空腔非局域反应声衬传播特性研究

本文对声波在空腔非局域反应声衬管道内的传播进行了研究。特征方程通过声衬内和管道内两部分声场的耦合求解得到,并采用积分方法对特征方程积分求解,通过模态匹配的方法建立并求解了有限长管道非局域反应声衬的声辐射数值模型,展示了在管道消声主动控制方面的应用潜力。     

利用声辐射模态重构任意目标的散射声场

水下目标散射声场的重构可以作为水下目标散射特性的研究基础。本文主要利用声辐射模态对水下目标进行散射声场重构研究。首先,在借助声传递矩阵给出的任意结构声辐射模态的流体域求解方法基础上,通过理论证明了目标的散射声压与声辐射模态具有函数关系。其次,借助声场分布模态的概念,同时考虑到声场分布模态病态及声压测量易受噪声污染,提出基于声辐射模态的正则化散射声场重构算法。仿真结果表明,波数越低,重构所需声辐射模态阶数越少,在较高波数时仅需总模态数的大约20%即可对声场进行重构。与基于边界元的声场重构算法相比,计算量减小了至少80%,且克服了赫姆霍兹积分方程最小二乘法仅对球壳结构的重构效果较好而不适用于长条形结构重构的缺陷。     

计入地面附加衰减的声屏障插入损失估算方法*

基于理论推导和计算,给出了公路声屏障声学设计中,在考虑地面附加衰减情况下计算插入损失的方法。该方法综合考虑了有限长线声源无限长声屏障绕射声衰减量、有限长线声源地面衰减量及遮蔽角对插入损失的影响。通过与《声屏障声学设计和测量规范》（HJ/T90-2004）的计算结果的对比,验证了本文所给方法的精确性及可行性,并对规范所给地面衰减修正量进行了商榷。最后,给出了当预测点位于有限长路段中央法线上时,通过计算线声源地面衰减量得到计算插入损失所需参数值,再计算插入损失的简便方法。本研究为存在地面附加衰减情况下有限长声屏障插入损失计算提供了一个新的参考方法。     

计算流管声场的三维有限元数值模型

发展了一种三维有限元数值模型和计算方法来对矩形流管声场进行整体的计算.与以往的二维方法相比,此种数值方法不仅全面反映了矩形流管内声波的传播情况,而且提高了网格精度,从而大大扩展了对铺设有声衬的流管的计算领域.结果表明,该数值模型是有效和准确的,与其它方法和文献的计算结果吻合得非常好.同时,在大大增加计算量的同时,也对程序代码进行了优化工作,提高了计算效率.     

半自由声场的全息重建和预测实验研究

半自由声场环境下的声源重建和声场预测研究对声全息技术走向实际应用具有非常重要的意义.在提出基于分布源边界点法的半自由声场全息重建和预测方法的基础上，对此展开了实验研究.并将重建和预测的结果与常规方法重建和预测的结果进行了比较和讨论，说明了重建预测过程中反射声压的影响和考虑反射声压的必要性，证明了所提出方法在解决半自由声场环境下存在地面反射时的声源重建和声场预测时的有效性和准确性.还提出了采用奇异值截断滤波和Tikhonov正则化方法来削弱测量误差的影响，从而进一步优化了重建结果，提高了全息成像的可信度. 关键词： 声全息 半自由场 边界点 声辐射 反射声     

基于瑞利积分叠加法的聚焦换能器声场特性研究

基于经典的瑞利积分,提出考虑非线性传播、各次谐波衰减的声场瑞利积分线性叠加算法.分析其原理,并以凹球面聚焦换能器为例,采用该算法研究媒质衰减和非线性传播特性影响声焦域的规律.并与Khokhov-Zabolotskaya-Kuznesov(KZK)数值算法,以及实验结果进行比较,验证瑞利积分线性叠加算法描述高强度聚焦超声(HIFU)声场的有效性.     

聚焦区域高声强声场分布的测量方法研究

常用的声场分布测量是采用水听器扫描声场的方法,该方法对于声能量密度较大的声场难以测量,因为在这种情况下声振幅比较大,水听器在这种声场中呈现非线性或遭到破坏。设计了一种用辐射压力测量高声强声场分布的方法,该方法利用一根微细管,直接测量声场的冲流压力,通过对声场进行扫描测量可以得到高声强声场压力分布。从理论上分析了这种测量方法的可行性,对测量基本要求及实验装置做了阐述。实验结果证实:该方法可以用来测量高声能密度声场压力分布;测量结果与水听器测量结果基本吻合;测量方法存在测量盲区。     

基于声粒子分布积分的无网格声场计算方法

小尺度封闭空间内部声场的数值计算是声学设计、噪声控制等领域的关键技术。由于波动声学及几何声学方法计算频率上的限制,中频段声场计算问题一直是个难点。本文以声学无网格法为基础,提出了一种基于声粒子分布积分的无网格声场数值计算方法。文中利用声线跟踪理论计算声场中的声粒子分布,并以某个时间点上的声粒子作为蒙特卡罗法中的积分点,将其应用于无网格法中,从而获得声场中的节点声压。利用该方法对一个矩形封闭空间的中低频声场进行了计算,并与模态叠加法、商用声场计算软件、经典无网格法的结果进行了对比,证明基于声粒子分布积分的无网格声场数值计算方法在中低频段相较于传统基于网格的方法具有更高的精度。     

有限液态介质样品声衰减的实验室测试新方法

本文介绍了一种在实验室条件下利用声脉冲测定有限容量液态介质样品声衰减系数的测试方法。该方法根据声纳方程原理导出了与测试系统参数和声场参数无关的计算式，从而避开了对测试系统参数和声场参数的测量，采用截取未被边界散射干扰的直达声脉冲的部分信号进行频谱分析，消除了在有限样品情况下必然存在的边界散射干扰，同时采用多次统计平均的做法抑制了噪声干扰。     

适用于任意形状封闭空间的声场预测相干模型

针对现有几何声学的方法对封闭空间内声场进行预测时在中低频段出现较大误差的问题,该文提出一种近似圆锥声束追踪法和相干反射场理论相结合的声场预测新模型。在近似圆锥声束追踪法基础上,考虑声束轴线在边界多次反射时声压和相位的改变,最后计算不同声波之间的干涉效应,建立一种适用于任意形状封闭空间的声场预测相干模型。利用该模型对某一矩形封闭空间进行声场预测,通过对边界元法、Raynoise软件相干和非相干算法的预测结果和本模型的数值模拟结果对比。结果表明,文中提出的方法和边界元法的计算结果在中低频段非常吻合,两者的计算结果平均绝对误差为1.1 d B。本模型在中低频率下与同样考虑了相位的Raynoise相干算法相比有更好的准确性,在较高频率上,本模型计算结果与Raynoise相干算法计算结果非常吻合。     

Predicting insertion loss of large duct systems above the plane wave cutoff frequency

If the dimensions of a silencer or muffler component are small compared to an acoustic wavelength, plane wave propagation can be assumed. This is not the case for HVAC (heating, ventilation, and air conditioning) duct systems, and large diesel engine mufflers commonly used in ship and generator sets. For such applications, the wave behavior in the inlet and outlet ducts is three-dimensional. In this paper, the finite element method is utilized to simulate large duct systems with an aim to predict the insertion loss. The boundary condition on the source side is a diffuse field applied by determining a suitable cross-spectral force matrix of the excitation. At the termination, the radiation impedance is calculated utilizing a wavelet algorithm. Simulation results are compared to published measurement results for HVAC plenums and demonstrate good agreement.     

Comparison of measured broadband noise attenuation spectra from circular flow ducts and from lined engine intakes with predictions

Sound propagation in lined circular ducts is investigated in the presence of uniform and sheared flow. The modal solutions are obtained by solving an eigenvalue equation which, in the case of sheared flow, is derived by using finite differences and by matching the pressure and the radial component of the particle velocity at the interface of the regions of uniform and sheared flow. For the uniform flow region, standard Bessel function solutions are used. The attenuation of acoustic energy at a given frequency and for a given liner length is computed on the assumption that at the inlet to the lined duct, the acoustic energy is equally distributed among the propagating modes. The total number of propagating modes is determined from the hard wall “cut off” condition. The failure to find some of the modal solutions on the attenuation computed in this way is discussed. It is shown that the reliability of this method of computing liner attenuation depends on the ability to successfully compute most of the modal solutions over a large range of frequencies, flow conditions and duct wall impedance values. A numerical technique is developed which uses a fraction of the total number of solutions to compute the total attenuations without appreciable loss of accuracy. Measured attenuation spectra from a flow duct facility and from lined intake ducts of the RB.211 engine are compared with predictions. In general very good agreement between predictions and measurements is obtained.     

Sound transmission across duct constrictions with and without tapered sections

The sound power transmission loss across duct constrictions with linearly tapered sections is studied with the finite element method. Results show that the acoustic energy distributions of transmitted waves at high frequency depend critically on the exit configuration of the constriction. The corresponding strengths of these waves are very much affected by the entrance setup of the constriction. The difference between inlet and outlet impedance of a constriction leads to weaker resonant sound transmission.     

Sound transmission across a smooth nonuniform section in an infinitely long duct

Sound transmission across a nonuniform section in an infinite duct is studied numerically using the finite element method. An impedance matched absorptive portion is added to each end of the computational domain so as to avoid the undesirable higher mode reflection that will otherwise take place there. Results suggest that the sound fields downstream of the nonuniform section inlet are complicated and cannot be easily described by the conventional solution of the wave equation. The distribution of acoustic energy among the various propagating modes well downstream from the outlet of the nonuniform section is also discussed. Results show that the first symmetrical higher mode is important for all cases. The plane wave becomes important at high frequency with high rate of change of the cross-sectional area when the section is a convergent one.     

Effect of flow on quasi-one-dimensional acoustic wave propagation in a variable area duct of finite length

The general equation for the velocity potential of quasi-one-dimensional acoustic wave motion in a variable area, finite duct with one-dimensional flow is derived by using a perturbation technique. The non-linear second-order partial differential equation is linearized and then solved, by either a power series expansion method or the Runge-Kutta fourth-order method, for harmonic time dependence. The boundary condition taken at the duct mouth is that of matching the impedance of the duct sound field to that of the radiation field at the duct opening. Three axial Mach number variations along the duct axis are considered and the results obtained are compared with those for the case of constant Mach number, to determine the influence of the axial velocity gradient on sound propagation. The effect of flow on the radiation impedance is also considered.     

插管结构对膨胀腔消声器声学性能的影响

将数值模态匹配法(NMM)拓展应用于计算和分析外插管膨胀腔消声器的声学性能,推导了相应的理论公式并编写了计算程序。使用二维有限元法提取横向波数和本征向量,应用模态匹配法计算消声器的传递损失。使用数值模态匹配法和三维有限元法(FEM)研究了插管长度和进出口位置对带有外插进出口管椭圆形非同轴膨胀腔消声器声学性能的影响,两种方法计算结果吻合良好,从而验证了本文数值模态匹配法的正确性。研究结果表明,设置特定的插管长度和进出口位置可以消除消声器的通过频率,进而改善消声器中低频的消声性能。      

Acoustic radiation impedance of duct-nozzle system

A method is described to evaluate the radiation impedance spectra of a duct-nozzle system with and without mean flow by using measured reflection coefficient data. In this method the impedance at the junction of the duct and nozzle is first evaluated by using complex reflection coefficient data measured experimentally with an impulse technique. This impedance is then transferred to the nozzle exit by using a solution of the wave equation appropriate for the duct-nozzle system. The application of this method is described and results are presented to show the effect of nozzle geometry and the effect of mean flow on the radiation impedance of the duct-nozzle system. The results derived by using this method are compared with the similar results derived by using some approximate methods.     

Thermal effect on the acoustic behavior of an axisymmetric lined duct

This paper deals with the effect of the temperature and the frequency on the acoustic behavior of lined duct partially treated with usual material used in acoustic insulation.First, the effect of frequencies and temperature on the acoustic impedance of usual materials used in lined duct such as glass or rock wools in order to reduce acoustic level is investigated.Secondly, the variational formulation of the acoustic duct problem taking into account velocity and temperature effects is established. Then, a numerical model is derived which permits to compute the reflection and the transmission coefficients of such duct for different temperatures and several flow velocities. The acoustic power attenuation is then computed from these coefficients and the effect of the temperature and flow velocities on this energetic quantity is evaluated.The numerical results are obtained for three configurations of a lined duct treated for different temperature ranges and several velocities. Numerical coefficients of transmission and reflection as well as the acoustic power attenuation show the relative influence of temperature.     

Effect of Mean Flow on Acoustic Wave Propagation in a Duct with a Periodic Array of Helmholtz Resonators

Sound propagation properties of a duct system with Helmholtz resonators(HRs) are affected by mean flow.Previous studies have tended to focus on the effects of mean flows on acoustic response of a duct system with a finite number of HRs. Employing an empirical impedance model, we present a modified transfer matrix method for studying the effect of mean flow on the complex band structure of an air duct system with an infinite periodic array of HRs. The efficiency of the modified transfer matrix is demonstrated by comparison between an example of transmission response calculation for a finite single HR loaded duct and the finite element simulation result calculated using the COMSOL software. Numerical results are presented to analyze the effect of mean flow on the band structure and transmission loss of the sound wave in the duct system. It is hoped that this study will provide theoretical guidance for acoustic wave propagation of HR silencer in the presence of mean flow.     

On the acoustic modes in a cylindrical duct with an arbitrary wall impedance distribution

The present paper considers the propagation of sound in a cylindrical duct, with a wall section of finite length covered by an acoustic liner whose impedance is an arbitrary function of position. The cases of (i) uniform wall impedance, and wall impedance varying along the (ii) circumference or (iii) axis of the duct, or (iv) both simultaneously, are explicitly considered. It is shown that a nonuniform wall impedance couples modes with distinct azimuthal l or axial m wave numbers, so that their radial wave numbers k can no longer be calculated separately for each pair (m,l). The radial wave numbers are the roots of an infinite determinant, in the case when the wall impedance varies either (i) circumferentially or (ii) radially. If the wall impedance varies (iv) both radially and circumferentially, then the radial wave numbers are the roots of a doubly infinite determinant, i.e., an infinite determinant in which each term is an infinite determinant. The infinite determinants specifying the radial wave numbers are written explicitly for sound in a cylindrical nozzle with a uniform axial flow, in which case the radial eigenfunctions are Bessel functions; the method of calculation of the radial wave numbers applies equally well to a cylindrical nozzle with shear flow and/or swirling flows, with the Bessel functions replaced by other eigenfunctions. The radial wave numbers are calculated by truncation of the infinite determinants, for several values of the aspect ratio, defined as the ratio of length to diameter. It is shown that a nonuniform wall impedance will give rise to additional modes compared with a uniform wall impedance. The radial wave numbers specify the eigenfrequencies for the acoustic modes in the duct; the imaginary parts of the eigenfrequencies specify the decay of the sound field with time, and thus the effectiveness of the acoustic liner.