阵列式阻性消声器传递损失计算模型*

针对气流通道彼此独立且截面尺寸较小的直管式阻性消声器,Belov基于声波导管理论推导了其消声量计算公式,该公式不适用于气流通道彼此连通且截面尺寸较大的阵列式阻性消声器。为此,本研究提出了一种阵列式消声器消声量计算方法。将阵列式消声器划分为周期性排列的消声单元,每个消声单元包含1个吸声柱。分别参照扩张式消声器和直管阻性消声器计算消声单元的抗性部分（进、出口气流通道截面突变处）和阻性部分消声量的理论值TL1和TL2。在此基础上,采用有限元法仿真得到消声器消声量仿真值TLs,基于阻性部分消声量仿真值和理论值的比值(TLs-TL1)/TL2,拟合确定各倍频带阻性消声量修正函数Nf,即修正后的消声量理论值计算模型为TLt''=TL1+TL2·Nf。作为算例,建立了多孔吸声材料流阻率为11425 Pa·s/m2时适用于不同结构尺寸的阵列式消声器消声量计算模型。实测结果验证表明,各倍频带修正后的消声量理论值与实测值绝对误差均小于3 dB。当吸声材料的流阻率与算例中取值相差较大时,消声量计算模型需参照本研究所述方法另行建立。     

On the acoustic performance of rectangular splitter silencers in the presence of mean flow

Dissipative splitter silencers are often used to reduce the noise emitted in ventilation and gas turbine systems. It is well known that the acoustic performance of a splitter silencer changes under the influence of the convective effects of a mean gas flow and so in this article a theoretical model is developed to include the effects of mean flow. The theoretical model is based on a hybrid finite element method which enables the inclusion of bull nose fairings and a perforated screen separating the mean gas flow from a bulk reacting porous material. Predictions are compared against experimental measurements obtained both with and without mean flow. Good agreement between prediction and measurement is generally observed in the absence of mean flow, although it is seen that for silencers with a low percentage open area the silencer insertion loss is over predicted at higher frequencies. When mean flow is present, problems with the experimental methodology are observed at relatively modest mean flow velocities, and so comparison between prediction and experiment is limited to relatively low face velocities. However, experiment and theory both show that the insertion loss reduces at low frequencies when mean flow is in the direction of sound propagation, and at high frequencies the influence of mean flow is generally much smaller. Following additional theoretical investigations it is concluded that the influence of mean flow on splitter silencer performance should be accounted for at low frequencies when silencer airway velocities are greater than about 20 m/s; however, at higher frequencies one may generally neglect the effect of mean flow, even at higher velocities. Predictions obtained using the hybrid method are also compared to a simplified point collocation approach and it is demonstrated that the computationally efficient point collocation method may be used to investigate the effects of mean flow in a splitter silencer without loss of accuracy.     

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。      

Impact of perforation impedance on the transmission loss of reactive and dissipative silencers

The effect of perforation impedance on the acoustic behavior of reactive and dissipative silencers is investigated using experimental and computational approaches. The boundary element method (BEM) is applied for the prediction of transmission loss of silencers with different perforation geometries. The variations are considered in the porosity (8.4 and 25.7%) and hole diameter (0.249 and 0.498 cm) of perforations for both reactive and dissipative silencers, as well as the fiber filling density (100 and 200 kg/m3) for the latter. The acoustic impedance for a number of perforations in contact with air alone and fibrous material has been incorporated into the predictions, which are then compared with the measured transmission loss using an impedance tube setup. The results demonstrate the significance of the accuracy of the perforation impedance in the predictions for both reactive and dissipative silencers.     

THE CALCULATION OF ACTIVELY ABSORBING SILENCERS IN RECTANGULAR DUCTS

Active silencers can provide effective solutions especially for the control of low-frequency noise in ducts. To evaluate the performance of this technology in the early design stages it is necessary to predict the insertion loss and adjust the silencer sufficiently precisely to the specific requirements of an application. This paper describes different models for the calculation of actively absorbing wall linings with proportional feedback control applied in splitter silencers as used in rectangular air-conditioning ducts. On the basis of well-established theories for the calculation of passive splitter silencers and a network model of electro-acoustic lumped elements for the wall impedance of each active cassette, it is conceivable to determine their insertion loss. Starting with a rather basic approach, the computational model is refined to increase its modelling accuracy. It is shown that a combination of active wall linings with passive linings yields a high attenuation for a wide frequency band. The theoretical findings compare well with experimental results from a laboratory set-up.     

Drum silencer with shallow cavity filled with helium

The motivation of this study is twofold: (a) to produce a flow-through silencer with zero pressure loss for pressure-critical applications, and (b) to tackle low frequency noise with limited sideway space using cavities filled with helium. The work represents a further development of our recently conceived device of a drum-like silencer with conventional air cavity [Huang, J. Acoust. Soc. Am. 112, 2014-2025 (2002); Choy and Huang, ibid. 112, 2026-2035 (2002)]. Theoretical predictions are validated by experimental data. The new silencer consists of two highly tensioned membranes lining part of a duct, and each membrane is backed by a cavity filled with helium. For a typical configuration of a duct with height h, membrane length L = 7h, cavity depth h = 0.2h, and tension T = 0.52rho0c0(2)h2, where rho0 and c0 are the ambient density and speed of sound in air, respectively, the transmission loss has a continuous stop band of TL > 6.35 dB for frequency 0.03c0/h to 0.064c0/h, which is much better than traditional duct lining. In addition to the mechanisms at work for drum silencers with air cavity, the low density of helium reduces the masslike reactance of the cavity on the second in vacuo mode of membrane vibration. The reduction greatly enhances the membrane response at this mode, which is found to be critical for achieving a broadband performance in the low-frequency regime.     

Comparison and implementation of the various numerical methods used for calculating transmission loss in silencer systems

Issues concerning the design and use of large-scale silencers are more prevalent today then ever before. With the increased use of large industrial machinery (such as gas turbines) and the increase in public awareness and concern for noise control, the desire to be able to properly design silencers for specific applications is increasing. Even today, most silencer design is performed by simply modifying existing designs without full confidence of the new performance characteristics. Due to the size and expense of these silencers, it would be beneficial to have means to predict the insertion loss (IL) or transmission loss (TL) characteristics at the design stage. To properly accomplish this, many factors such as geometry, absorptive material properties, flow effects, break out noise, and self-generated noise must be considered. The use of the finite element method (FEM) and the boundary element method (BEM) can aid in the prediction and design. This paper examines three of the different methods used in calculation of TL values; namely the “traditional” laboratory method, the 4-pole transfer matrix method and the 3-point method. A comparison of these methods based on such criteria as accuracy, computation time, and ease of use was conducted. In addition, the idiosyncrasies and problems encountered during implementation are presented. The conclusions were that the FEM is better suited for this kind of application and that the 3-point method was the fastest method and was easier to use than the 4-pole method.     

电子器件散热风扇气动噪声管道声学模态截止控制技术

深度学习输入特征的选择直接影响其分类性能,为了进一步提高基于深度学习的鸟类物种识别模型的分类性能,该文提出一种多特征融合识别方法。该方法首先通过短时傅里叶变换、梅尔倒谱变换和线性调频小波变换分别计算得到鸣声信号的3种语图样本集,然后分别利用3种语图样本集训练3个基于VGG16迁移的单一特征模型,将3个模型的输出进行自适应加权求和实现融合,并修正了加权交叉熵函数以克服样本不平衡的问题,最后对语图进行分类实现鸟类物种的识别。以ICML4B鸣声库的35种鸟类为研究对象,对比了4种模型的平均识别准确率(MAP),结果表明特征融合模型较单一特征模型的MAP最大提高了0.307;选择输入语图的持续时间分别为100 ms、300 ms以及500 ms,对比不同持续时间下4种模型的测试MAP值,结果表明持续时间为300 ms时4种模型的MAP值均为最高;对比了不同信噪比下4种模型的识别效果,多特征融合模型的识别准确率随着信噪比的下降降低最少。说明在选择合适的语图持续时间后,该文提出的特征融合模型能得到更高的识别准确率,具有一定的抗噪能力,且训练参数少,更适合于少样本鸟类的识别。     

A mode matching method for modeling dissipative silencers lined with poroelastic materials and containing mean flow

A mode matching method for predicting the transmission loss of a cylindrical shaped dissipative silencer partially filled with a poroelastic foam is developed. The model takes into account the solid phase elasticity of the sound-absorbing material, the mounting conditions of the foam, and the presence of a uniform mean flow in the central airway. The novelty of the proposed approach lies in the fact that guided modes of the silencer have a composite nature containing both compressional and shear waves as opposed to classical mode matching methods in which only acoustic pressure waves are present. Results presented demonstrate good agreement with finite element calculations provided a sufficient number of modes are retained. In practice, it is found that the time for computing the transmission loss over a large frequency range takes a few minutes on a personal computer. This makes the present method a reliable tool for tackling dissipative silencers lined with poroelastic materials.     

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway that is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in the computational expenditure when compared to a three-dimensional finite element analysis.     

A network approach for analysis of silencers with/without absorbent material

The object of this work is to establish a general approach that can analyze the performance of most of the silencers with/without sound absorbent material. Under the assumption of plane wave propagation, the transfer matrices between the two ends of straight pipe and two-duct perforated section are derived and taken as the basic elements. Based on the conditions of continuity of pressure and of mass velocity, the silencer is modeled as a network formed by the two basic elements. Then the sound attenuation characteristic of the silencers can be investigated. With this scheme the multiply connected acoustic filters can also be analyzed. Further, the porous sound absorbent material is also included in this scheme. The effect of sound absorption material on the performance of silencers is analyzed and discussed.     

Experimental investigation of metal foam for controlling centrifugal fan noise

This paper presents an experimental investigation on the metal foam for controlling a centrifugal fan noise. Nine samples of metal foam with different types of cells, i.e., open, semi-open and close, are employed to compare their effects on the aerodynamic performance and noise level of the centrifugal fan. Experimental data confirms that the open cell metal foam is the most effective to control the fan noise because it not only significantly suppresses the tonal noise but also attenuates the broadband noise. Moreover, the geometrical parameters of the open cell metal foam, i.e., pores per inch and porosity, are studied to investigate their effects on the aerodynamic performance and noise level of the centrifugal fan.     

Multiple drumlike silencer for low frequency duct noise reflection

Low frequency duct noise is difficult to tackle by passive means within a certain space limitation. Drumlike silencer offers broadband sound reflection in the low-frequency region, e.g. 0.1–0.3 times the first cut-on frequency of the central duct. Optimization was carried out for the logarithmic bandwidth in which the transmission loss (TL) is greater than a pre-determined criterion value. The prediction was validated in laboratory but it was anticipated that the required high level of tension is difficult to implement in practice. It is found that the optimal tension is approximately proportional to the 3rd power of membrane length, so a partitioned membrane is expected to reduce the required tension. In order to optimize the partitioned drumlike silencer, a new cost function of total transmission loss, or insertion loss, with a frequency weighting in favour of a certain wide band of low frequencies is introduced. The new cost function removes the undesirable discontinuous behaviour occurring in the previous optimization process. It is also found that the partitioned silencer can drastically improve the weighted total TL and the tension required is indeed much reduced. The main performance improvement derives from around the lower frequency limit of interest.     

Analysis and design of pod silencers

Parallel baffle mufflers or split silencers are used extensively in heating, ventilation and air conditioning systems for increased attenuation of noise within a short or given length. Acoustic analysis of rectangular parallel baffle mufflers runs on the same lines as that of a rectangular duct lined on two sides. This simplification would not hold for circular configurations. Often, a cylindrical pod is inserted into a circular lined duct to increase its attenuation (or transmission loss), thereby making the flow passage annular and providing an additional absorptive layer on the inner side of this annular passage. This configuration, called a pod silencer, is analyzed here for the four-pole parameters as well as transmission loss, making use of the bulk reaction model.The effect of thin protective film or a highly perforated metallic plate is duly incorporated by means of a grazing-flow impedance. Use of appropriate boundary conditions leads to a set of linear homogeneous equations which in turn lead to a transcendental frequency equation in the unknown complex axial wave number. This is solved by means of the Newton-Raphson method, and the axial wave number is then used in the expressions for transmission loss as well as the transfer matrix parameters. Finally, results of a parametric study are reported to help the designer in optimization of a pod silencer configuration within a given overall size for minimal cost.     

A displacement-pressure finite element formulation for analyzing the sound transmission in ducted shear flows with finite poroelastic lining

In the present work, the propagation of sound in a lined duct containing sheared mean flow is studied. Walls of the duct are acoustically treated with absorbent poroelastic foams. The propagation of elasto-acoustic waves in the liner is described by Biot's model. In the fluid domain, the propagation of sound in a sheared mean flow is governed by the Galbrun's equation. The problem is solved using a mixed displacement-pressure finite element formulation in both domains. A 3D implementation of the model has been performed and is illustrated on axisymmetric examples. Convergence and accuracy of the numerical model are shown for the particular case of the modal propagation in a infinite duct containing a uniform flow. Practical examples concerning the sound attenuation through dissipative silencers are discussed. In particular, effects of the refraction effects in the shear layer as well as the mounting conditions of the foam on the transmission loss are shown. The presence of a perforate screen at the air-porous interface is also considered and included in the model.     

The use of a hybrid model to compute the nonlinear acoustic performance of silencers for the finite amplitude acoustic wave

In the present study, a hybrid method is proposed for predicting the acoustic performance of a silencer for a nonlinear wave. This method is developed by combining two models: (i) a frequency-domain model for the computation of sound attenuation due to a silencer in a linear regime and (ii) a wavenumber space model for the prediction of the nonlinear time-evolution of finite amplitudes of the acoustic wave in a uniform duct of the same length as the silencer. The present method is proposed under the observation that the physical process of the nonlinear sound attenuation phenomenon of a silencer may be decoupled into two distinct mechanisms: (a) a linear acoustic energy loss that owes to the mismatch in the acoustic impedance between reactive elements and/or the sound absorption of acoustic liners in a silencer; (b) a nonlinear acoustic energy loss that is due to the energy-cascade phenomenon that arises from the nonlinear interaction between components of different frequencies. To establish the validity of the present model for predicting the acoustic performance of silencers, two model problems are considered. First, the performance of simple expansion mufflers with nonlinear incident waves has been predicted. Second, proposed method is applied for computing nonlinear acoustic wave propagation in the NASA Langley impedance duct configuration with ceramic tubular liner (CT57). Both results obtained from the hybrid models are compared with those from computational aero-acoustic techniques in a time-space domain that utilize a high-order finite-difference method. Through these comparisons, it is shown that there are good agreements between the two predictions. The main advantage of the present method is that it can effectively compute the nonlinear acoustic performance of silencers in nonlinear regimes without time-space domain calculations that generally entail a greater computational burden.     

An Exact Solution for Acoustic Simulation Based Transmission Loss Optimization of Double-Chamber Silencer

The optimization of the acoustic silencer volume is very important to develop it and to get high-performance, the importance of the silencer was appeared in industrial field to eliminate the noise of the duct by efficient and economical method. The main goal of this research is to optimize the transmission loss (TL) by analytical method of the Double-Chamber Silencer (DCS), the TL has been selected as the main parameter in silencer because it does not based on the source or the termination impedances. First we calculated the power transmission coefficient (PTC) and the TL of an acoustic silencer, then used the Lagrange method to optimize the silencer length. All calculation of silencer data is obtained by solving the governing equations in commercial software Matlab^®. A several calculations for different silencer length at many frequency ranges were performed simultaneously. Finally, this research supports the efficient and rapid techniques for DCS optimal design under narrow space. The results show that the acoustic TL is maximized at the desired frequency.     

Active membrane-based silencer and its acoustic characteristics

A membrane-based silencer using dielectric elastomer absorbers (DEAs) was developed and explored in the present study. Dielectric elastomer, a soft smart material, was used to fabricate this actuator. It has the characteristic of lightweight, high elastic energy density and large deformation under external direct current (DC) or alternating current (AC) voltages. The typical acoustic performances of this membrane-based silencer were experimentally identified using a transmission loss (TL) duct acoustic measurement system. It was found that the resonance peaks of this membrane-based silencer could be controlled by applying different external voltages, a maximum resonance frequency shift of 59.5 Hz for the resonance peaks was achieved which indicated that this membrane-based silencer could be adjusted to absorb the target noise without any addition mechanical part. Furthermore, the resonance shift and multiple resonances mechanisms using DEAs were proposed and discussed which was aiming to achieve multi-peaks noise reduction. The present results also provide insight into the appropriateness of the absorber for possible use as an acoustic treatment to replace the traditional acoustic treatment in the noise reduction technology.     

A comparison between analytic and numerical methods for modelling automotive dissipative silencers with mean flow

Identifying an appropriate method for modelling automotive dissipative silencers normally requires one to choose between analytic and numerical methods. It is common in the literature to justify the choice of an analytic method based on the assumption that equivalent numerical techniques are more computationally expensive. The validity of this assumption is investigated here, and the relative speed and accuracy of two analytic methods are compared to two numerical methods for a uniform dissipative silencer that contains a bulk reacting porous material separated from a mean gas flow by a perforated pipe. The numerical methods are developed here with a view to speeding up transmission loss computation, and are based on a mode matching scheme and a hybrid finite element method. The results presented demonstrate excellent agreement between the analytic and numerical models provided a sufficient number of propagating acoustic modes are retained. However, the numerical mode matching method is shown to be the fastest method, significantly outperforming an equivalent analytic technique. Moreover, the hybrid finite element method is demonstrated to be as fast as the analytic technique. Accordingly, both numerical techniques deliver fast and accurate predictions and are capable of outperforming equivalent analytic methods for automotive dissipative silencers.     

Measured light vehicle noise reduction by hedges

The acoustical effects of hedges result from a combination of physical noise reduction and their influences on perception. This study investigates the physical noise reduction so as to enable estimation of its relative importance. Different in-situ methods have been used to measure noise shielding by hedges. These include a statistical pass-by experiment where the real insertion loss of a hedge could be measured, three controlled pass-by experiments using a reference microphone at close distance, and transmission loss measurements using a point source. Thick dense hedges are found to provide only a small total A-weighted light vehicle noise reduction at low speeds. Measured insertion losses range from 1.1 dBA to 3.6 dBA. The higher noise reductions are found to be associated with an increased ground effect.