穿孔板的声学厚度修正

使用有限元法计算穿孔板的声学厚度修正系数,研究穿孔率、穿孔板厚度、孔径和穿孔排列形式对声学厚度的影响,获得了穿孔率低于40%的穿孔板的声学厚度修正系数.计算结果表明:穿孔板厚度、孔径和排列形式对声学厚度的影响不大;穿孔率对声学厚度影响很大.基于数值计算结果给出了声学厚度修正系数的近似表达式,利用由该表达式形成的穿孔声阻抗公式计算了穿孔管消声器的传递损失,计算结果与实验结果吻合良好,从而验证了应用该表达式预测穿孔管消声器消声性能的准确性.     

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。      

阻抗复合消声器声学性能有限元预测方法及分析

为计算和分析具有复杂结构的阻抗复合式消声器的宽频消声性能,建立了一种高效声学有限元方法,给出了不同边界条件下的边界积分处理细节,得到有限元全局系数矩阵表达式,设计出计算程序框架以实现这些算法,其求解规模和计算速度与商业软件相比有优势。为计算阻抗复合式消声器的传递损失,通过阻抗管测量和数据拟合得到了吸声材料声学特性的经验公式。计算和测量了两通穿孔阻抗复合式消声器的传递损失,二者良好的吻合验证了声学有限元方法和计算程序的正确性。研究表明,插管长度影响消声器在中高频段的消声特性,右侧隔板上穿孔会消除共振峰,中高频消声性能随着出口管穿孔率的增加而提升。      

直通穿孔管消声器声学特性预测的数值模态匹配法

将数值模态匹配法应用于计算横截面为任意形状的直通穿孔管抗性消声器的声学特性。应用二维有限元法计算横截面的本征值和本征向量,应用模态匹配技术求解模态幅值系数,进而得到所需的声学量。对于圆形和椭圆形直通穿孔管消声器的传递损失,数值模态匹配法计算结果与三维有限元计算结果和相应的实验测量结果吻合良好,表明数值模态匹配法能够精确计算直通穿孔管消声器的声学特性。计算结果表明,穿孔管的偏移影响消声器在中高频段的消声特性,同轴结构消声器的消声性能好于非同轴结构。      

膨胀腔消声器声学仿真的一维修正方法

传统一维方法计算消声器声学性能随频率增加误差加大,本文应用管道末端修正系数改善一维方法的计算结果.应用二维轴对称解析方法计算管道末端修正系数,研究结构因素和频率对修正系数的影响规律,研究表明修正系数随截面半径比增加而降低,随频率增加而增加,超过截止频率后修正系数无效.根据计算结果给出修正系数拟合公式并修正传统一维方法,应用一维修正方法计算膨胀腔消声器的传递损失并和实验结果、三维有限元结果进行比较,证明一维修正方法结果精度有明显提高.     

声波导管对圆柱型共鸣腔声场扰动的研究

声学共鸣法是目前测量流体热物性、热力学温度和玻尔兹曼常数最精确的方法之一,声波导管为共鸣腔提供声源和气体进入口,但是会破坏共鸣腔的理想表面从而导致腔体内气体介质共鸣频率的偏移(△f)和共鸣峰半宽的增加(g)。本文从一阶声学微扰出发建立了声波导管对共鸣腔声场的扰动模型,分析了导管位置、长度和半径大小对腔体中介质共振频率的影响。进一步测量了52～1763 mm之间六个不同长度的导管在T=332 K和p=50～500 kPa时,圆柱共鸣腔中轴向共鸣模式的共鸣频率和半宽的变化,测量结果与理论计算值吻合较好,证明了理论模型的正确性。     

圆柱声学共鸣腔边界层扰动的二阶修正模型

圆柱声学共鸣法是测量Boltzmann常数和研究声学温度计的重要方法.热边界层和黏性边界层是影响圆柱声学共振频率的最主要因素。目前已有的声学一阶微扰理论已不能满足圆柱声学共鸣法精确测量Boltzmann常数的需求(不确定度小于1×10~(-6)).本文建立了基于声学二阶微扰理论的边界层扰动修正模型,计算结果表明,与一阶修正相比,二阶修正不影响圆柱声学腔的共振频率,但对频率半宽产生不可忽略的影响,且随着圆柱腔内的声学共振模式而变化,压力越低影响越大.对于长度80 mm、半径40 mm的圆柱腔,在273.16 K、50 kPa,二阶修正对Ar的(2,0,0)频率半宽的影响接近7×10~(-6).采用二阶修正模型,更符合真实物理规律,满足精确测量的需求.     

均匀流直通穿孔消声器的声学特性分析

将数值模态匹配法(NMM)扩展应用于计算有均匀流存在时直通穿孔管抗性和阻性消声器的声学特性,编写了相应的计算程序。对于圆形同轴穿孔管抗性和阻性消声器,应用数值模态匹配法计算得到的传递损失结果与实验测量结果吻合良好,从而验证了计算方法和计算程序的正确性。进而应用数值模态匹配法研究了运流效应和穿孔阻抗以及穿孔管偏移对穿孔管抗性和阻性消声器传递损失的影响。研究结果表明,马赫数越高,穿孔管抗性消声器在中高频的消声量越高,阻性消声器在整体频段内的消声性能越差;低马赫数时运流效应对穿孔管抗性消声器的影响可以忽略,马赫数较高时运流效应和穿孔阻抗的影响比较明显;对于穿孔管阻性消声器,穿孔阻抗对消声器声学特性的影响比运流效应的影响小,但是与真实值的差别不可忽略;穿孔管偏移对消声器声学特性的影响与频率和消声器结构均相关。      

均匀流直通穿孔消声器的声学特性分析

将数值模态匹配法(NMM)扩展应用于计算有均匀流存在时直通穿孔管抗性和阻性消声器的声学特性,编写了相应的计算程序。对于圆形同轴穿孔管抗性和阻性消声器,应用数值模态匹配法计算得到的传递损失结果与实验测量结果吻合良好,从而验证了计算方法和计算程序的正确性。进而应用数值模态匹配法研究了运流效应和穿孔阻抗以及穿孔管偏移对穿孔管抗性和阻性消声器传递损失的影响。研究结果表明,马赫数越高,穿孔管抗性消声器在中高频的消声量越高,阻性消声器在整体频段内的消声性能越差;低马赫数时运流效应对穿孔管抗性消声器的影响可以忽略,马赫数较高时运流效应和穿孔阻抗的影响比较明显;对于穿孔管阻性消声器,穿孔阻抗对消声器声学特性的影响比运流效应的影响小,但是与真实值的差别不可忽略;穿孔管偏移对消声器声学特性的影响与频率和消声器结构均相关。     

薄纤维层对穿孔板的声学影响研究

为了提高附着薄纤维层穿孔板的声学预测精度,通过实验及理论分析方法研究薄纤维层对穿孔板的声学影响。采用声学阻抗管系统测试穿孔饰面板及背腔的吸声系数。测试结果显示,薄纤维层对吸声层的吸声性能具有明显的提高作用。以穿孔板声阻抗模型为基础构建附着薄纤维层的穿孔饰面板等效声阻抗模型,以附加修正参数等效薄纤维层的声学影响。依据吸声系数测试结果计算薄纤维层的等效声学修正参数,结果显示薄纤维层造成的附加阻性修正比较显著。     

基于低频吸声超构材料的复合消声器研究*

针对管道内低频噪声难以抑制的问题，本文基于亥姆霍兹共振腔（HR）阵列吸声板和穿孔管消声器组合，设计了一种复合式宽带消声器。首先利用有限元法仿真分析传统穿孔管消声器，发现中低频消声能力较差，通过嵌入HR阵列吸声板吸收中低频噪声。采用仿真与实验的方式研究吸声板的声学性能：在400-1000 Hz频段内的平均吸声系数达到了0.88。然后对复合式消声器进行数值模拟及3D打印阻抗管实验测试对比：复合式消声器在400-1718Hz频率范围内的平均传递损失为18.15 dB ，最终实现了管道内全频带噪声有效控制。     

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.      

含有侧孔阵列圆柱管的发声频谱

利用末端导纳匹配方法和格林函数方法,对含有侧孔的律管进行了声学谱分析,得到了均匀侧孔长管在两个音阶上的共振频谱。发现了随有效管长的均匀递减,其发声频率的递增是不均匀的。主要共振效应发生在离吹孔最近的侧孔处,随着共振频率的升高,气流更加集中地在第一侧孔辐射能量,次共振几乎可以忽略。附管上的侧孔对共振频率影响甚微,但是对音色、音强以及各侧孔的透射率都有一定的影响。该理论给出的共振频率与数值模拟结果符合地很好,第一音阶和第二音阶的平均偏差分别为5.03%和2.23%。由此认为末端导纳匹配方法适用于各类含有侧孔的管状发声物声学特性的研究。      

Experimental investigation of holes interaction effect on the sound absorption coefficient of micro-perforated panels under high and medium sound levels

This paper experimentally investigates the holes interaction effect on the sound absorption coefficient of micro-perforated panels under high and medium sound levels. The theoretical formulations are based on a semi-empirical approach and the use of Fok’s function to model the acoustic surface impedance. For the high sound level regime, an empirical power law involving three coefficients is adapted. It is shown theoretically and experimentally that these coefficients can lead to optimized absorption performance and particularly, a formula relating the critical Reynolds number (Reynolds number value after which the absorption coefficient decreases with the increase of sound level) and the center-to-center distance between the perforations is derived. It is demonstrated that the first coefficient of the nonlinear acoustic resistance strongly depends on the separation distance between the apertures and decreases with a decrease of this latter distance. Analysis of the data reveals the fact that even with Holes Interaction Effect (HIE), the nonlinear reactance dependence on velocity is still very low compared to the resistance-velocity dependence. Four perforated panels of 1.5 mm thickness with different separation distances between the holes (from widely to closely separation) were built and tested. Experimental results performed with an impedance tube are compared with the described model for HIE. To test the dependence of the coefficients on frequency, the experiments are carried out for two different excitation frequencies (292 Hz and 506 Hz). The results can be used for designing optimal perforated panels for ducts, silencers and for the automotive industry.     

Reduction of acoustic radiation by impedance control with a perforated absorber system

A model of sound radiation from an infinite plate with an absorptive facing is proposed and investigated theoretically from the viewpoint of acoustic power. Acoustic characteristics on the plate surface are represented by impedance derived from iso-absorption curves. A parametric study is carried out to clarify the effect of the impedance on the acoustic power. Results derived from this model show that acoustic radiation depends on change in impedance as well as the absorption coefficient, and there is a possibility of reducing the radiation from vibrating surface by introducing an appropriate impedance surface. In order to realize this effect, a model using a perforated board with a back cavity attached to the vibrating surface is proposed, in which the motion of the perforated board is made equal to that of the vibrating surface. To obtain fundamental data, a theoretical study is performed under a simplified condition, assuming an infinite plane piston. The calculated results are compared to experimental data measured by using an acoustic tube. The results, which are in good agreement in the reduction effect, show that this system can achieve the reduction of radiated sound power at arbitrary frequencies.     

锥形颈部赫姆霍兹共振器声学性能预测

锥形颈部赫姆霍兹共振器具有更好的低频消声能力,而其声学性能尚无准确解析预测方法。为了研究其声学性能,在声学长度修正的基础上,利用一维解析方法建立了用于计算传递损失的一维修正模型。运用分割法计算锥形管内部声传播的声学长度修正,并给出了声学修正长度计算公式。采用得到的锥形管声学修正长度和一维修正模型,计算出的锥形颈部赫姆霍兹共振器频率与有限元及实验测试结果偏差在2 Hz以内,明显优于不修正的计算结果。表明锥形管声学长度修正法提高了一维解析方法的精度,从而可以快捷准确的预测锥形颈部赫姆霍兹共振器的消声性能。      

Panel of resonators with variable resonance frequency for noise control

The article focuses on acoustic resonators made of perforated sheets bonded onto honeycomb cavities. This kind of resonators can be used in adverse conditions such as high temperature, dirt and mechanical constraints. For all these reasons, they are, for example, widely used in aeronautic applications. The acoustic properties are directly linked to the size, shape and porosity of holes and to the thickness of air gaps. Unfortunately, the acoustic absorption of these resonators is selective in frequency and conventional acoustic resonators are only well adapted to tonal noises. In case of variable tonal noise, the efficiency is limited if the resonators are not tunable. One common solution is to control the depth of cavities based on the noise to be attenuated. This article proposes another technology of tunable resonators with only a very small mass and size increase. It consists of two superposed and identically perforated plates associated with cavities. One plate is fixed and bonded to the cavities and the other plate is mobile. The present concept enables to change the internal shapes of the holes of the perforated layers. The article describes this system and gives a theoretical model of the normal incidence acoustic impedance that allows to predict the acoustic behavior, in particular the resonance frequency. The model shows that the resonance frequency varies with hole profiles and that the absorption peak moves towards the lower frequencies. The proposed model is validated by measurements on various configurations of resonators tested in an impedance tube. The perspectives of this work are to adapt the hole profiles using an actuator in order to perform active control of impedance.     

Broadband noise reduction by circular multi-cavity mufflers operating in multimodal propagation conditions

In this study, sound propagation through a circular duct with non-locally lining is investigated both numerically and experimentally. The liner concept is based on perforated screens backed by air cavities. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. This gives rise to complex scattering mechanisms among duct modes which renders the muffler more effective over a broader frequency range. This work emanates from the Cleansky European HEXENOR project which aim is to identify the best multi-cavity muffler configuration for reduction of exhaust noise from helicopter turboshaft engines. Here, design parameters are the cavity dimensions in both longitudinal and azimuthal directions. The best cavity configuration must in addition fit weight specifications which implies that the number of walls separating each cavity should be chosen as small as possible. To achieve these objectives, the scattering matrix of the lined duct section is obtained experimentally for two specific muffler configurations operating in multimodal propagation conditions. The good agreement with numerical predictions serves to validate the perforate plate impedance model used in our calculation. Finally, given an incident acoustic pressure which is representative of typical combustion noise spectrum, the best cavity configuration achieving the maximum overall acoustic Transmission Loss is selected numerically. The study also illustrates how the acoustic performances are dependent on the nature of the incident field.