基于等效源法和双面质点振速测量的声场分离方法

目前基于等效源法的声场分离方法有两种输入方式, 一种以双测量面上的声压为输入, 另一种以单测量面上的声压和质点振速为输入. 本文以双测量面上的质点振速为输入, 提出一种新的基于等效源法的声场分离方法. 首先给出了该方法的理论推导, 然后通过数值仿真和实验验证了该方法的有效性. 通过与基于双面声压测量的声场分离方法的比较, 证明了该方法在分离质点振速方面的优越性. 此外, 在仿真中还研究了干扰声源强度和测量面间距对分离精度的影响. 关键词： 声场分离 质点振速测量 等效源法 近场声全息     

非自由声场中目标声场还原与重建的等效源方法

为消除在非自由声场中重建声场时干扰声源对重建效果的影响,提出一种采用单个测量面上的声压和质点振速作为输入、等效源法作为分离和重建算法的非自由声场中目标声场还原与重建方法。该方法首先利用单面声压-质点振速测量和基于等效源法的声场分离技术将测量的混合声场分离为来自目标声源的向外传播的声场和来自干扰声源的向内传播的声场,然后利用向内传播的声场和目标声源的边界条件计算出干扰声在目标声源表面产生的散射声场,并将其从向外传播的声场中去除,还原出目标声源在自由声场条件下的辐射声场,最后利用还原的声场实现目标声场重建。通过数值仿真和实验检验了该方法的有效性和必要性。仿真和实验的结果表明,该方法可以在非自由声场的测量条件下,有效地去除干扰声的影响,实现目标声场的准确重建。      

声场匹配波叠加法的水下结构声辐射预报

提出了一种适用于典型结构声辐射预报的声场匹配波叠加方法。该方法利用少量的参考点声压,通过声场匹配搜索等效源分布,得到最小二乘意义下的最优等效源位置。并研究了表面振动测点位置、数目及振动分布的离散性对声场预报精度的影响。最后在半空间消声水池中,对两端带帽圆柱壳的声辐射预报进行了试验验证。结果表明:等效源最佳位置一经确定,即可利用结构的表面振速,对不同激励下的该结构进行声辐射预报。该方法在较宽频段内对不同的振动分布有较好的适应性。      

利用源强密度声辐射模态重建声场

为了利用声场中少量测点声压数据精确重建复杂结构的辐射声场,提出了源强密度声辐射模态分析理论和声场重建公式.在结构表面定义的空间上,利用以源强密度分布函数为参量的结构辐射声功率泛函表达式定义了一个线性自伴正辐射算子,该算子的特征函数为结构的源强密度声辐射模态.然后通过对矩形平板和带有半球帽的圆柱体的源强密度声辐射模态的分析,证明了源强密度声辐射模态具有空间滤波特性,并利用该性质建立了声场重建公式.球体仿真和平板实验验证了所提出的声场重建方法的可行性和稳健性.基于源强密度声辐射模态的声场重建方法简单,利用较少测点数据就可以获得较高的声场重建精度,特别适合于复杂结构的低频声场重建.     

基于声压-振速测量的平面近场声全息实验研究

常规的近场声全息均是采用全息面声压或质点振速作为输入量求解,由于采用单一输入量无法分离来自全息面背向声波的干扰,因此要求所有声源均位于全息面的同一侧,即测量声场为自由声场,这种要求大大限制了近场声全息的实际应用.基于声压-速度测量的近场声全息以全息面上声压和质点振速同时作为输入量,通过建立和求解两侧声源在全息面上的声压和质点振速耦合关系,可以实现全息面两侧声波的分离,从而解决上述问题.文中在前期对声场分离技术研究的基础上,基于欧拉公式和有限差分近似,推导了新的基于声压-速度测量的平面近场声全息理论公式.随后通过实验检验了该方法在有背景源干扰情况下实现声场分离和重建的有效性.     

基于双面质点振速测量的声场分离技术

现有的声场分离技术或是基于双面声压测量或是基于单面声压和质点振速测量。本文以二维空间傅里叶变换法作为声场分离算法,提出一种基于双面质点振速测量的声场分离技术。论文首先推导出了该方法的理论模型,随后通过仿真研究验证了该方法的有效性,并通过与基于双面声压测量的声场分离方法的比较说明了其优越性。结果表明:采用基于双面质点振速测量的声场分离方法可以有效实现声场分离;基于双面质点振速测量的声场分离方法可以获得更高的质点振速分离精度,而基于双面声压测量的声场分离方法可以获得更高的声压分离精度。      

一种辐射声场近似计算方法——单元辐射叠加法

提出一种基于表面振速预报辐射声场的近似方法--单元辐射叠加法.研究表明,声传递向量中的每项元素等于对应单元以单位速度振动、其它单元振速都为零时的辐射声压,即刚性障板上活塞面以单位速度振动时的辐射声压.在此基础上,利用刚性障板上单位速度振动活塞面的辐射声压直接建立表面振速与辐射声压之间的传递关系,根据这个传递关系对表面振速进行加权、求和便得到总的辐射声压.计算时采用规则形状障板面去拟合实际障板面,以规则形状障板上振动活塞面的辐射声压去近似实际障板上振动活塞面的辐射声压.相对于边界元方法,该方法在计算速度与存储空间上具有很大的优势.计算表明该近似方法是合理的、可行的.     

考虑任意阻抗壁面条件管腔结构声场特性分析

针对任意阻抗壁面条件一维管腔声学系统建模,对系统动力学特性进行预报。为了满足阻抗边界条件对声压一阶导数连续性要求,管腔声压函数通过在标准傅里叶级数端点位置引入边界光滑辅助多项式进行构建。结合壁面阻抗声学边界和管腔声学Helmholtz控制微分方程得到强形式标准特征值问题,获得相应的声学模态信息。在数值仿真中,通过算例给出各种边界条件下管腔声学模态频率、声压振型、声压和质点振速频率响应曲线,与现有文献中相关结果进行对比,充分验证了本文求解方法的正确性和有效性,证明该方法可对任意阻抗壁面条件管腔系统声学特性进行准确预报。     

基于单元辐射叠加法的结构声源声场重建方法

为了提高分布式结构声源的声场重建精度,本文提出了基于单元辐射叠加法的结构声源声场重建方法.该方法首先利用声场叠加原理和结构振声传递特性,建立了结构声源表面振动与辐射声场之间的振声传递关系解析表达式,得到便于快速计算的振声传递矩阵,能够解决连续分布、相干结构噪声源的声传播模型精细化表征问题.然后利用振声传递矩阵作为传递算子进行声场重建,并与迭代加权算法相结合.通过将基于单元辐射叠加法的声场预报结果与解析法预报结果进行比较,验证了单元辐射叠加法具有较高的准确性.并将基于单元辐射叠加法的声场重建方法与传统等效源法近场声全息和迭代加权等效源法相比较,通过仿真分析与矩形板声场重建实验证明了基于单元辐射叠加法的声场重建方法能够改善结构声源的声场重建精度并增大近场声全息的有效测试距离范围.     

波叠加声场重构精度的影响因素分析

波叠加法是利用等效源的思想来求解声辐射问题的一种数值方法.其原理并没有确切地规定如何配置等效源.正由于这些不确切性的存在,当将它应用到声场重构时,有必要检验重构的准确性,分析影响重构精度的各种因素,目的是通过分析而获得一些提高重构精度的规律.通过理论分析和数值仿真,分析了各因素对重建准确性的影响.证明等效源必须配置在辐射体内部,而且最好能与辐射体表面共形;但测量面并不需要与辐射体表面共形,而且也不需要在近场测量;还发现,Tikhonov和TSVD正则化方法很适合与波叠加法结合用于声场重构.依据这些规律能更好地应用波叠加法进行声场重构,也将拓宽其应用范围. 关键词： 声场重构 波叠加法 误差影响因素 正则化方法     

Equivalent source method for identification of panel acoustic contribution in irregular enclosed sound field

For the interior sound field formed by the complex vibrating structure,an identification approach of panel acoustic contribution based on equivalent source method(ESM)was presented.The normal velocity on the surface of vibrating structure was first reconstructed by using interior nearfield acoustic holography based on ESM and the prediction of whole interior enclosed sound field was realized.Then the sound pressure produced by each panel at the interested field point was respectively replaced by the radiated pressure of the enclosed interior sound field which is formed by the equivalent virtual sources located near the surface of the cavity.Combining with the reconstructed normal surface velocity,the acoustic contribution of each panel to any position in the cavity was obtained by transforming the complex enclosed non-free field into the simple interior free field.Numerical simulations and experiments are conducted,and the influences of the number of the equivalent sources and the distance between them and the reconstructed surface have been investigated.The results show that the proposed method is easier to be implemented with the same accuracy than the traditional analysis method.     

Study of the comparison of the methods of equivalent sources and boundary element methods for near-field acoustic holography

Boundary element methods (BEM) based near-field acoustic holography (NAH) has been used successfully in order to reconstruct the normal velocity on an arbitrarily shaped structure surface from measurements of the pressure field on a nearby conformal surface. An alternative approach for this reconstruction on a general structure utilizes the equivalent sources method (ESM). In ESM the acoustic field is represented by a set of point sources located over a surface that is close to the structure surface. This approach is attractive mainly for its simplicity of implementation and speed. In this work ESM as an approximation of BEM based NAH is studied and the necessary conditions for the successful application of this approach in NAH is discussed. A cylindrical fuselage surface excited by a point force as an example to validate the results is used.     

封闭空间声场重构的多层等效源法

对于封闭空间内的多途反射声,传统的等效声源法将其等效为距离边界一定距离的单层等效声源体进行声场重构,然而等效源与边界的距离选取依据不确定。因此,为获得等效声源配置的最优距离,在等效声源法(ESM)的基础上构建多层等效声源,提出一种适用于封闭环境声场重构的多层等效声源法(MESM),并依据等效声源的空间分布的稀疏性来获得等效声源强度信息。首先给出多层等效源法的理论依据,其次通过数值计算以及实验测试两种方式对比验证了所提方法。数值结果表明:MESM相比于ESM可在600 Hz以上频段获得低5~10 dB左右的重构误差,但是200 Hz以下的低频重构误差会增加5 dB左右。实验结果表明:MESM可比ESM获得更低的重构误差。文章最后基于数值计算研究了所提方法的主要影响因素。研究表明:虽然MESM会比ESM耗费2倍的计算时间,但在整体频率范围内,MESM可在ESM基础上提升600 Hz以上的重构性能。另外,等效声源的层数和层内数目的改变不会影响声场重构性能,而当传声器数目较多、阵列位置随机、空间边界的吸声系数不是很大时,MESM可获得比ESM更低重构误差,特别是600 Hz以上的中频段区间。      

基于近场声全息的圆柱壳体内部声场重构效果的影响因素

在Neumann边界条件下基于近场声全息原理(NAH)重构了两端封闭的有限长圆柱壳体内部声场,计算出的结果与基于模态叠加法构建的内部声场进行比较,证明了重构方法的正确性,然后分别讨论了激励频率和全息面选择对NAH重构声场精度的影响以及重构面上不同位置的重构效果。     

Automotive panel noise contribution modeling based on finite element and measured structural-acoustic spectra

The radiated noise contributions of automotive body panels to the interior sound pressure levels are modeled using an approximate spectral formulation that applies the theoretical interior acoustic sensitivity terms derived from a finite element model and measured spatial-averaged structural-acoustic spectra. The finite element calculation is validated by comparison to a set of experimental acoustic transfer functions. A measurement set-up for the sound intensity and structural-acoustic response is applied to acquire the cross and auto power spectra needed to predict the relative mean-squared velocity term of each control plane near the panel surface, and to obtain the individual panel contribution function. The proposed approach also computes the noise spectra in 1/12 octave band form at selected positions in the passenger compartment, which matches well with the overall experimental results. Through an actual passenger car application, the approximate computational scheme is proven to be generally quite robust and effective for analyzing higher frequency interior noise problems.     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

Structure normal velocity reconstruction withsparse measurement points

To achieve normal velocity reconstruction of a vibrating surface with sparse measurement points, a reconstruction method is proposed by exploiting of acoustic radiation modes as expansion functions, which are capable of describing the geometric shape of a vibrating surface. Firstly, acoustic radiation modes of the vibrating surface are calculated and the relationship between normal velocity and acoustic radiation modes is built. Then actual measured normal velocity values are expressed by corresponding acoustic radiation modes and the expansion coefficients are calculated. Subsequently, all normal velocity values can be reconstructed by the obtained expansion coefficients. Experimental validations have been performed by a double-layer steel cylindrical shell with enclosed ends in.an anechoic water tank. Two cases with different wavenumber components distribution were designed by a vibration shaker and a rotor device respectively. Two experimental results both show that actual vibration distribution cannot be revealed exactly by the sparse measurement points, which corresponds to severe loss of vibration related wavenumber components. On the other hand, normal velocity and corresponding wavenumber components can be restored accurately in both two wavenumber components distribution cases according to the proposed method, which demonstrates obvious effectiveness of the proposed method.     

稀疏测点条件下的结构法向速度重建

为了实现稀疏测点条件下结构表面法向速度的准确重建,利用声辐射模态包含结构表面几何形状信息的性质,以声辐射模态作为基函数提出了一种稀疏测点条件下的结构法向振速重建方法。首先对结构表面的声辐射模态进行计算,并建立结构表面法向振动速度与声辐射模态之间的关系;在此基础上,由实际布置情况形成测点位置处的振速与其声辐射模态值的关系,并通过最小二乘法求得展开系数;最后由展开系数重建出结构表面的全部法向振速。利用两端封闭的双层钢质圆柱壳体在消声水池中进行了试验验证,分别开启激振器和转子台进行激励,两个试验的结果均表明,当测点数目较少时,所测的结果不能准确地表示结构的实际振动情况,在波数域内就表现为与振动相关的波数成份的丢失;利用所提出的方法,可以较为准确地重建和恢复结构的表面法向速度及其波数成份,由此验证了所提出方法的有效性。      

Vibro-acoustic analysis of a rectangular-like cavity with a tilted wall

In this paper, a fully coupled vibro-acoustic model is developed to characterize the structural and acoustic coupling of a flexible panel backed by a rectangular-like cavity with a slight geometrical distortion, which is introduced through a tilted wall. The combined integro-modal approach is used to handle the acoustic pressure inside the irregular-shaped cavity. Based on the model proposed, the distortion effect on the vibro-acoustic behavior of the coupled system is investigated using the averaged sound pressure level inside the enclosure and the averaged quadratic velocity of the vibrating plate. Simulations are conducted to examine the distortion effect on acoustic natural frequencies, acoustic pressures and structural responses. Effects of the wall inclination on coupling coefficients are also assessed, and an index is proposed to quantify the degree of variation of coupling strength.     

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.