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。     

空腔流动的大涡模拟及气动噪声控制

大涡模拟(LES)和三维的Ffowcs Williams-Hawkings声学比拟方法相结合,研究空腔过流的一种噪声控制措施.空腔的底板/后墙使用多孔壁板,因此流体可以穿透空腔壁面,多孔效果使用Darcy压力-速度关系模拟.声源流场由LES计算,声辐射和远声场由声学比拟获得.结果表明,这种措施有效地减弱了空腔内的压力脉动和远场声辐射,低频脉动Rossiter模数对应的波动幅值被有效抑制,声源中偶极子占优项大幅度减小,从而抑制了声辐射.     

Controlling sound absorption by an upstream resistive layer

Resistive screens, or perforated plates, are widely used upstream of porous materials. They can be used either for protection or decoration, or for acoustic properties enhancement. This study points out the role that a resistive layer can have upstream on a porous material. Based on numerical simulations, this work gives the guidelines for rational use of high resistive layers in order to maximize the normal sound absorption of porous multilayers. Two major results emerge: (i) the upstream resistive layer can control the sound absorption of the porous multilayer, while nullifying the acoustic properties of downstream layer and (ii) this upstream layer may be detrimental to sound absorption of porous multilayer. Experimental validation on a porous multilayer, controlled by a woven textile, supports these findings. The sound absorption of material with poor sound absorption performance can be enhanced with a conveniently designed resistive layer.     

散砂及表层海洋(砂质)沉积物中的声速

根据作者的颗粒介质中的声传播理论,应用熄灭定理,给出了颗粒介质中的声速公式。对表层海洋(砂质)沉积物及气砂中的声速作了数值计算,结果表明,在大气压力下,气砂中的声速比空气中的声速小。而在表层海洋沉积物中,散射相互作用的影响不可忽视,粘滞波相互作用对声速的影响不大。刚性粒子理论结果与发表的实验数据比较符合,弹性粒子理论模型应该说是符合实际的,但给出的结果反而比实验数据小,这表明更多的实验测量是需要的。还比对分析了已发表的实验数据,结果表明,颗粒介质理论是更适宜于研究气砂中的声传播问题。     

Sound speed in air-filled sand and 03ine shallow-layer sandy sediments

Based on the author's theory for acoustic propagation in granular media and by employing the extinction theorem, the sound speed formulae in these media were derived. The numerical computations of sound speed in 03ine sediments and air-filled sand were carried out, and the results demonstrated that under the normal atmosphere the sound speed in air- filled sand is lower than the sound speed in the air. The numerical results also indicated that the influence of scattering interaction between the grains upon the sound speed in the 03ine shallow-layer sandy sediments has to be taken into account; however, the influence of the viscous-wave interaction can be neglected. The theoretical results obtained from the rigid-granular model seem to match the measured data better than from the elastic-granular model, even the latter model fits better for the real situation, indicating further measurements are necessary in order to gain an insight into this problem thoroughly. Through an analysis of experimental data published in journals a conclusion can be drawn that the theory of granular media is suited to deal with the problems of the sound propagation in air-filled sand better than the theory of porous media.     

Acoustical estimation of parameters of porous road pavement

In the simplest case, porous road pavement of a known thickness is described by such parameters as porosity, tortuosity, and flow resistance. The problem of estimating these parameters is investigated in this paper. An acoustic signal reflected by the pavement is used for this. It is shown that this problem can be solved by an experiment conducted in the time domain (i.e., the pulse response of the media is recorded). The incident sound wave is thrown at a grazing angle to the surface between the pavement and the air to improve penetration into the porous medium. The procedure of computing of the pulse response using the Morse-Ingard model is described in detail.     

Near field acoustic holography with microphones on a rigid sphere (L)

Spherical near field acoustic holography (spherical NAH) is a technique that makes it possible to reconstruct the sound field inside and just outside a spherical surface on which the sound pressure is measured with an array of microphones. This is potentially very useful for source identification. The sphere can be acoustically transparent or it can be rigid. A rigid sphere is somewhat more practical than an open sphere. However, spherical NAH based on a rigid sphere is only valid if it can be assumed that the sphere has a negligible influence on the incident sound field, and this is not necessarily a good assumption when the sphere is very close to a radiating surface. This Letter examines the matter through simulations and experiments.     

The acoustic emission of a distributed mode loudspeaker near a porous layer

Experimental and theoretical modeling of the vibro-acoustic performance of a distributed mode loudspeaker (DML) suggest that their acoustic emission can be significantly affected by the presence of a porous layer. The amplitude of the surface velocity of the panel and the acoustic pressure on the porous surface are reduced largely in the vicinity of structural resonances due to the additional radiation damping and visco-thermal absorption phenomenon in the porous layer. The experimental results suggest that a porous layer between a rigid base and a DML panel can considerably alter its acoustic emission in the near field and in the far field. This is illustrated by a reduction in the level of fluctuations in the emitted acoustic pressure spectra. These fluctuations are normally associated with the interference between the sound emitted by the front surface of the speaker and that emitted from the back. Another contribution comes from the pronounced structural resonances in the surface velocity spectrum. The results of this work suggest that the acoustic boundary conditions near a DML can be modified by the porous layer so that a desired acoustic output can be attained.     

Characterization of snow by acoustic sounding: A feasibility study

The feasibility of characterizing the structural properties of snow from its acoustic behavior is demonstrated. Snow has characteristics similar to many other acoustically porous media. An in situ experimental procedure, which meets the requirement not to disturb the deposited snow sample on the ground during the experiments is devised by means of acoustic sounding. Acoustic pulses incident perpendicularly on the snow surface and their return echos can be analyzed by the cepstral technique. A two-phase mechanical model, similar to that developed by Zarek for acoustically porous media, may also be formulated for snow.     

高声压级时多孔金属板的吸声特性研究

针对高声压级下有限厚度多孔金属板在线性阻抗背衬条件下(背衬表面声压与声质点速度为线性关系)的吸声问题,提出了一个描述不同声压级下材料层法向吸声性能的一维模型,并给出求解材料层内部声质点速度的线化与差分方法,以预测多孔金属板在高声压级下的非线性吸声特性。在阻抗管中对两块多孔金属板进行了声学测试,得到了材料层法向表面阻抗和吸声系数随入射声压级变化的实验结果。研究表明:实验与理论预测符合良好,验证了模型与数值方法的正确性。本文所提原理和方法,可用于一般硬质多孔材料。     

Prediction of the sound field above a patchwork of absorbing materials

The aim of this paper is to investigate the acoustic performance of sound absorbing materials through a numerical wave based prediction technique. The final goal of this work is to get insight into the acoustic behavior of a combination of sound absorbing patches. In order to address a wide frequency range, a model based on the Trefftz approach is adopted. In this approach, the dynamic field variables are expressed in terms of global wave function expansions that satisfy the governing dynamic equations exactly. Therefore, approximation errors are associated only with the boundary conditions of the considered problem. This results in a computationally efficient technique. The main advantage of this method is the fact that the sound absorbing patches do not have to be locally reacting. In this article, the wave based method is described and experimentally validated for the case of normal incidence sound absorption identification in a standing wave tube. Afterwards, the method is applied to simulate some interesting setups of absorbing materials.     

Prediction of airborne sound and impact sound insulation provided by single and multilayer systems using analytical expressions

In this work, the authors use analytical solutions to assess the airborne sound and impact insulation provided by homogeneous partitions that are infinite along their plane. The algorithm uses Green’s functions, derived on the basis of previous work by the authors on the prediction of airborne sound insulation provided by single and double panels. The model is now extended to handle multilayer systems, allowing the simulation of three-dimensional loads applied in both the acoustic and solid media.The model is validated against experimental results and compared with simplified expressions for single, double and triple panels. The results provided by the analytical model were found to provide a good agreement with the experimental results, except in the vicinity of the coincidence effect in the presence of thicker panels.The applicability of the proposed tool is then illustrated by analyzing the acoustic behavior provided by single layers and by a suspended ceiling. Different variables are studied, such as the mass, the stiffness of the layers, the position and direction of the load within the elastic medium and the presence of porous material in the fluid layers. It was found that the model is able to simulate the acoustic phenomena involved in single and multilayer systems.     

Effects of a multi-layered poro-elastic ground on attenuation of acoustic waves and ground vibration

Ground conditions affect the propagation of outdoor sound and vibration. This paper focuses on the interaction between air pressure and porous ground at low frequencies- where mechanisms other than the rigid porous effects used in locally reacting models may be important. A 2-D analytical model has been developed in this study for the calculation of acoustic and acousto-seismic admittances in a multi-layered poro-elastic ground. The model can be used as a prediction tool both for the ground effect on the sound and the generation of ground vibration by the sound. The modelled acoustic admittance is validated successfully against established rigid frame admittance models over a frequency range of 1 Hz-3 kHz. Moreover, the acousto-seismic impedance is verified against full scale airblast field test data measured during the Norwegian Trials full scale field test program. For certain ground types, the predicted acoustic admittance illustrates a different behaviour compared with predictions from the traditional rigid frame acoustic impedance models. This emphasises the importance of including a deformable frame in the model to obtain realistic results for these conditions.     

The sound speed and attenuation in loose and consolidated granular formulations of high alumina cements

Clinkers of high alumina cements are separated into three granular formulations with particle sizes in the range 0.6-0.71 mm, 0.71-1.18 mm and greater than 1.18 mm. These are used to manufacture consolidated samples of porous concrete in an autoclave. The acoustic and microscopic properties of loose and consolidated porous samples of concrete are investigated using both experimental methods and mathematical modelling. Values of porosity, flow resistivity, tortuosity and parameters of the pore size distribution are determined and used to predict closely the sound speed, acoustic attenuation and normal incidence absorption coefficient of these materials. It is shown that high alumina cements do not require additional binders for consolidation and that the structural bonds in these cements are developed quickly between individual clinkers in the presence of water. The hydration product build-up during the consolidation process is insignificant which ensures good acoustic performance of the consolidated samples resulting from a sufficient proportion of the open pores. The value of porosity in the consolidated samples was found to be around 40%, which is close to that measured in some commercial acoustic absorbers. This work provides a foundation for the development of acoustically efficient and structurally robust materials, which can be integrated in environmentally sustainable concrete and masonry structures.     

High-frequency acoustics of 3He in aerogel

High-frequency ( approximately 15 MHz) acoustics were performed on 3He in 98% porous silica aerogel using an acoustic cavity technique. Measurements of the sound attenuation in the normal Fermi liquid and superfluid display behavior quite different from the bulk owing to strong elastic scattering of quasiparticles. The transition from first-to-zero sound is completely obscured with a quasiparticle mean-free path estimated to be in the range of 200-300 nm. No collective mode attenuation peak was observed at or below the superfluid transition.     

多层多孔吸声材料结构参数优化设计

高开孔率的发泡材料(如三聚氰胺、聚氨酯发泡材料)具有优良的吸声、隔热防火、防腐及环保性能,可以作为吸声、阻尼等材料应用于建筑、航空、交通工具等领域.该文基于Biot理论和多层介质声波传播理论(传递矩阵法),建立多层多孔吸声结构背衬刚性壁的理论模型,利用遗传算法优化多层结构厚度和质量.将理论模型计算结果与阻抗管测试结果进...     

非均匀流体介质内部散射声场重建的逐层计算方法*

入射声波激励下非均匀流体介质内部散射声场的重建方法对超声层析成像具有重要意义。以往采用矩量法求解,但该方法全域离散形成的复数满秩矩阵规模随着分辨率与计算精度的提高而急剧增大,对算力具有很高的要求,一定程度上限制了其在实际中的应用。为克服上述缺陷,本文以逐层离散、逐层计算为核心思想,以声散射基本公式与近场声全息理论为基础,推导出逐层计算非均匀流体介质内部散射声场的理论公式并给出对应的几何离散模型。为验证该方法的可行性,以矩量法为参照,对同样的介质模型进行介质内部声场重构仿真。结果表明,逐层算法不仅可以有效地重建非均匀流体介质内部散射声场,且大幅度减小了求解规模。     

Nonlinear wave interactions in porous media filled with a quantum fluid

The problem of the nonlinear interaction between the fourth sound and an acoustic wave propagating in a porous medium filled with superfluid helium is solved. Based on the Landau equations of quantum fluid dynamics and on the Biot theory of mechanical waves in a porous medium, nonlinear wave equations are derived for studying the aforementioned interaction. An expression is obtained for the vertex that determines the excitation of an acoustic wave by two waves of the fourth sound. The possibility of an experimental observation of this process is estimated.     

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。     

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.