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,但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,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TL_s,基于阻性部分消声量仿真值和理论值的比值(TL_s-TL₁)/TL₂,拟合确定各倍频带阻性消声量修正函数N_f,即修正后的消声量理论值计算模型为TL′_t=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...     

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.     

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.     

Reducing Occupational Noise Propagated from Centrifugal Fan through Dissipative Silencers: A Field Study

Acoustic performance of dissipative silencer was evaluated to determine the effectiveness of perforated duct porosity and absorbent material density in reducing occupational noise exposure propagated from centrifugal fan. Design charts were applied to predict noise reduction and length of a dissipative silencer. Dissipative silencers with various punched duct porosity (14%, 30% and 40%) and sound absorbent density (80 Kg/m³, 120 Kg/m³, and 140 Kg/m³) were designed and fabricated. According to ISO9612 and ISO11820, noise level was measured before and after installing all nine test silencers at fixed workstations around the discharge side of a centrifugal fan in a manufacturing plant. On average, the noise level at the discharge side of a fan without silencer was measured to be 93.6 dBA, whereas it was significantly mitigated by 67.4 dBA to 70.1 dBA after installing all silencers. Dynamic insertion loss for a dissipative silencer with 100 cm length was predicted to be 27.9 dB, which was in agreement with experimental ones. Although, there was no significant differences between insertion loss of silencers, the one with 30% porosity and 120 Kg/m³ rock wool density had the highest insertion loss of 26.2 dBA. Dissipative silencers noticeably reduced centrifugal fan noise exposures. Increasing sound absorbent density and duct porosity up to a certain limit could probably be effective in noise reduction of dissipative silencers.     

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。      

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.     

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。      

Evaluation of acoustical and non-acoustical properties of sound absorbing materials made of polyester fibres of various cross-sectional shapes

Four types of fibrous material samples made of polyester fibres of various cross-sections, i.e. circle, hollow, flat and triangle, have been prepared. The surface impedance and the non-acoustical parameters of them have been measured.The effect of the variation in the cross-sections on the acoustical properties and the non-acoustical parameters of the polyester fibre samples have been examined. The effects of the ‘circle’ and the ‘hollow’ samples are almost the same. However, for the ‘flat’ and the ‘triangle’ samples, the effects different from the ‘circle’ sample are found in the measured flow resistivity and the two characteristic lengths. This paper also discusses the equivalent diameters of the materials made of non-circular cross-sections.     

Aircraft noise annoyance modeling: Consideration of noise sensitivity and of different annoying acoustical characteristics

Noise annoyance due to aircraft flyover noise was assessed under laboratory conditions. The main objectives of the study were: (i) to identify influential acoustical features of noise annoyance, (ii) to propose noise indices to characterize these acoustical features and (iii) to enhance annoyance models including influential acoustical and non-acoustical variables. Therefore, a verbalization task was performed by the participants of the experiment to collect their whole impression concerning the aircraft flyover noises for which they rated annoyance. This verbalization task highlights that noise annoyance was influenced by three main acoustical features: (i) the spectral content, (ii) the temporal variation and (iii) the perceived sound intensity. Four combinations of noise indices were used to propose multilevel annoyance models, in combination with the individual noise sensitivity. Noise sensitivity was found to highly contribute to annoyance models and should therefore be considered in future studies dealing with noise annoyance due to aircraft noise. Different combinations of noise indices coupled with noise sensitivity were found to be promising for future studies that aim to enhance current annoyance models.     

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.     

Inverse acoustical characterization of natural jute sound absorbing material by the particle swarm optimization method

For modeling of jute as acoustic material, knowledge of its non-acoustical parameters like porosity, tortuosity, air flow resistivity, thermal and viscous characteristic lengths is a prime requisite. Measurement of these non-acoustical parameters is not straightforward and involves a dedicated measurement setup. So in order to overcome this issue, the inverse acoustical characterization can be used. In this paper, the particle swarm optimization method (PSO) is used as an optimization method. This method estimates the non-acoustical parameters of jute material in felt form by minimizing the error between experimental and theoretical sound absorption data. In this work, the impedance prediction models for fibrous materials like Johnson–Champoux–Allard model with rigid and limp frame and Garai–Pompoli model is used for sound absorption coefficient calculation by the transfer matrix method along with the PSO. The inverse estimated non-acoustical parameters for jute material are then compared with estimated and experimentally measured parameters for jute felts. Using these inversely predicted parameters, sound absorption of multilayer sound absorbers is also studied.     

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.     

Wheel/rail noise—means for control

Wheel/rail noise is one of the primary sources of noise from rail system operations—in many cases, the primary source. This paper is a review of the methods available for controlling all three types of wheel/rail noise: squeal, impact, and roar (rolling noise). The acoustical performance, non-acoustical benefits, problems, constraints, and additional research and testing requirements for each noise control treatment are presented.     

The attenuation of lined plenum chambers in ducts,II: Measurements and comparison with theory

Experimental measurements of the acoustic performance of single and three-pass lined plenum chambers are compared to calculations based on theoretical models described in a companion paper [1]. Generally, quite good agreement is obtained, subject to the limitations of the theories. For the sake of completeness, comparison is made between the performance of a single plenum chamber and that of an equivalent splitter silencer. The two are seen to differ somewhat in their attenuation characteristics. The aerodynamic pressure losses of all three silencers are compared, and observations are made concerning the relative mass, construction time, et cetera, of the single chamber and the splitter. A tentative design procedure for plena is suggested.     

Attenuation of a resonant dissipative silencer in a rigid duct

Sound attenuation characteristics of a resonant-type dissipative silencer consisting of a reactive chamber with a porous facing have been considered. Such a silencer provides a high degree of attenuation within a narrow frequency range. Predicted attenuation values are compared with experiment for plane waves propagating in a rigid duct containing the dissipative silencer. The sound field is described by one-dimensional acoustical expressions taking into account the effect of boundary conditions and the presence of the silencer. The theoretical model incorporates the acoustical properties of porous materials and inertance of the sound field in the duct adjacent to the silencer. Good agreement was achieved between theoretical predictions and actual measurements. Results presented indicate the dependence of the attenuation spectrum upon flow resistivity and thickness of the porous material.     

Acoustic attenuation analysis of silencers with multi-chamber by using coupling method based on subdomain division technique

For multi-chamber silencers with non-uniform cross-sections at axial direction, the coupling method based on subdomain division technique is proposed. The silencer is divided into several subdomains which impedance matrixes are calculated by using suitable 3-D methods. Combining with the continuity conditions at the interfaces to solve the integral impedance matrix of the silencer, thereby the transmission loss is derived. For the double-chamber silencers, the transmission loss and computational speed results from the proposed method, the numerical mode matching (NMM) method, 3-D finite element (FE) method predictions and experimental measurements are compared, which verifies the accuracy and efficiency.     

Numerical studies on venting system with multi-chamber perforated mufflers by GA optimization

Research on new techniques of perforated silencers has been well addressed and developed; however, the research work in shape optimization for a volume-constrained silence requested upon the demands of operation and maintenance inside a constrained machine room is rare. Therefore, the main purpose of this paper is to not only analyze the sound transmission loss of a multi-chamber perforated muffler but also to optimize the best design shape under space-constrained condition.In this paper, both the generalized decoupling technique and plane wave theory are used. The four-pole system matrix in evaluating the acoustic performance of sound transmission loss (STL) is also deduced in conjunction with a genetic algorithm (GA). To demonstrate the precision of the tuning ability in a muffler, various targeted pure tones are proposed in numerical cases. Results reveal that the maximal acoustical performance precisely occurred in the desired frequency. Furthermore, a noise reduction with respect to full-band exhausted noise emitted from a diesel engine is also introduced and assessed. To achieve a better optimization in GA, several test parameter values were used. Before a GA operation can be carried out, the accuracy of the mathematical models have to be checked by experimental data.The optimal result in eliminating full-band noise reveals that the overall noise reduction of a multi-chamber muffler can achieve 68 dB under space-constraint conditions. Consequently, the approach used for the optimal design of the STL proposed in this study is indeed easy, economical and quite effective.     

Sound absorption properties of a sunflower composite made from crushed stem particles and from chitosan bio-binder

A recent study investigated the mechanical, thermal and acoustical properties of a bio-based composite made from crushed particles of sunflower stalks binded together by chitosan, a bio-based binder. The acoustical performance in absorption was found to be poor as the material was highly compacted and with low porosity. The present study focuses on the acoustical properties of a higher porosity composite, with lower density while the mechanical rigidity remains fairly high. A higher absorption coefficient is obtained. The experimental results on the absorption coefficient are compared to the prediction of a model involving 5 physical parameters (porosity, tortuosity, airflow resistivity, thermal and viscous characteristic lengths). The characterization methods to determine these parameters are described. The comparison between experimental and theoretical results shows that this material exhibits peculiar microstructural features. It is found that the sound absorption properties can involve dead-end pores or clusters and multiple porosity scales in the material.     

A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials

A method for evaluating the acoustical properties of homogeneous and isotropic porous materials that may be modeled as fluids having complex properties is described here. To implement the procedure, a conventional, two-microphone standing wave tube was modified to include: a new sample holder; a section downstream of the sample holder that accommodated a second pair of microphone holders and an approximately anechoic termination. Sound-pressure measurements at two upstream and two downstream locations were then used to estimate the two-by-two transfer matrix of porous material samples. The experimental transfer matrix method has been most widely used in the past to measure the acoustical properties of silencer system components. That procedure was made more efficient here by taking advantage of the reciprocal nature of sound transmission through homogeneous and isotropic porous layers. The transfer matrix of a homogeneous and isotropic, rigid or limp porous layer can easily be used to identify the material's characteristic impedance and wave number, from which other acoustical quantities of interest can be calculated. The procedure has been used to estimate the acoustical properties of a glass fiber material: good agreement was found between the estimated acoustical properties and those predicted by using the formulas of Delany and Bazley.     

The technique of active/inactive finite elements for the analysis and optimization of acoustical chambers

In this work are investigated two topics associated with numerical calculations of the transmission loss in acoustical silencers: analysis of acoustic chambers employing active/inactive finite elements and its optimization using the GA (genetic algorithm) with integer variables. The technical information on the use of active/inactive elements and the definition of all the design variables used for the entire control of the finite element mesh are detailed. Although simple, the numerical results for the examples analyzed show excellent convergence achieved with the combination of these two techniques for the optimization of symmetrical acoustic chambers.