Effects of compression on the sound absorption of porous materials with an elastic frame

The absorption characteristics of a porous material are well known to vary during compression. The transfer matrix method is applied with an elastic frame to explore the effect of compression on absorption properties. In this work, the materials are treated as elastic rather than being made of rigid models. The absorption coefficients of the uncompressed and compressed porous material are initially calculated and verified from the experimental measurements. Then, numerical predictions of absorption coefficient are made for the compressed porous material.     

多孔弹性介质三层夹心板的隔声性能研究

应用Biot关于流体饱和多孔弹性介质的声传播理论,采用传递矩阵的分析方法,就复合多孔弹性材料夹心三层板在不同结构情况下的隔声性能进行了理论研究和实验分析,并与同等条件下双层夹心板的隔声性能进行了比较。数值计算和实验结果均表明,与双层夹心板相比,三层夹心板在中高频段隔声性能有明显优势,但低频段隔声性能有一定程度上的下降。研究还表明不同结构的复合三层夹心板在隔声效果上也各有特色。     

含三聚氰胺多孔材料分层复合介质吸声特性*

三聚氰胺泡沫材料是一种具有高开孔率的多孔材料,具备优良的吸音、防火隔热及环保性能,可以作为吸声材料与弹性板、空腔介质形成复合结构,在建筑、航空、交通工具等工程领域有广泛的应用。该文基于Biot理论和分层介质在交界面处的不同边界条件,建立非均匀复合介质背衬刚性壁面结构的理论声学模型,详细分析了多孔材料布局对复合结构吸声特性的影响。该文理论模型计算的结果与阻抗实验得到的垂直入射吸声系数基本一致,验证了理论模型的正确性。结果表明:在多孔材料前面增加空气层可以改善高频吸声特性;在多孔材料后面增加空气层可以改善复合结构低频吸声特性。通过合理配置多孔材料,可以在应用需求频段上达到满意的吸声效果。     

Sound absorption performance of gradiently slit-perforated double-porosity materials

A gradiently slit-perforated double-porosity material is proposed by introducing macro-scale periodic gradient slit-perforations into traditional porous materials with singleporosity.This material is one kind of multiscale material since it includes two scales of matrix micro-pore size and slit-perforation size.A theoretical model is developed for the sound absorption of the gradiently slit-perforated double-porosity material.In the model,the material is divided into lots of thin layers and each layer is approximated to be straight slit-perforated material.The equivalent density and dynamic compressibility of each thin layer are given by using the low or high permeability contrast double-porosity theory.Then the sound pressure and particle velocity relations between adjacent thin layers are obtained by employing the transfer matrix method.Finally,the surface acoustic impedance and the sound absorption of the gradiently slit-perforated porous material can be calculated.A finite element model is further established to validate the accuracy of the theoretical model.In the considered frequency range of 100-3000 Hz,the simulation results agree well with theoretical results.The influence of multiscale structural parameters on the sound absorption performance of the porous materials is analyzed theoretically and numerically.It is proved that the multiscale structure design can significantly improve the sound absorption performance of porous materials.Compared to the slit-perforation gradient,the slit-perforation width plays a more significant influence on sound absorption.The sound absorption enhancement mechanism of the multiscale structure design is revealed by the analysis of the sound pressure and energy dissipation distributions in the material.This work provides a multiscale structural design method for improving the sound absorption performance of traditional porous materials at broadband frequency.     

Sound absorption characteristics of a high-temperature sintering porous ceramic material

This article is dedicated to sound absorption properties of porous zeolite with macropores, a ceramic material fabricated by high-temperature sintering. Acoustical properties of this ceramic material are studied by two analytical models, Delany–Bazley model and Johnson–Allard model, where the latter one shows a better fit to the experimental results. Moreover increasing the thickness of samples would improve the sound absorption in the low frequency ranges. Raising the porosity could increase the highest sound absorption coefficient. The resonance frequencies of the materials with 3–5 mm particles are more obvious. Comparing with glass wool, porous zeolite has a better sound absorption.     

梯度穿缝型双孔隙率多孔材料的吸声性能

在传统单一孔隙率多孔材料中引入宏观尺度的周期性梯度穿缝结构设计,构造出梯度穿缝型双孔隙率多孔材料,其包含多孔材料基体微孔尺度与穿缝尺度两个尺度。采用分层等效的理论建模方法,将复杂梯度渐变问题变为多层均匀等效层叠加问题。针对不同特征尺寸的多孔材料薄层,分别采用低、高两种渗透率对比度双孔隙率理论,给出了其等效密度和动态压缩系数,再应用传递矩阵方法得到了相邻薄层之间的声压和质点速度传递关系并求得其表面声阻抗,从而建立了梯度穿缝型双孔隙率多孔材料的吸声理论模型。发展了多尺度材料声学有限元数值模型,在所考虑的100~3000 Hz频段范围内数值模拟结果完全吻合理论模型结果。理论与模拟分析了多尺度结构参数对双孔隙率多孔材料吸声性能的影响,结果表明引入多尺度梯度结构设计能够显著提高单一孔隙率多孔材料的吸声性能,且穿缝尺度比穿缝梯度影响更为显著;精细数值模拟获得的声压和能量密度分布云图揭示了多尺度结构设计的吸声增强机制。该工作可用于指导双孔隙率多孔材料的多尺度结构设计,从而提高多孔材料的中低频吸声性能。     

四边简支含多孔材料双层板隔声特性

有限大含多孔材料和空气层复合板结构隔声特性的研究尚不充分。该文旨在研究四边简支边界条件下复合板结构隔声特性。首先基于流体饱和多孔弹性介质的声传播理论计算了声波在多孔介质中的传播波数；继而采用四边简支边界条件下板结构的模态函数，利用模态叠加法和伽辽金法推导了复合板结构隔声系数理论模型，并数值求解复合板隔声量。将理论模型得到的四边简支复合板隔声量与实验结果对比，验证了理论模型的正确性。最后，详细讨论了边界条件、板结构尺寸和多孔材料厚度等主要参数对隔声特性的影响。结果表明：四边简支复合结构隔声特性曲线上“谷值”出现得较少，并且简支复合板隔声特性的第一个“吻合频率”比固支支撑复合板更靠近低频，当频率超过3000 Hz以后，简支和固支边界条件复合板结构隔声特性趋于一致。     

Sound transmission in pipes with porous walls

Pipes with porous (permeable) walls have received the attention of several authors as a noise control element in automotive intake systems; however, a closed theory of sound transmission including the effect of the coupling of the internal and external acoustic fields and the presence of mean flow does not appear to be available. The present paper proposes an integro-differential system for the propagation of plane sound waves in pipes with porous walls, and presents its general numerical solution, as well as an approximate analytical solution. The predicted effect of the coupling between the internal and external acoustic fields in a circular pipe made of reinforced woven fabric walls is shown, and the transmission loss predictions are compared with the existing experimental data in the literature.     

Diffusive liquid propagation in porous and elastic materials: the case of foams under microgravity conditions

We report the results of fluid transport experiments in aqueous foams under microgravity. Using optical and electrical methods, the capillary motion of the foam fluid and the local liquid fractions are monitored. We show that foams can be continuously wetted up to high liquid fractions ( approximately 0.3), without any bubble motion instabilities. Data are compared to drainage models: For liquid fractions above 0.2, discrepancies are found and identified. These new results on foam hydrodynamics and structure can be useful for other poroelastic materials, such as plants and biological tissues.     

Characterization of elastic parameters of acoustical porous materials from beam bending vibrations

Dynamic visco-elastic characterization of porous material parameters shows a revival of interest since a few years. In this article, a methodology for the assessment of the visco-elastic parameters of soft, highly damped porous materials is presented. This method uses a frequency response function of a free-free bending beam. The global method is based on an inversion procedure. It consists of fitting the experimental data to the GLM computation model. In the experimental part of this work, it is explained how to take the attachment system and sensor mass into account and how to measure it. The assessment of the visco-elastic parameters needs a discrete laminae analytic model. The GLM model is described and used for a sensitivity study in relation to the visco-elastic parameters. This study gives indications for the beam dimensions design. The results, bending modulus and loss factor in the beam's main direction for various frequencies, obtained for two polymeric foams are shown and compared to results obtained with other techniques. The originality can be found in the experimental developments but also on technical developments of parameters assessment.     

Flexural vibration of perforated plates and porous elastic materials under acoustic loading

This paper presents a method of theoretical treatment of acoustic coupling due to flexural vibration of perforated plates and plates of porous elastic materials. The analytical model is developed by introducing flow continuity at the plate surface in a spatially mean sense and air-solid interaction within the plate material. To demonstrate the method of application, some fundamental acoustic problems based on a classical thin-plate theory are analyzed and discussed in relation to the interactive effect of flexural vibration and plate permeability. For acoustic radiation from a vibrating plate excited by a harmonic point-force, the attenuation effect of power radiation appears at frequencies below the critical frequency of coincidence. In the problem of sound absorption of a perforated plate or a plate of porous elastic material backed by an air layer, as permeability decreases, the effect of plate vibration increases. For perforated absorber systems including plate vibration effects, the trend of variation from ordinary theory depends on plate thickness.     

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.     

基于(u,p)方程含多孔材料有源声学结构的声学性能研究

为准确分析含多孔材料有源声学结构的声学性能,利用一种结合(u,p)方程和有限元技术的计算方法对有源声学结构建模,分析声波在多孔材料中传播规律,并进行了实验验证.基于固相位移和流相声压的(u,p)方程对多孔材料固、流两相间强烈耦合进行分析,建立了多孔材料的有限元模型,并对含有多孔材料双层板有源声学结构的声学性能进行了测量...     

Sound absorption of cellular metals with semiopen cells

A combined experimental and theoretical study is presented for the feasibility of using aluminum foams with semiopen cells for sound-absorption applications. The foams are processed via negative-pressure infiltration, using a preform consisting of water-soluble spherical particles. An analytical model is developed to quantify the dependence of pore connectivity on processing parameters, including infiltration pressure, particle size, wetting angle, and surface tension of molten alloy. Normal sound-absorption coefficient and static flow resistance are measured for samples having different porosity, pore size, and pore opening. A theory is developed for idealized semiopen metallic foams, with a regular hexagonal hollow prism having one circular aperture on each of its eight surfaces as the unit cell. The theory is built upon the acoustic impedance of the circular apertures (orifices) and cylindrical cavities due to viscous effects, and the principle of electroacoustic analogy. The predicted sound-absorption coefficients are compared with those measured. To help select processing parameters for producing semiopen metallic foams with desirable sound-absorbing properties, emphasis is placed on revealing the correlation between sound absorption and morphological parameters such as pore size, pore opening, and porosity.     

Influence of elastic strains on the adsorption process in porous materials. Thermodynamics and experiment

If we disregard the shape of the boundary hysteresis loop, H1 for SBA-15, MCM-41 and KIT-6, H2 for p⁺-type porous silicon and porous glass, the hysteretic features inside the loop are qualitatively the same for all these systems and show that none of them are composed of independent pores whether the pores are interconnected or not. We hence believe that the physical parameter which couples the pores is not the interconnectivity but the elastic deformation of the porous matrix. The thermodynamic approach we develop includes the elastic energy of the solid. We show that the variation of the surface free energy, which is proportional to the deformation of the porous matrix, is an important component of the total free energy. With porous silicon, we experimentally show that a stress external to the porous matrix exerted by the substrate on which it is supported significantly increases the total free energy and the adsorbed amount and decreases the condensation pressure compared to that of the same porous matrix detached from its substrate which is the relaxed state of the supported layer. This stress can be partly relaxed by making thicker porous layers due to the breaking of Si-Si bonds. This results in the shift of the isotherms towards that of the membrane. We propose a new interaction mechanism occurring through the pore wall elastic deformation in which the external mechanical stress is imposed on a given pore by its neighbours.     

Vibration and stability of sandwich beams with elastic bonding

This paper is a study of the vibration and stability of symmetrical sandwich cantilevers with elastic bonding. The horizontal displacements of the face layer and the core are discontinuous due to the elasticity of the interface bond. The shear traction at the interface is assumed to be proportional to the relative horizontal displacement of the layers. The core layer is subjected to a horizontal (conservative) or tangential (non-conservative) axial force at its free end. It is shown that a symmetrical sandwich beam with elastic bonding has two kinds of vibration modes: i.e., bending modes and longitudinal modes. Numerical results are given for a beam composed of FRP face layers and a syntactic-foam core layer. It is shown that the divergence and the flutter type instability loads, as well as the natural frequency, are affected considerably by the stiffness of the interface bond.     

On the use of perforations to improve the sound absorption of porous materials

The concept of meso-perforations in appropriately chosen porous media can help enhance their sound absorption performance. The meso-perforated materials are also referred to as “double porosity materials” since they are made up of two interconnected networks of pores of different characteristic size. Several theoretical, numerical and experimental works have been accomplished on the subject by the authors. The purpose of this paper is to give a synthetic review of these works and establish practical design rules to develop optimized noise control solutions based on this concept. The paper presents two complementary models to deal with this kind of materials: an analytical model based on homogenization techniques and a numerical model relying on a finite element discretization of the domains. The limits of these models are discussed. The choice of the design parameters is then been investigated in order to provide practical design rules. This choice relies on a criterion which is evaluated from the knowledge of the resistivity, porosity and tortuosity of the micropous medium, and the calculation of a geometrical parameter defined from the chosen mesoscopic structure. Experimental and numerical results regarding the influence of the mesopore profile along the thickness performed in a appropriately chosen substrate microporous medium are presented. The agreement between the models and the experiments is satisfactory. Results show that significant enhancements of the absorption properties can be obtained over a selected frequency band by adjusting the mesopore profile. It is also shown that interesting absorbing properties can be obtained when coating a double porosity medium with an impervious screen.     

Specific features of infrared absorption in films materials subjected to a magnetic field

The reflection spectra and magnetorefractive effect (MRE) of metal (Co, CoFe), semiconductor (Si, GaAs), and granular and amorphous Co₃₀Ag₇₀ and Co₅₉Fe₅Ni₁₀Si₁₁B₁₅ films are studied in the IR spectral region at λ=2.5–25 μm. It is found that the IR optical properties of the ferromagnetic metal films can be described with regard to light absorption due to electron transitions in the two spin systems. The MRE is found to occur in both the ferromagnetic (Co, CoFe) and semiconductor (Si, GaAs) films. The amplitude and shape of the MRE are determined for the p and s polarizations of light. It is shown that, to a first approximation, the IR optical properties of the films with giant magnetoresistance can be described by the Drude theory, while the MRE is explained on the basis of a modified Hagen-Rubens relation. Variations in the IR reflection of semiconductor or amorphous metal films in the magnetic field are found to depend on the degree of polarization of localized electron states at the Fermi level.