Dissipative Silencers Research Articles

Optimization of dissipative mufflers

The method of selecting the configuration of dissipative mufflers with the required acoustic efficiency is considered. The peculiarity of the considered approach is the use of an integral indicator of acoustic efficiency and dimensionless geometric parameters. The studies were carried out using finite element calculations. In the finite element model of a dissipative mufflers, acoustic characteristics of a fibrous sound-absorbing material obtained from experimental studies were used.

Optimization of Dissipative Mufflers

Optimization of Dissipative Mufflers

Review on attenuation methods of low-frequency noise in passive silencers

Noise reduction in various industrial and building systems such as ventilation ducts, vehicles, wind tunnels, and test facilities for jet engines, automobiles, and factories has always interested engineers, especially the problematic noise reduction at low frequencies. This problem is even more important with passive silencers, which require absorbent materials and more space. Passive silencers include reactive and dissipative silencers. Among reactive silencers, Helmholtz resonators, expansion chambers, flute-like, drum-like, and plate silencers have proven effective at low frequencies. Combining some of these silencers with a dissipative silencer, called a hybrid silencer, can achieve excellent performance at a wide range of low to high frequencies. In these silencers, the reactive part reduces noise at low frequencies, and the dissipative part reduces noise at medium and high frequencies. The aim of this work, which focuses more on experimental studies, is to introduce the mentioned silencers, investigate the presented methods to improve the performance of these silencers at low frequencies, and offer the practical advantages and disadvantages of these methods.

Noise Reduction from Energy Facilities by Using Energy-efficient Dissipative Silencers in the Turns of Gas-air Paths

Noise Reduction from Energy Facilities by Using Energy-efficient Dissipative Silencers in the Turns of Gas-air Paths

Acoustic Attenuation Prediction of Perforated Reactive and Dissipative Mufflers With Flow by Using Frequency-Domain Linearized Navier-Stokes Equations

To comprehensively consider the impact of complex airflow on sound propagation, a numerical approach by solving the three-dimensional (3-D) frequency-domain linearized Navier-Stokes equations (LNSEs) is proposed to accurately predict the transmission losses of perforated tube reactive and dissipative mufflers in the presence of complex airflow. The numerical computations are performed in two steps: (1) solve the time averaged flow by steady-state computational fluid dynamics (CFD) simulation and then map the results into acoustic mesh; (2) treat sound-absorbing material as an equivalent fluid and obtain acoustic perturbation variables by solving the frequency-domain LNSEs. The transmission losses of perforated tube reactive and dissipative mufflers are measured in the presence of flow and compared with the numerical predictions. The excellent consistencies between predictions and measurements validate the present approach. The influence of Mach number on acoustic attenuation performance of perforated tube reactive and dissipative mufflers is then investigated. With Mach number increases, the transmission loss of perforated tube reactive muffler increases somewhat, while the transmission loss of dissipative mufflers decreases obviously in the lower frequency range and change is limited at higher frequencies.

Design and demonstration of composite mufflers based on dissipative and reactive units

We have proposed and validated a design of a composite muffler. By a combination of dissipative and reactive units with different operating mechanisms, the advantages of both structures are utilized to achieve high transmission loss (TL) from low to high frequency in the ventilation duct system. A dissipative muffler composed of porous sound absorbing materials (PSAM) is chosen for noise attenuation above 1000 Hz, and a reactive muffler composed of Helmholtz resonators is used to reduce the noise below 1000 Hz. The combination of the two can achieve a TL above 20 dB in the broadband range of 244–1600 Hz and up to 30 dB in the vast majority of the frequency band, demonstrating excellent noise reduction effect. The proposed composite muffler has great advantages in broadband noise reduction for ventilation systems.

Computational approach for the acoustic modelling of large aftertreatment devices with multimodal incident sound fields

The influence of multimodal incident sound fields on the acoustic behaviour of large aftertreatment devices incorporating a monolith is modelled and analysed in detail. The analytical mode matching method is applied to the compatibility conditions of the three-dimensional acoustic fields at the device geometric discontinuities, leading to the computation of the complex wave amplitudes in all the subdomains involved and the corresponding device transmission loss. To have a realistic model, three-dimensional propagation must be considered in the inlet/outlet ducts and chambers, while one-dimensional wave propagation has to be assumed along the small capillaries of the aftertreatment device monolith (such catalytic converters and particulate filters); therefore, the monolith can be replaced by a plane wave four-pole transfer matrix from an acoustical point of view. On the other hand, for large aftertreatment device inlet ducts such as those found in heavy-duty and off-road engines, the usual models with plane incident wave excitation are not accurate since the onset of higher order incident modes in the inlet duct is expected for the frequency range of interest. Therefore, a variation of the acoustic attenuation performance is likely to occur depending on these modes, similar to the results previously found in the case of large dissipative silencers. Results are presented for three different multimodal incident sound field hypotheses: equal modal amplitude, equal modal power and equal modal energy density. A relevant influence on the sound attenuation is found for the test problems considered in the current investigation.

Modeling the Aerodynamic and Acoustic Characteristics of Plate Dissipative Silencers

The acoustic and aerodynamic properties of dissipative silencers with plates of constant and variable shape are compared. The purpose of the study was to jointly consider the aerodynamic and acoustic characteristics of silencers of various shapes. The effect the sound-absorbing material density has on the noise attenuation in the silencer is considered. It is shown that the use of variable-shape plates makes it possible to reduce the silencer flow friction. This results in a lower energy consumption for driving the fans and exhaust pumps and in a lower influence of flow friction on the technological processes at power and other industrial enterprises. It is shown that in the case of using plates with a constant shape, a drop in static pressure occurs in the channel between them, whereas in the case of using plates with a variable cross-section, the static pressure increases. With a free area ratio of 38%, the flow friction is reduced by up to 79%. The dependence of noise attenuation in the channels of plates with variable cross-section and constant shape on the free area ratio has been determined. In designing the silencer structures for noise reduction, it is recommended to use silencers with plates of variable cross-section, which have a significantly lower flow friction with the same acoustic efficiency in comparison with plates of a constant shape.

A study on acoustic performance of dissipative silencer for fluid-filled pipe system according to acoustic models of absorbing material

In many industrial systems involving pipe or duct structures, sound transmission modeling is critical to noise treatment and control. A typical noise treatment method applied to pipe structures is to use a silencer. In particular, a dissipative silencer with a sound absorbing material has a noise control ability to reduce noise through successive sound reflections by impedance mismatch while dissipating incident sound energy as heat. In this study, the acoustic performance of a dissipative silencer lined with multi-layered absorbing materials for fluid-filled pipe system is investigated. Herein, the absorbing materials are modeled applying poroelastic and viscoelastic theories. In the analysis, the eigenvalues and eigenfunctions in the dissipative silencer are determined from the rigid wall boundary condition and the interfaces between the absorbing materials, and then the acoustic performance is compared by obtaining the transmission loss using the mode matching method. Also, a parametric study is conducted to investigate the effect of the number and arrangement of layers on the acoustic performance of the silencer according to the absorbing material properties.

Neck-embedded acoustic meta-liner for the broadband sound-absorbing under the grazing flow with a wide speed range

Acoustic liners are the most commonly used dissipative muffler for aero-engines but remain a challenge to design a broadband acoustic liner under the grazing flow. In this work, a novel neck-embedded meta-liner is proposed for the broadband sound-absorbing under the grazing flow with a wide speed range. The meta-liner is composed of many neck-embedded Helmholtz Resonators, and the idea of bending the subsequent cavity is introduced to improve low-frequency sound absorption performance. The acoustic impedance model of the meta-liner with neck-embedded slit is first detailly established by analyzing the acoustic characteristics of the equivalent elliptical pipe and using the matrix transfer method. Then by designing the local coupling between channels, efficient broadband impedance modulation and broadband matching with air impedance under grazing flow conditions are realized. The proposed meta-liner exhibits the excellent sound attenuation capacity in the frequency domain of 500–3000 Hz under the grazing flow speeds from 0 m s−1 to 98 m s−1 and the sound absorption performance is greatly boosted compared with the traditional Double- Degree-Of-Freedom liner. Features mentioned above have been verified numerically and in a series of comparative experiments. Besides, the thickness of the meta-liner is only 50 mm. Our work provides a possible design reference for the new generation of acoustic liners and has potential applications in noise reduction engineering.

Acoustical optimization of mufflers hybridized with spiral perforated tubes using finite element method, Artificial Neural Network, and Genetic Algorithm

Because venting noise emitted from high pressure valve often occurred in industry and is huge, the noise abatement of the venting noise to protect human’s hearing health is necessary. In order to depress the high speed noise, a muffler internally equipped with spiral perforated tube is presented. To mitigate this noise, a dissipative muffler with perforated spiral tube installed on the pipeline is proposed. To evaluate the acoustical performance, a finite element method using COMSOL software is adopted. Also, the sensitivity analyses of Transmission Loss (TL) of the proposed muffler with respect to (1) diameter of the spiral and perforated tube ( D), (2) pitch of the spiral tube ( L), (3) acoustical impedance of the dissipative acoustic material ( R), and (4) perforation rate of a perforated tube (σ) has been carried out. For the optimization studies, both Artificial Neural Network (ANN) and Genetic Algorithm (GA) to optimize the muffler parameters L and D are applied. Here, two high target frequencies (2000 and 3500 Hz) to optimize the proposed muffler are exemplified. Consequently, the hybrid design of perforated spiral tube optimized using ANN and GA will mitigate the high pressure valve noise efficiently.

A Case Study on Annoyance Noise Caused by Metro Railway at a TOD Developed Depot

As an importance strategy to finance the transport infrastructure investment, transit‐oriented development (TOD) for railway depot is rapidly growing in China. However, railway‐induced noise emerged as the leading environmental nuisance triggering one of the most common public complaints. In order to discuss the influence of railway‐induced noise on the neighbourhood residents in a TOD developed depot, a special case study is provided in this study. Measurement campaigns were conducted for responding to the complaints, and noise map was calculated for finding out a noise control solution. Based on the recommendation awakening threshold value of 42 dB (A) summarized from the Night Noise Guidelines for Europe (WHO, 2009), the measurement results confirmed that the neighbourhood residents were indeed suffering from the instantaneous railway noise, and the sleep disturbance correlated indicator LAmax was more than 56 dB (A) at outside of buildings. Furthermore, calculation results indicated that the lower office buildings became mirrors which reflected the noise across over the cover structure to the higher stories of residents’ buildings. At the ends, a kind of dissipative splitter muffler was proposed as a potential solution, and its performance was also simulated.

Pressure Loss in Ducts by Dissipative Splitter Silencers: Comparative Study of Standardized, Numerical and Experimental Results

Dissipative splitter silencers are widely used in industry for the reduction of propagated sound waves in ducts. Even though these systems are effective from the acoustics point of view when they are properly designed, they also introduce a pressure loss in the system, due to the modification of the properties of the flow circulating inside the duct. This effect is not desired in some industrial applications, so it is necessary to be able to predict the pressure loss as precisely as possible to design silencers according to the needs. Nevertheless, the prediction made by standards are usually limited to given geometries or flow speed. In this work, we present a comparative study on the results obtained for the pressure loss by means of the standards ISO 14163 and VDI 1801-1, numerical simulations with the finite element method, and experimental measurements. Additionally, two different profile shapes and four input face velocities are tested in order to know the influence of these parameters in the variations of the flow and the accuracy of the prediction of the different methods.

Optimization of a reactive muffler used in four-cylinder petrol engine into hybrid muffler by using CFD analysis

Four-wheeler automotive vehicle is the basic need of people in today’s life all over the world. But the noise coming from the exhaust system is highly affecting the human health. Among all the noise produce in vehicle, exhaust system noise is maximum. Muffler is one such acoustic filter used for reducing this exhaust noise. Hybrid muffler is one type of muffler which combines the properties of both reactive muffler and dissipative muffler and it operates at wide frequency range. In this study an already existing muffler (Reactive type muffler) used in 91bhp, 6000 RPM four-cylinder petrol engine was optimized to hybrid muffler. Aerodynamic and acoustic performance parameters were the main performance parameter and were determined by CFD analysis and acoustic analysis. The existing reactive muffler of the four-cylinder petrol engine could be replaced by the hybrid muffler. Here the mufflers were designed in ANSYS design modular and analyses were performed in ANSYS Fluent and COMSOL software. CFD analysis was performed for the existing muffler and three other newly designed muffler by considering inlet velocity boundary condition at inlet velocity 40 m/s and 80 m/s. It was observed that increase in pressure drop (aerodynamic performance) in model 4, model 3 and model 2 is around 38%, 33.2% and 29% respectively. Again, from acoustic analysis the transmission loss (TL) in model 4 was observed to be 47 dB whereas in other models it was around 20 dB. Further it was observed that the sound pressure level from model 1 at the exit is around 75–78 dB whereas in model 4 it was observed to be around 45–50 dB at the exit. Hence from both the analysis it is concluded that model 4 which is hybrid type muffler is the most optimized muffler.

Transmission loss calculation of a pipe with dissipative silencer containing porous materials

Various types of silencers have been used to reduce noise transmitted through pipe system. In particular, a dissipative silencer containing absorbing materials inside has an ability of noise control by transferring acoustic/vibrational energy to thermal energy. In this study, the acoustic performance of a dissipative silencer lined with two-layer absorbing materials is investigated. Herein, the absorbing materials in the form of a polymer foam are considered as a porous medium composed of a solid polymer skeleton and internal pores. From the boundary conditions at the rigid wall and the interfaces between the porous materials, eigenvalues and eigenfunctions in the dissipative silencer are determined. Finally the transmission loss is obtained through the mode matching method. The effects of geometry and acoustic properties of the porous medium are also discussed.

Numerical analysis of the transmission loss of dissipative mufflers with polygonal cross-section

Dissipative mufflers are often used for the reduction of broadband noise transmitted in ducts. Many common calculation procedures for the transmission loss of such mufflers require conventional shapes like rectangular or circular cross-sectional areas. In an effort to analyze the effect of the cross-sectional area of dissipative mufflers on the resulting noise reduction, the transmission loss of axially uniform mufflers with polygonal cross-sectional areas was investigated using the finite element method. The mufflers are designed to have the same open area, and hence in a practical application would lead to a similar pressure drop. The results were compared to those obtained with the well known approximative method of Piening. Good agreement between simulation and estimation was found regarding basic trends at low frequencies, while notable differences were revealed regarding the maximum transmission loss.

A time-domain simulation method to predict insertion loss of a dissipative muffler with exhaust flow

Insertion loss (IL) is a more useful acoustic index than noise reduction and transmission loss for muffler design, but it is harder to predict because the prediction of IL requires knowing the characteristics of the muffler itself and the acoustic source. A muffler with porous material and complex meanflow makes the prediction even more challenging. Although calculation of noise reduction and transmission loss with the transient computational fluid dynamics (CFD) can naturally solve the flow–acoustic interaction problem, the published CFD methods have not been adopted to evaluate the IL of the dissipative muffler with exhaust flow. In this paper, a time-domain simulation method is developed to predict the IL of a complex engine muffler with porous material. The noise reduction and load impedance of the muffler were first calculated using a three-dimensional CFD model, in which the porous material was simulated by the governing equations modified with the material porosity and flow resistivity. The IL was then predicted from the noise reduction, the load impedance, and the engine source extracted from the measured exhaust noise. This developed method explores the application of the three-dimensional CFD method in calculating noise reduction, load impedance, and insertion loss of complex dissipative mufflers with exhaust flow.

Passive noise control in buildings: An engineering case study of ducted systems

The most common noise sources in buildings are related to Heating, Ventilating and Air Conditioning (HVAC) systems, plumbing systems, electrical systems and exterior sources. Passive Noise Control (PNC) techniques in buildings have been implemented in several ways. The aim of this work is to analyses the use of silencer to attenuate the noise in the ducts that are part of the ventilation systems in buildings, internal combustion systems, fans, gas conduction systems, boilers, etc. The main objective of a silencer is to reduce the transmission of noise, disturbing as little as possible the circulation of gas or liquid. In the first instance, the silencers are classified as reactive and dissipative, depending on whether the attenuation of the noise is produced by reflective or dissipative mechanisms, respectively. In a reactive silencer, the losses occur essentially due to the reflections of the sound waves in impedance discontinuities, such as widening or narrowing of the tube. In dissipative silencers, the flow is in contact with a large surface of absorbent material. The attenuation of the noise is then produced by visco-thermal losses in the porous material. In this work, a practical issue will be addressed with a noise reduction of 19 dBA in 60 Hz. Practical application Noise is a current issue in residential areas that could lead to health problems for people. The origin of these noises within buildings is very diverse, one of them is produced by ducts. Appling the PNC technique in modern building construction would be a good prevention practice. For this reason, in this project a PNC system was carried out in the ducts of a residential building, which could be used as a praiseworthy solution, avoiding problems for the inhabitants of these spaces.

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.

OPTIMIZATION OF COMPLEXLY SHAPED DISSIPATIVE SILENCERS

The article is about a comprehensive study of dissipative silencers of complex shapes. Analyzed the acoustic efficiency of such silencers. The sound absorption coefficient determined from experimental data. The acoustic properties of plate silencers take into account that coefficient. Presented the results of verification of models with plates of constant thickness. Considered the plates of complex shapes: uniform thickness change, convex and concave. Obtained results of modeling the acoustic efficiency of silencers with complex shape plates. These results are for different values of the mean integral thickness for the octave bands frequencies. Determined the dependence of the length of complex shapes silencers on their mean integral thickness. Presented the results of mathematical modeling of the airflow in silencers to determine their aerodynamic drag. The calculations showed that there is an optimal shape and mean integral thickness of the plates. Under which conditions the aerodynamic resistance of the silencer is minimal.