Dissipative Silencers Research Articles

PETAL-SHAPED SILENCERS USAGE FOR NOISE REDUCTION IN CYLINDRICAL CHANNELS

The article presents data on the results of petal-shaped silencer mathematical modeling. Dissipative silencers of complex shapes are the most promising technology. This design allows for achieving the required noise reduction. At the same time, geometric shape optimization allows reducing the aerodynamic drag to the smallest values. The article compares the variants of the silencer with the rotation angles of the plates 45 and 70 degrees. The use of such plates allows you to increase the acoustic efficiency of the silencer by eliminating the radiation effect. In the article, there are the results of noise calculation from a dry cooling tower. The use of a petal-shaped silencer will significantly increase acoustic efficiency. The extra aerodynamic drag will be minimal. For comparison presented the results of modeling a cylindrical silencer. The cylindrical silencer has an equal through-section and long. Justified the approach to choosing the most preferred option. The initial velocity field of the gas or air flow in the channel determines the choice of silencer design.

Topology and shape optimization of dissipative and hybrid mufflers

This article presents a topology optimization (TO) method developed for maximizing the acoustic attenuation of a perforated dissipative muffler in the targeted frequency range by optimally distributing the absorbent material within the chamber. The finite element method (FEM) is applied to the wave equation formulated in terms of acoustic pressure (chamber) and velocity potential (central duct, due to the existence of thermal gradients and mean flow) in order to evaluate the acoustic performance of the noise control device in terms of transmission loss (TL). Sound propagation through the chamber fibrous material is modeled considering complex equivalent acoustic properties, which vary spatially not only as a function of temperature but also as a function of the filling density, since non-homogeneous density distributions are considered. The acoustic coupling at the perforated duct is performed by introducing a coordinate-dependent equivalent impedance. The objective function to maximize is expressed as the mean TL in the targeted frequency range. The sensitivities of this function with respect to the filling density of each element in the chamber are evaluated following the standard adjoint method. The method of moving asymptotes (MMA) is used to update the design variables at each iteration of the TO process, keeping the weight of absorbent material equal or lower than a given value, while maximizing attenuation. Additionally, several particular designs inferred from the topology optimization results are analyzed. For example, the sizing optimization of a number of rings is carried out simultaneously with the aforementioned TO process (density layout). A reactive chamber is added in order to evaluate the TL of a hybrid muffler and its shape optimization is also carried out simultaneously with the aforementioned TO. Results show an increase in the muffler’s mean TL at target frequencies, for all cases under study, while the amount of absorbent material used is maintained or even reduced.

Virtual Herschel-Quincke tube using the multiple small resonators and acoustic metamaterials

This study is on the practical application of the acoustic metamaterials to the design of a silencer with high acoustic and geometric efficiencies, and negligible pressure drop. The object is to achieve broadband and low-frequency attenuation simultaneously by combining the effects of resonance, periodicity, phase difference, and impedance mismatch. An array of multiple small resonators, which is periodically applied to the wall of a duct, invokes the dispersion of sound propagating in the duct. This phenomenon is implemented to construct a virtual Herschel-Quincke (HQ) tube system. The main duct is split into two parallel ducts of the same length: a rigid duct and a duct with a wall covered by the periodic resonators. For a modelling of sound propagation in a dispersive duct, the phase speed of sound is analytically derived in the presence of a mean flow. Also, the attenuation conditions of the virtual HQ tube are proposed to establish a guideline in selecting the proper design parameters for achieving the required transmission loss (TL) in the desired frequency range. The predicted TL spectra are compared with the test results, for a virtual HQ tube system with 9 identical quarter-wavelength tube resonators, and they generally agree well. With increasing flow speed, the amount of attenuation decreases a bit, but the general spectral characteristics are maintained for |M|≤0.1. Based on the same principle, the acoustic metamaterials (AMM) are applied to the practical silencer design to achieve the wide-band sound reduction at low- to mid-frequencies for a given small space. A virtual HQ tube having 26 cells of AMM is tested, of which a cell is composed of 3 types of quarter-wavelength tube resonators. The experiment well validates the predicted TL. The results show that TL is at least 5 dB for a wide frequency range of 230–1000 Hz. The additional volume due to the attachment of the AMM layer is only 40%, while the TL is far larger than that of the simple expansion chamber or the dissipative silencer having the same excess volume.

Development of test bench with airflow for resonator acoustic performance measurement

Acoustic properties of resonators installed in vehicle intake system are influenced by the high-speed airflow passing through, which causes errors between practical application and bench tests. In this paper, a test bench with airflow for measuring the resonator transmission loss is developed based on the principle of two-load method. Equipment types are selected and parameters calculation is presented. Effects of sound source protection devices on the performance of sound source are studied experimentally. A resistance resonator and several dissipative mufflers are mounted at the outlet of the vortex air pump to reduce airflow noise and verified to be effective. Finally, the transmission loss of a multi-chamber perforated resonator is measured with the developed test bench and effects of airflow on resonator acoustic properties are analyzed.

STUDY ON SILENCER ACOUSTIC EFFICIENCY IN GAS-AIR FLOW DUCT BENDS

The study examines a topical problem of increased noise nuisance originating from energy facilities at thermal mechanical equipment tight integration. Noise negatively affects residents of large cities and could contribute to development of chronic diseases characterized by such symptoms as reduced hearing sensitivity, changes in digestive function, cardiovascular insufficiency and neuroendocrine disorder. At the heat supply energy facilities, main sources of noise and vibration include pumping equipment, boiler burners and gas-air flow ducts, as well as ventilation systems. This paper considers the problem of noise reduction in the boiler gas-air flow ducts. Original design of a dissipative silencer in the form of a bent is proposed, which use is relevant, if extended straight sections of gas-air flow ducts are missing. Results of the developed silencer full-scale bench tests are presented. According to test results, the silencer acoustic efficiency constituted 18 dBA.

Improving the acoustic efficiency of laminated dissipative noise silencers for boiler gas-air paths

The operation of thermal power facilities is associated with the negative impact on the environment and humans of such factors as air, water, soil pollution, electromagnetic radiation, and noise. This article is devoted to improving the noise reduction efficiency of gas and air paths of boilers of thermal power plants. In world practice, to reduce noise in gas-air ducts, dissipative noise silencers are used. The authors propose a new design of sound attenuation elements for dissipative plate-type silencers. The article defines the acoustic and aerodynamic characteristics of noise silencers with new sound attenuation elements. We compare the characteristics of silencers made of new silencing elements and the characteristics of silencers consisting of plates of common design with flat surfaces of the side walls, and demonstrate an increase in acoustic efficiency and a decrease in pressure loss when using the proposed new silencing elements in noise silencers. Recommendations on the use of dissipative plate-type silencers with developed sound attenuation elements for gas-air paths of boilers are given.

Numerical mode matching for sound propagation in silencers with granular material

This work presents an efficient numerical approach based on the combination of the mode matching technique and the finite element method (FEM) to model the sound propagation in silencers containing granular material and to evaluate their acoustic performance through the computation of transmission loss ( TL ). The methodology takes into account the presence of three-dimensional (3D) waves and the corresponding higher order modes, while reducing the computational expenditure of a full 3D FEM calculation. First, the wavenumbers and transversal pressure modes associated with the silencer cross section are obtained by means of a two-dimensional FEM eigenvalue problem, which allows the consideration of arbitrary transversal geometries and material heterogeneities. The numerical approach considers the possibility of using different filling levels of granular material, giving rise to cross sections with abrupt changes of properties located not only in the usual central perforated passage, but also in the transition between air and material, that involves a significant change in porosity. After solving the eigenvalue problem, the acoustic fields (acoustic pressure and axial velocity) are coupled at geometric discontinuities between ducts through the compatibility conditions to obtain the complete solution of the wave equation and the acoustic performance ( TL ). The granular material is analysed as a potential alternative to the traditional dissipative silencers incorporating fibrous absorbent materials. Sound propagation in granular materials can be modelled through acoustic equivalent properties, such as complex and frequency dependent density and speed of sound. TL results computed by means of the numerical approach proposed here show good agreement with full 3D FEM calculations and experimental measurements. As expected, the numerical mode matching outperforms the computational expenditure of the full 3D FEM approach. Different configurations have been studied to determine the influence on the TL of several parameters such as the size of the material grains, the filling level of the chamber, the granular material porosity and the geometry of the silencer cross section. • Numerical mode matching is applied to sound waves in silencers with granular medium. • It allows efficient computation of geometries with arbitrary cross section. • TL curves show good agreement with full 3D FEM results and experimental measurements. • The acoustic influence of granular size and silencer cross section is analysed.

Analytical modeling of dissipative silencers.

This paper applies ideas originally developed in the field of acoustics of multiscale porous media to the analytical modeling of dissipative silencers. Analytical models for the long-wavelength acoustical properties of different types of dissipative silencers, such as pod silencers, lined ducts, splitters, and bar silencers, are introduced in this work. Some of these models account for the possibility of using different porous materials in a given silencer. For instance, a model for a splitter silencer comprising baffles made of different porous materials is derived. Examples of the sound transmission behavior of the said types of silencers are presented to evidence that the dissipation of sound energy is largely influenced by pressure diffusion occurring in the porous constituents. The predictions of the analytical models are in good agreement with finite element calculations.

Numerical optimization of straight multi-array side-branched mufflers using differential evolution method

To overcome this noise, traditional reactive as well as dissipative mufflers have been used. However, the allowable backpressure and the extra load acting on the cooling tower are limited. So, in order to efficiently reduce the broad noise under a low-pressure drop and a small dead weight, a straight multi-array resonator muffler without sound absorbing wool is required. Multi-array Helmholtz mufflers with complicated Helmholtz resonators were found to be too expensive in manufacturing costs and can be easily accumulated with duct and water. Therefore, a straight multi-array side-branched muffler with an empty chamber is proposed. Here, to efficiently reduce the broadband noise level, eight kinds of mufflers (muffler A–H) with 1–8 raw chambers internally partitioned with concentric perforated plate will also be introduced. To delineate the best acoustical performance of a space-constrained muffler, a numerical assessment using a four-pole matrix method in conjunction with a differential evolution method is adopted. To verify the availability of the differential evolution optimization, a numerical optimization of mufflers A–H at a pure tone (400 Hz) is exemplified. Before the differential evolution operation in broadband noise elimination can be carried out, the accuracy of the mathematical model has been checked using the experimental data. Consequently, a successful approach in eliminating a broadband noise with a low-pressure drop and a dead load using optimally shaped straight multi-array side-branched mufflers and a differential evolution method within a constrained space has been demonstrated.

Parametric optimization and analysis of metallic porous material used for discharge system of hermetic compressor

Gas pulsation is an important noise source in household refrigerators. The pulsation is generated in the compressor discharge chamber and it is transferred to the cabinet through the condenser, which is directly connected to the back of the system. The discharge muffler makes a considerable contribution to refrigerator noise. The noise can be controlled through the use of metallic porous material in the expansion chamber of the muffler. The macro-acoustic parameters of porous medium (airflow resistivity, porosity, tortuosity, viscous and thermal length) were characterized experimentally to validate numerical and analytical results. The porous material was modeled, using Johnson-Champoux-Allard (JCA) approach, as an equivalent fluid. Parametric optimization was employed to maximize the transmission loss of the dissipative muffler. The optimized frequency range was 500 Hz to 5 kHz. Prototypes were developed with different distributions of porous material in the chamber and they showed improved performance compared with the reactive muffler.

Hybrid silencer transmission loss above a duct’s plane wave region

For large ducts, the removal of low frequency and tonal noise is normally achieved through the use of inefficient dissipative silencers; however, a combination of dissipative and reactive solutions could be more effective. But reactive noise control solutions are rarely applied to large diameter duct systems since it is commonly assumed that the low cut-on frequency of higher order modes severely restricts their efficiency. However, it is possible for a reactive silencer to remain operational outside of the plane wave region, provided the reactive elements are distributed across the cross-section of the duct. Of course, at higher frequencies, the sound field within a duct will have nonplane wave modal content, and the transmission loss is expected to differ compared to the plane wave condition. This effect is investigated here using numerical (FEM) predictions for hybrid dissipative-reactive parallel baffle silencers and the performance of the reactive elements is explored under different excitations. The effects of non-planar fields and individually excited modes are analyzed, and it is found that the frequency range over which quarter wave resonators contribute to transmission loss can be extended above the cut-on frequency of the duct by increasing the number of baffles.

The Application of Coconut Fiber as Dissipative Silencer

Heat ventilation air conditioning system (HVAC) is one of the ducting systems that broadly applied in the building. There are HVAC silencers in the market, however the sound absorptive material commonly used is mineral wool. In this research study, a sound absorptive material made of coconut fiber was tested to identify its performance as a potential replacement of green material for ducting silencer. The experiment was carried out in a testing apparatus that follows the BS EN ISO 11691:2009 standard. Different configurations of sound absorptive material and contents of coconut fiber were investigated in the study. The trend of insertion loss at 1/3 octave frequency was identified where at frequency below 3000Hz, the insertion loss of dissipative silencer is observed high at certain frequency with a very narrow range. At 3000Hz, the insertion loss of 4dB to 6dB is constant until 4000Hz and drops until 5000Hz before it increases again steadily up to 13dB at 10000Hz. A similar trend was observed for different configuration of sound absorptive material. Despite the configuration different, the outcome shows that the insertion loss is increasing with higher content of coconut fiber.

Experimental investigation of noise characteristics for HVAC silencers

Heating, ventilating and air-conditioning (HVAC) systems represent a major source of inside buildings noise with a negative effect on the acoustical environment. Therefore, it is necessary to add attenuation to the system. This is provided by sound attenuators or silencers manufactured in different geometrical configurations and with various absorptive lining materials. The paper deals with the insertion loss, as a performance criterion for in-duct noise, of some dissipative silencers types. The measured data are compared in order to establish the efficiency of every tested silencer type. Third-octave band analysis of random noise of an air handling unit (AHU) from a HVAC system is realized without and with silencers (ducts fitted). Insertion loss measurements are made in an anechoic room using a test facility that agrees the requirements of ISO 7235:2009. The results highlight the effects of the silencers geometry, pressure-air flow and loading relations upon the sound attenuation level. The silencers selection could be a difficult task for the designers which have to consider all these parameters.

Alternative method for acoustical characterization of absorbent materials based on the assessment of the acoustical efficiency of dissipative silencers

Alternative method for acoustical characterization of absorbent materials based on the assessment of the acoustical efficiency of dissipative silencers

Optimisation of micro-perforated cylindrical silencers in linear and nonlinear regimes

This paper describes analytical and experimental studies conducted to understand the potential of lightweight non-fibrous alternatives to dissipative mufflers for in-duct noise control problems, especially under high sound pressure levels (SPLs) and in the low frequency domain. The cost-efficient multi-modal propagation method has been extended to predict nonlinear effects in the dissipation and the transmission loss (TL) of micro-perforated cylindrical liners with sub-millimetric holes diameter. A validation experiment was performed in a standing wave tube to measure the power dissipated and transmitted by a nonlocally reacting liner under moderate and high SPLs. Although nonlinear effects significantly reduce the dissipation and TL around the liner maximum damping frequency, these power quantities may be enhanced below the half-bandwidth resonance. An optimal value of the in-hole peak particle velocity has been found that maximizes the TL of locally reacting liners at low frequencies. Optimisation studies based on dissipation or TL maximization showed the sensitivity of the liner constituting parameters to variations in the design target range such as the center frequency, the levels of acoustic excitation and the nature of the surface impedance (locally or nonlocally reacting). An analysis is proposed of the deviation observed at low frequencies between the optimum impedance of the locally reacting liner under moderate SPLs and Cremer׳s optimum impedances.

해상용 가스터빈의 스플리터 소음기 성능에 대한 실험적 연구

Based on ISO 7235, an experimental setup to measure the acoustic performance of splitter type dissipative silencers was fabricated. The length of each duct, sound source, microphone locations, modal filter, and anechoic termination were considered in the design of this setup. The modal filter is a particularly important factor because it affects the limit of measurement. The effects of number of splitters, absorptive material density, perforate plate, and media on the noise reduction of the sample silencers were experimentally investigated. The experimental results show that the insertion loss of a silencer with media, high perforate opening, and higher number of splitters increases especially at higher frequencies.

Finite element based acoustic analysis of dissipative silencers with high temperature and thermal-induced heterogeneity

A mixed finite element model has been derived for the acoustic analysis of perforated dissipative silencers including several effects simultaneously: (1) High temperature and thermal gradients in the central duct and the outer absorbent material; (2) a perforated passage carrying non-uniform axial mean flow. For such a combination, the properties of sound propagation media and flow are inhomogeneous and vary with position. The material of the outer chamber can be modelled by its complex equivalent acoustic properties, which completely determine the propagation of sound waves in the air contained in the absorbent medium. Temperature gradients introduce variations in these properties that can be evaluated through a heterogeneous temperature-dependent resistivity in combination with material models obtained at room temperature. A pressure-based wave equation for stationary medium is then used with the equivalent density and speed of sound of the absorbent material varying as functions of the spatial coordinates. Regarding the central air passage, a wave equation in terms of acoustic velocity potential can be used to model the non-uniform moving medium since the presence of temperature variations introduce not only heterogeneous acoustic properties of the air but also a gradient in the mean flow velocity. The acoustic connection between the central passage and the outer chamber is given by the acoustic impedance of the perforated duct. This impedance depends on the heterogeneous properties of the absorbent material and the non-uniform mean flow, leading to a spatial variation of the acoustic coupling and also to additional convective terms in the governing equations. The results presented show the influence of temperature, thermal gradients and mean flow on the transmission loss of automotive silencers. It has been found that high temperature and thermal-induced heterogeneity can have a significant influence on the acoustic attenuation of an automotive silencer and so should be included in theoretical models. In some particular configurations it may be relatively accurate to approximate the temperature field by using a uniform profile with an average value, specially for low resistivity materials. It has been shown, however, that this is not always possible and attenuation overestimation is likely to be predicted, mainly for high radial thermal gradients and high material flow resistivities, if the temperature distribution is not taken into account.

Point collocation scheme in silencers with temperature gradient and mean flow

This work presents a mathematical approach based on the point collocation technique to compute the transmission loss of perforated dissipative silencers with transversal temperature gradients and mean flow. Three-dimensional wave propagation is considered in silencer geometries with arbitrary, but axially uniform, cross section. To reduce the computational requirements of a full multidimensional finite element calculation, a method is developed combining axial and transversal solutions of the wave equation. First, the finite element method is employed in a two-dimensional problem to extract the eigenvalues and associated eigenvectors for the silencer cross section. Mean flow as well as transversal temperature gradients and the corresponding thermal-induced material heterogeneities are included in the model. In addition, an axially uniform temperature field is taken into account, its value being the inlet/outlet average. A point collocation technique is then used to match the acoustic fields (pressure and axial acoustic velocity) at the geometric discontinuities between the silencer chamber and the inlet and outlet pipes. Transmission loss predictions are compared favorably with a general three-dimensional finite element approach, offering a reduction in the computational effort.

Determination of the characteristic impedance and the complex wavenumber of an absorptive material used in dissipative silencer

The silencers are acoustic filters that have the purpose of reducing unwanted noise emitted by engines or equipment to acceptable levels. The vehicular silencers have large volume and dissipative properties. Dissipative silencers have absorptive material inside. These materials are typically fibrous and have good acoustic dissipation. However, few works depict the acoustic behavior of silencers with absorptive materials. The difficulty in evaluating this type of silencer is determining the acoustic properties of the absorptive material: the characteristic impedance and the complex wavenumber. This work shows an inverse methodology for determining the acoustic properties of the absorptive material used in silencers. First, it is found the silencer's acoustic efficiency in terms of the experimental sound transmission loss. Second, the absorptive material properties are determined with a parameters adjustment through a direct search optimization algorithm. In this step, the adjustment is done by applying The Finite Element Method in the search for the silencer's computational efficiency. The final step is to verify the difference between the experimental and computational results. For this work is used the acoustic efficiency of a silencer that has already been published in the literature. The results show good agreement.

Research and Application of Special Shaped Muffler Structure

The special-shaped silencing sheet and its derivatives of folded plate and expansion muffler structure, improve the acoustic attenuation performance in low frequency. The technical difficulties how dissipative silencer can effectively control the low frequency noise, is solved, the noise band of dissipative silencer is broadened, transmission loss increases, aerodynamic performance is optimized.