mass reactance of the MPP is considered, (2) the case in which the losses of the panel is considered, and (3) the case in which the sound absorption of the back wall surface is considered. The results suggest that the microperforated panel absorption, which is Helmhotz-type resonance, and the panel/membrane-type absorption can be regarded as phenomena of the same kind which can be smoothly transformed into each other by changing a parameter, and can be consistently modelled and comprehensively discussed.     

A sound absorption panel containing coiled Helmholtz resonators

This study proposes a promising design of sound absorption panels containing acoustic resonators. Each resonator is comprised of one small-sized and one large-sized tubes in series. The former can be viewed as the neck of a Helmholtz resonator, while the latter serves as a resonant chamber. Both tubes are bent up to fit in limited space of the panel. The 3D printing technology is exploited to fabricate samples for measurements. Frequency manipulation for the absorption peak can be achieved by adjusting the geometric parameters of tubes. Moreover, two pairs of resonators with different dimensions can broaden the bandwidth of absorption. Theoretical predictions on absorption characteristics agree well with numerical and experimental results. The proposed structure offers a feasible way of absorbing low-frequency sound without the need to use thick panels.     

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 insulation characteristics of multi-layer structures with a microperforated panel

Multi-layer structures have issues with sound insulation at low and mid-frequencies due to mass-air-mass resonance. The purpose of this study is to investigate improvements to the sound insulation performance of multi-layer structures using a microperforated panel (MPP), which can absorb well over a wide frequency range. Although MPPs have been investigated over the last several decades, almost all studies have been conducted in terms of sound absorption. Herein the sound transmission loss of multi-layer structures with flexible MPPs of infinite extent is theoretically investigated. The calculation is based on the wave equation and the equation of panel vibration including the effect of perforation of the panel. Additionally to consider a more realistic sound insulation performance, the effect of the directional distribution of the incident energy in a reverberation chamber is taken into account. Experiments are conducted using an acoustic tube to validate the calculated results and the reverberation chamber method to verify the actual sound insulation characteristics. Both experiments agree well with the theoretically calculated perforation effects. Consequently, MMPs are confirmed to improve the deterioration of sound insulation performance due to mass-air-mass resonance of multi-layer structures.     

A pilot study on improving the absorptivity of a thick microperforated panel absorber

Microperforated panel (MPP) absorbers are promising as a basis for the next-generation of sound absorbing materials. MPPs are typically made of a thin metal or plastic panel. However, thin limp panels are generally not suitable as an interior finish of room walls because they do not have sufficient strength, which prevents practical application of MPPs as an interior finish of room walls. In order to overcome the lack of appropriate strength required for room walls, it is possible to make an MPP out of a thick panel. However, thick MPPs are usually not efficient because the resistance and/or reactance become too high. In this study, trial production of thick MPPs and measurement of their normal absorption coefficients were carried out. Results show that efficient absorption can be given with a thick MPP by using a tapered perforation.     

Sound absorption of a double-leaf micro-perforated panel with an air-back cavity and a rigid-back wall: Detailed analysis with a Helmholtz-Kirchhoff integral formulation

So far the electro-acoustical equivalent circuit analysis has been widely used to analyse micro-perforated panel (MPP) absorbers, however, as for the double-leaf MPP the equivalent circuit analysis inevitably includes an approximation. In this paper, the sound absorption characteristics of a double-leaf MPP absorber backed by a rigid wall are analysed by wave theory using Helmholtz-Kirchhoff integral formulation to obtain a strict solution. The present wave theory is experimentally validated with existing measured results. The theory is also compared with the equivalent circuit solutions so that the differences between the two theories appear and the effect of the approximation is clarified. The comparison shows that the difference mainly appears in the vicinity of the resonance peaks: the differences occur in the resonance frequencies and the absorption coefficient at frequencies between the two resonance peaks.     

Estimation of effective parameters for microperforated panel absorbers and applications

Microperforated panel absorbers are used in many noise control applications as an alternative to fibrous materials. In the past, perforations were circular in shape. However, perforations are now often pressed or cut into the metal. They are non-circular and are irregular shaped and are difficult to model. A nonlinear least squares data-fitting algorithm is utilized to estimate geometric parameters from measured absorption coefficient data. The circular perforation model proposed by Maa is used in the algorithm. The estimated geometric parameters are then used to calculate transfer impedance with good agreement compared to measured data. The procedure is then used to aid in understanding the effect of manufacturing variations, and dust and fluid contamination on the performance of microperforated panel absorbers.     

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.     

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.     

Optimization of multiple-layer microperforated panels by simulated annealing

Sound absorption by microperforated panels (MPP) has received increasing attention the past years as an alternative to conventional porous absorbers in applications with special cleanliness and health requirements. The absorption curve of an MPP depends on four parameters: the holes diameter, the panel thickness, the perforation ratio, and the thickness of the air cavity between the panel and an impervious wall. It is possible to find a proper combination of these parameters that provides an MPP absorbing in one octave band or two, within the frequency range of interest for noise control applications. However, when a wider absorption frequency band is required, it is necessary to design multiple-layer MPP (ML-MPP). The design of an N-layers MPP depends on 4N parameters. Consequently, the tuning of an optimal ML-MPP by exhaustive search within a prescribed frequency band becomes impractical. Therefore, simulated annealing is proposed in this paper as a tool to solve the optimization problem of finding the best combination of the constitutive parameters of an ML-MPP providing the maximum average absorption within a prescribed frequency band.     

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.     

Experimental study for control of sound transmission through double glazed window using optimally tuned Helmholtz resonators

This paper presents an experimental investigation of passively control of sound transmission through a double glazed window by using arrangement of Helmholtz resonators (HRs), which are commonly used for narrow band control application. The laboratory experiments were performed placing the window between reverberation chamber and anechoic chamber. The window was subject to diffuse field, approximate normal wave and oblique wave acoustic excitations. Three sets of HRs were designed and installed in cavity of window. The sound control performances at far-field were measured. The control performances from varying the number of HRs, incident acoustic field, excitation sources (band-limited white noise and traffic noise examples) are presented and discussed in detail. It is shown that a considerable reduction of the transmitted sound pressure levels has been achieved around the mass–air–mass resonance frequency (50–120 Hz). The obtained reductions in the transmitted sound pressure illustrate the potentials of HRs for improving the sound insulation characteristics of double glazed window. The experimental results also indicate that only tuning the HRs to the mass–air–mass resonance frequency does not guarantee the best possible insulation of the sound transmission.     

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.     

管束穿孔板的管腔耦合共振吸声机理研究

为了揭示管束穿孔板共振吸声结构的吸声机理,利用热黏性条件下基于有限元算法的管束穿孔板仿真模型,研究了平面声波正入射条件下,管束穿孔板内部声场分布特征,并利用阻抗管对吸声系数的理论仿真结果进行了试验验证.结果表明,管束穿孔板在低频主要靠腔体共振吸声,在高频主要靠管共振吸声,管束穿孔板整体呈现出较为明显的管腔耦合共振吸声特征。管束穿孔板共振时管中声强和质点法向振速较大,高频次吸声峰频点处管中和腔中均有驻波形成,频率越高驻波数量越多.管束穿孔板的耦合共振受到管长、腔深、穿孔率和管内径等参数变化的影响,管长对高频耦合共振的影响最大,管长增大使高频主吸声峰频点移向低频,并使相邻主吸声峰之间的间距减小.     

水下弹性微穿孔吸声结构吸声系数研究

利用模态叠加法建立了水介质微穿孔板的数学模型,基于声电类比法得到其等效电路模型。研究了弹性微穿孔板和弹性背腔对垂直入射吸声系数的影响。与空气介质中的微穿孔板不同,水下微穿孔板因结构阻抗不足,难以取得满意的吸声效果,为此提出了增强型微穿孔吸声结构,并在水介质阻抗管内对理论结果予以验证。结果表明,随着增强型弹性微穿孔板弯曲刚度的增大,其在[20,2000]Hz范围内的平均吸声系数得到提高,逐步趋近于刚性微穿孔板的结果,弹性背板使微穿孔吸声结构的吸声峰向低频移动,低频吸声效果得到提高。     

超材料谐振子间的电耦合谐振理论与实验研究

通过将两个金属开口环谐振器口对口地放置, 实现了超材料谐振子间的电耦合谐振. 对电耦合谐振的微波等效电路进行了理论分析和数值计算, 结果表明耦合后的超材料谐振子能产生两个谐振频率, 其中一个随耦合强度的增加逐渐向低频方向移动, 而另一个固定在单谐振子的谐振频率处不变. 微波透射谱的实验测试和电磁仿真结果表明, 两个谐振峰随耦合强度的增加分别向低频和高频方向移动. 分析表明: 低频谐振峰的位置主要是由超材料谐振子间的电耦合强度决定的; 高频谐振偏离单谐振子的谐振频率主要是由不可避免的磁耦合引起的, 而且在耦合间距越小时磁耦合影响越大. 提出的基于超材料谐振子间的电磁耦合实现的双频谐振及其可调性极大地增加了超材料的设计与应用空间.     

Acoustical properties of aerated autoclaved concrete

This work analyses acoustic qualities of autoclaved aerated concrete (AAC). Three the most widely used types of AAC are chosen for the analysis: gas cement concrete, gas cement concrete with combined binder (Portland cement and lime), and foam cement concrete. The procedure and technique of the materials’ formation is presented in this work. The evaluation of acoustic qualities of AAC is based on the material’s air permeability and porosity (i.e., ratio of the volume of the interconnected pores to the total volume of pores). For this purpose the measurements obtained by an acoustic interferometer are used. The results of the experiment show that regression equations for the AAC types, which density ranges from 250 to 500 kg/m³, may be used to estimate the materials’ normal incidence absorption coefficient values, which depend on the air permeability and porosity. Results show that absorption coefficient of not specially treated AAC is rather low. According to the measurements obtained in a special reverberation room of 202 m³, a sound absorption coefficient may increase up to 0.6, provided that slits of Helmholtz resonator’s type are made in the slabs of AAC gas cement concrete with combined binder.     

窄脉冲声用于大样品的吸声测量

本文利用逆滤波器原理,在空间产生了波形可控、长度在毫秒量级的窄脉冲声信号,分别采用脉冲分离法和脉冲叠加法,对一种毛毡材料和三种不同厚度的海绵材料进行了吸声系数的测量。实验证明,基于窄脉冲声信号的吸声测量结果与ISO13472-1:2002中的MLS脉冲法及阻抗管的测量结果基本吻合。采用窄脉冲进行吸声测量,可以减少样品边缘和周围环境对测量信号的干扰,提高现场测量的准确性。     

液体火箭发动机中声腔抑制不稳定燃烧的声学分析

发展了声腔的分析和数值模型,对液体火箭发动机不稳定燃烧的抑制作用进行了评定,通过迭代计算研究了二维流动和温度分布变化对声腔调 谐和稳定性能的影响,对不同的声腔几何尺寸和温度梯度的稳定性计算结果表明,燃烧带有较大开口面积的声腔会更大程度地改变振荡的空间分布,这种改变而且影响了驱动和抑制燃烧的机理,讨论了在声腔设计安排中正确选择声腔的几何尺寸,且比较了不同长度和不同直径声腔的阻尼特性,通过考察声吸收系统的方法来最优化系统的阻尼,得到了可供设计参考的结论。