Dissipative Silencers Research Articles

Numerical Mode Matching Approach for Acoustic Attenuation Predictions of Double-Chamber Perforated Tube Dissipative Silencers with Mean Flow

The transfer matrix method (TMM) based on numerical mode matching (NMM) approach is developed to investigate the acoustic behavior of double-chamber perforated tube dissipative silencer with mean flow. The present method is verified by comparing the transmission loss (TL) predictions and experimental data. Then the effects of mean flow, perforated tube offset and lengths of perforations are studied computationally. As the Mach number increases, the TL of dissipative silencer is lowered at most frequencies. The perforated tube offset may change the acoustic behavior in the mid-high frequency range. Increasing the total length of perforations increases TL at the mid-high frequencies.

3D Acoustic Modelling of Dissipative Silencers with Nonhomogeneous Properties and Mean Flow

A finite element approach is proposed for the acoustic analysis of automotive silencers including a perforated duct with uniform axial mean flow and an outer chamber with heterogeneous absorbent material. This material can be characterized by means of its equivalent acoustic properties, considered coordinate-dependent via the introduction of a heterogeneous bulk density, and the corresponding material airflow resistivity variations. An approach has been implemented to solve the pressure wave equation for a nonmoving heterogeneous medium, associated with the problem of sound propagation in the outer chamber. On the other hand, the governing equation in the central duct has been solved in terms of the acoustic velocity potential considering the presence of a moving medium. The coupling between both regions and the corresponding acoustic fields has been carried out by means of a perforated duct and its acoustic impedance, adapted here to include absorbent material heterogeneities and mean flow effects simultaneously. It has been found that bulk density heterogeneities have a considerable influence on the silencer transmission loss.

Расчет и проектирование диссипативных глушителей шума методом конечных элементов

Расчет и проектирование диссипативных глушителей шума методом конечных элементов

Study of micro-perforated tube mufflers with adjustable transmission loss

Micro-perforated tube muffler has the advantages of both the reactive and dissipative mufflers. The paper proposed that the change of micro-perforated tube length could change the transmission loss behaviors of the micro-perforated tube mufflers. The acoustic impedance of micro-perforated plate was derived in this paper, which was used in the finite element method to calculate the transmission loss of mufflers with various micro-perforated tube lengths. Then, a relation formula of the maximum muffling frequency and the micro-perforated tube length was fitted. This kind of adjustable mufflers could be used efficiently to control the noise of the rotating machines, whose main noises are always proportional to the rotating speed.

Shape Optimization of One-Chamber Perforated Mufflers Filled with Wool Using Simulated Annealing

Research on dissipative and reactive mufflers has been addressed. However, the acoustical performance -sound transmission loss (STL)- of mufflers within a constrained space is often insufficient. In this paper, to improve the acoustical efficiency, a one-chamber perforated muffler filled with sound absorbing wool optimized by using simulated annealing (SA) in conjunction with the numerical decoupling technique is presented. A numerical case in eliminating a broadband noise is also introduced. To verify the reliability of SA optimization, optimal noise abatements for the pure tone are exemplified. Before the SA operation can be carried out, the accuracy of the mathematical models has been checked using the experimental data. Results indicate that the maximal STL is precisely located at the desired target tone. Moreover, the STL can be improved when the ratio of the perforated tube’s length and the porosity of the perforated tube and the acoustical flowing resistance of the wool increase and the Mach number and the expansion ratio decrease. Consequently, a successful approach used for the optimal design of the one-chamber dissipative mufflers within a constrained space has been demonstrated.

The prediction of silencer acoustical performances by 1D, 1D–3D and quasi-3D non-linear approaches

The design of silencers for internal combustion (I.C.) engines is a key issue to attenuate or emphasize certain spectral components of tailpipe noise. The optimization of complex shape silencing systems is generally a time-consuming operation, which must be carried out by means of concurrent experimental measurements and numerical simulations. This paper describes the development and application of different non-linear models: a coupled 1D–multiD model and a coupled 1D-quasi-3D model, to predict the silencer behavior in the time and frequency domains. Second order time and space discretization were adopted in the 3D and quasi-3D approaches, whereas specific coupling strategies were developed to realize the interface between them and the 1D model. In particular, since the 3D relies on a collocated grid discretization, a Riemann solver based method was developed to realize the coupling with the 1D code, while a cell overlapping procedure was exploited to interface the 1D code with the quasi-3D method, in order to fit with the pseudo-staggered grid arrangement. Both a white noise and a single impulse boundary condition have been imposed upstream of the pipe system to excite the wave motion. The integrated 1D–multiD and the quasi-3D approaches were applied to predict the transmission loss of reactive and dissipative mufflers in which the pressure waves can be significantly non-planar, to point out the influence of higher order modes on the acoustical performance. Reverse chambers with extended inlet and outlet and perforates have been investigated, showing the potential of both the hybrid 1D–3D code and the quasi-3D code with respect to a simple, fully 1D model. A comparison between predicted results of transmission loss and experimental measurements has pointed out the importance of correctly capturing multi-dimensional wave effects at mid and high frequencies as well as the effects of high amplitude perturbations and mean flow.

On the use of micro-perforates for machinery and vehicle noise control

A micro-perforated plate (MPP) is a perforated plate with holes typically in the sub-millimeter range and perforation ratio around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: To create a surface with a built in damping that effectively dissipates sound waves. To achieve this, the specific acoustic impedance of a MPP is normally tuned to be of the order of the characteristic wave impedance in the medium (400 Pa*s/m in air at STP). The traditional application for MPP's has been building acoustics, normally in the form of a so called panel absorber to create an absorption peak at a selected frequency. However, MPP's made of metal are also well suited for machinery and vehicle noise control. For instance MPP's have the potential to be used instead of porous materials in dissipative mufflers, which not only can save weight but also offer a non-fibrous alternative. Furthermore, since MPP's have a large steady flow re...

A finite element approach for the acoustic modeling of perforated dissipative mufflers with non-homogeneous properties

In this work, a finite element approach is presented for modeling sound propagation in perforated dissipative mufflers with non-homogeneous properties. The spatial variations of the acoustic properties can arise, for example, from uneven filling processes during manufacture and degradation associated with the flow of soot particles within the absorbent material. First, the finite element method is applied to the wave equation for a propagation medium with variable properties (outer chamber with absorbent material) and a homogeneous medium (central passage). For the case of a dissipative muffler, the characterization of the absorbent material is carried out by means of its equivalent complex density and speed of sound. To account for the spatial variations of these properties, a coordinate-dependent function is proposed for the filling density of the absorbent material. The coupling between the outer chamber and the central passage is achieved by using the acoustic impedance of the perforated central pipe, that relates the acoustic pressure jump and the normal velocity through the perforations. The acoustic impedance of the perforated central duct includes the influence of the absorbent material and therefore a spatial variation of the impedance is also taken into account. A detailed study is then presented to assess the influence of the heterogeneous properties and the perforated duct porosity on the acoustic attenuation performance of the muffler.

Genetic Algorithm Optimization on a Venting System With Three-Chamber Hybrid Mufflers Within a Constrained Back Pressure and Space

Recently, research on new techniques for dissipative mufflers in dealing with the higher frequencies has been addressed. However, the shape optimization of hybrid mufflers in reducing broadband noise within a constrained space as well as a pressure-drop limit which are both concerned with the necessity of operation and system venting in practical engineering work was rarely tackled. Therefore, this study will not only analyze the sound transmission loss (STL) of a space-constrained multichamber hybrid muffler but also optimize the best design shape under a specified pressure drop. In this paper, the generalized decoupling technique and plane wave theory used to solve the coupled acoustical problem of perforated mufflers with/without sound absorbing material are presented. The four-pole system matrix used to evaluate acoustic performance is also introduced in conjunction with a genetic algorithm (GA). A numerical case for eliminating a broadband venting noise emitted from a pressure relief valve using four kinds of hybrid mufflers is also introduced. To verify the reliability of the GA optimization, optimal noise abatement for a pure tone (1000 Hz) is exemplified. Before the GA operation can be carried out, the accuracy of the mathematical models need to be checked using the experimental data. The optimal result in eliminating broadband noise reveals that the overall noise reductions with respect to various mufflers under a maximal allowable pressure drop of 100 Pa can achieve 62.6, 54.8, 32.3 and 87.8 dB. Consequently, the approach used for the optimal design of the multichamber hybrid mufflers under space and back pressure constrained conditions is indeed easy and quite effective.

Study on Theoretical Design and Simulation of a Tuned Frequency Muffler

The criterions of parameter selection for a tuned frequency muffler are proposed. The experimental results show that the radius of a cylindrical perforated tube dissipative muffler meets the criterions. Based on the conclusion, through installing the rotatable circumferential separate plates, an actuator of the tuned frequency muffler is designed to realize automatic adjustment of the radius, which is available for two kinds of working conditions of acoustic attenuation and can both attain the optimal acoustic attenuation performance. Utilizing the finite element analysis and the experiment test, variation rules of transmission loss versus the muffler’s radius is further investigated. The results show that the variable-radius method used in the tuned frequency muffler can shift the frequency band of acoustic attenuation and optimize the acoustic attenuation performance, which expands the application fields of mufflers and has a great engineering value.

Noise Reduction of a Fan and Air Duct by Using a Plenum Chamber Based on ASHRAE Guidelines

Noise propagates from sources through air distribution duct systems, structures, and combinations of paths, reaching the occupants. A plenum chamber is similar to a dissipative silencer, but it is also similar to a small room with a noise source in one wall. The dimensions of the plenum chamber are usually larger than the wavelength of the sound being attenuated. Above the cutoff frequency, as defined by the plenum's inlet duct dimensions, the wavelength of sound is small compared to the characteristic dimensions of the plenum. All values are in terms of sound pressure level with “A” frequency weighting and “slow” time weighting measured by a B&K 2231 sound level meter and 1625 analyzer. Input and output in this compartment as opposed to a two-sided compartment was based on the ASHRAE instructions on the frequency of cutoff. This frequency indicates that the chamber dimensions that channel input and output frequencies higher than 436 Hz will be effective in reducing noise.

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.

A New Type of Muffler Based on Microperforated Tubes

Microperforated plate (MPP) absorbers are perforated plates with holes typically in the submillimeter range and perforation ratios around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: It is to create a surface with a built in damping, which effectively absorbs sound waves. To achieve this, the specific acoustic impedance of a MPP absorber is normally tuned to be of the order of the characteristic wave impedance in the medium (∼400 Pa s/m in air at STP). The traditional application for MPP absorbers has been building acoustics often combined with a so called panel absorber to create an absorption peak at a selected frequency. However, MPP absorbers made of metal could also be used for noise control close to or at the source for noise control in ducts. In this paper, the possibility to build dissipative silencers, e.g., for use in automotive exhaust or ventilation systems, is investigated.

Optimization Design of Hybrid Mufflers on Broadband Frequencies Using the Genetic Algorithm

Recently, there has been research on high frequency dissipative mufflers. How- ever, research on shape optimization of hybrid mufflers that reduce broadband noise within a constrained space is sparse. In this paper, a hybrid muffler composed of a dissipative muffler and a reactive muffler within a constrained space is assessed. Using the eigenvalues and eigenfunctions, a coupling wave equation for the perfo- rated dissipative chamber is simplified into a four-pole matrix form. To efficiently find the optimal shape within a constrained space, a four-pole matrix system used to evaluate the acoustical performance of the sound transmission loss (STL) is eval- uated using a genetic algorithm (GA). A numerical case for eliminating a broadband venting noise is also introduced. To verify the reliability of a GA optimization, optimal noise abatements for two pure tones (500 Hz and 800 Hz) are exemplified. Before the GA operation can be carried out, the accuracy of the mathematical models has been checked using experimental data. Results indicate that the maximal STL is precisely located at the desired target tone. The optimal result of case studies for eliminating broadband noise also reveals that the overall sound power level (SWL) of the hybrid muffler can be reduced from 138.9 dB(A) to 84.5 dB(A), which is superior to other mufflers (a one-chamber dissipative and a one-chamber reactive muffler). Consequently, a successful approach used for the optimal design of the hybrid mufflers within a constrained space has been demonstrated.

The use of axial splitters inside ducts for active noise control of higher order acoustic modes

Studies of noise reduction in ducts have become, over the years, a field of intense application mainly because of the industrial potential and feasibility of practical systems. The traditional approach to noise control in ducts uses passive techniques such as dissipative and reactive silencers. However, with the advance of the technology and limitations of conventional noise control treatments, active noise control (ANC) has become a practical and economic method due to powerful hardware for the digital signal processing in real time. However, in industrial applications, the design and employment of active noise control not always is simple and practicable. In exhaust systems, the ducts have geometry which allows high order acoustic modes to propagate at the frequencies to be controlled. These characteristics make it difficult to implement an ANC system. Higher order modes in duct sometimes result in lower ANC performance. In this paper, a methodology based on splitting the duct into parallel axial sections is presented in order to ensure that only plane waves are present in the region of the ANC system. The aim of this work is to show that this artifice can be a viable and cost effective solution. Numerical acoustic simulations were carried out in order to demonstrate the viability of the suggested technique. A finite elements method is used. An optimization using evolutionary algorithms is also used to select optimal actuators positions. Attenuation of 26 dB was obtained for tonal noise in a duct with splitters, while simulations with the conventional duct had 3 dB of mitigation

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.

Boundary element acoustic analysis of hybrid expansion chamber silencers with perforated facing

The substructure boundary element approach is developed to predict and analyze the acoustic attenuation characteristics of hybrid expansion chamber silencers with perforated facing. The silencers are divided into a number of acoustic domains with single medium (air or sound-absorbing material), and treating the sound-absorbing material as an equivalent fluid with complex-valued density and speed of sound (or complex-valued characteristic impedance and wavenumber), and then the boundary element method (BEM) may be applied to each domain leading to a system of equations in terms of acoustic pressure and particle velocity. Using the specific acoustic impedance of perforate, which takes into account the effect of sound-absorbing material, the relationship of acoustic pressures and particle velocities between the inlet and outlet of silencer may be obtained and then transmission loss is determined. For the straight-through perforated tube reactive and dissipative silencers, the predictions of transmission loss agree reasonably well with experimental measurements available in the literature, which demonstrated the applicability and accuracy of the present approach. The BEM is then used to investigate the effect of internal structure on the acoustic attenuation characteristics of hybrid expansion chamber silencers with perforated facing. The numerical results demonstrated that the hybrid expansion chambers may provide higher acoustic attenuation than the reactive expansion chamber in the mid to high frequency range.

Modeling the acoustical characteristics of silencers for suppressing noise from a gas-turbine unit compressor

An acoustic model constructed for silencers of noise from the air intake paths of gas-turbine units is proposed, which correlates indicators characterizing the effectiveness of noise suppression with dimension-less parameters that depend on linear dimensions of the construction, current frequency of sound, as well as factors characterizing the properties of working medium and sound-absorbing material. Universal acoustic characteristics of extended dissipative plate silencers filled with a fibrous sound absorber are constructed. The influence of protective fabric coating on the sound-proofing properties of silencers is analyzed.

The development of technologies and devices for protection from noise generated by power equipment

The main lines of currently conducted research and development activities on suppressing noise produced by power-generating equipment are presented. Matters related to preventing the occurrence of aeroacoustic self-excited vibrations, optimizing dissipative noise silencers, using structural methods for damping acoustic vibrations, suppressing low-frequency noise, and analyzing the effectiveness of soundproof coatings are considered. The process diagrams and parameters of devices for suppressing noise generated during discharge into the atmosphere of high-pressure gaseous media are discussed.

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.