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Aiming at the problems of large volume, high exhaust resistance and difficulty in suppressing noise in the 500 Hz-100 Hz frequency band of traditional internal combustion engine exhaust mufflers, a noise reduction unit design based on acoustic metamaterials is proposed. Based on the equivalent medium theory, an acoustic model with a ring structure and multi-region variable refractive index was established. Phase control is achieved by helically winding the acoustic channel to change the refractive index, and the basic dimensions of the acoustic metamaterials muffling unit are calculated. The sound field distribution, transmission loss and flow field characteristics of the muffling unit are simulated and analyzed. This structure utilizes a multi-layer acoustic channel structure, effectively alleviating the problem of insufficient low-frequency noise elimination caused by the asymmetry of Fano interference. It achieved a transmission loss of over 10 dB within 85% of the 500 Hz to 1000 Hz frequency band, and still maintained excellent noise reduction performance under high-frequency conditions through multi-level phase control. By connecting multiple units in series, a transmission loss of 10 dB can be achieved within 85% of the 500 Hz to 1000 Hz frequency band. The exhaust flow field of the muffling unit was simulated and analyzed. Whether used alone or in series with the traditional muffling structure, the exhaust resistance remained within the range of 360 Pa to 370Pa. Experimental tests show that when the metamaterial muffler unit is used in combination with the traditional muffler, it effectively achieves targeted noise elimination in the 500 Hz-1000 Hz frequency band, and also demonstrates clear noise reduction capabilities in higher frequency ranges. The high noise suppression characteristics, high gas passage characteristics and compact volume characteristics of this structure provide more potential analysis methods and design schemes for the research and development of internal combustion engine mufflers and noise reduction accessories.

Traditional sound-absorbing materials, which are intended to address the issue of low-frequency noise control in automobile air-conditioning duct mufflers, have limited noise reduction effects in small spaces. Because of their straightforward structure and excellent controllability, acoustic metamaterials—particularly Helmholtz resonators—have emerged as a research hotspot in low-frequency noise reduction. However, existing technologies have issues such as restricted structural scale, narrow absorption frequency bands, and conflicts with ventilation requirements. To address these, this paper proposes a new type of Helmholtz perforated and tortuous-characteristic duct muffler for the unit cell of acoustic metamaterials. Through the innovative structural design combining a perforated panel with a multi-channel tortuous cavity, the length of the channel is changed in a limited space, thereby extending the sound wave propagation path and enhancing the dissipation of sound wave energy. Meanwhile, for the muffler, acoustic theoretical modeling, finite element simulation, and parametric optimization methods are adopted to systematically analyze the influence of its key structural parameters on the sound transmission loss (STL) of the muffler. Compared with the traditional folded-channel metamaterial, the two differ in resonance frequency by 38 Hz, in transmission loss by 1.157 dB, and in effective bandwidth by 1 Hz. This research provides theoretical support and design basis for solving the problem of low-frequency noise control in ventilation ducts, improves low-frequency broadband sound absorption performance, and promotes the engineering application of high-efficiency noise reduction devices.

South Africa’s electrical infrastructure deficit and poor maintenance plans have resulted in frequently occurring power outages, which has caused a decline in productivity, alongside an increase in vandalism. As a result, many households and businesses have resorted to alternative energy sources, such as petrol generators. However, petrol generators are loud, reaching acoustic levels of roughly 90 dB, and have proven negative impacts on society and the environment. To reduce the noise intensity produced by small petrol generators, a noise reduction mechanism has been designed. The mechanism incorporates several noise attenuation materials such as porous absorbers and perforated panels. The design also features both passive and active ventilation features for optimal cooling of the generator during use. With flow simulating software embedded in SolidWorks, the temperature, pressure and acoustic power level within the noise reduction mechanism was investigated. The sound box is shown to reduce the noise of the generator to approximately 10 dB, whereas the muffler, can reduce the noise intensity to 37 dB. The overall temperature within the sound box is roughly 75 ˚C, and the maximum pressure was determined to be 103 kPa, which is sufficient for operation of the generator, without compromising its performance and functionality. A modified experimental model was built to validate the numerical results. The results from the experiment produced similar results in terms of decrease in sound when the insulator is added.

Over the last two decades, several experimental and numerical studies have been performed in order to investigate the acoustic behavior of different muffler materials. However, there is a problem in which it is necessary to perform large, important, time-consuming calculations particularly if the muffler was made from advanced materials such as composite materials. Therefore, this work focused on developing the concept of the indirect dual-chamber muffler made from a basalt fiber reinforced polymer (BFRP) laminated composite, which is a monitoring system that uses a deep learning algorithm to predict the acoustic behavior of the muffler material in order to save effort and time on muffler design optimization. Two types of deep neural networks (DNNs) architectures are developed in Python. The first DNN is called a recurrent neural network with long short-term memory blocks (RNN-LSTM), where the other is called a convolutional neural network (CNN). First, a dual-chamber laminated composite muffler (DCLCM) model is developed in MATLAB to provide the acoustic behavior datasets of mufflers such as acoustic transmission loss (TL) and the power transmission coefficient (PTC). The model training parameters are optimized by using Bayesian genetic algorithms (BGA) optimization. The acoustic results from the proposed method are compared with available experimental results in literature, thus validating the accuracy and reliability of the proposed technique. The results indicate that the present approach is efficient and significantly reduced the time and effort to select the muffler material and optimal design, where both models CNN and RNN-LSTM achieved accuracy above 90% on the test and validation dataset. This work will reinforce the mufflers’ industrials, and its design may one day be equipped with deep learning based algorithms.

This research aims to measure OEM, HPLPM and Free flow mufflers under several noise level testing. In this study, the factory-installed exhaust was replaced with a free-flow type and HPLPM exhaust. The research employed a quantitative method with an experimental approach. Testing was carried out under three different vehicle conditions that correspond to their usage: normal condition, exhaust condition, and acceleration condition. Each type of exhaust was tested three times for each testing condition. Subsequently, the collected data were analyzed and compared against the established quality standard in Indonesia, which stipulates that the noise level must not exceed 90 db. Based on the average noise levels produced by the free-flow exhaust type, it was found that under normal vehicle conditions, the average noise level was 28.08% higher compared to the OEM exhaust type. Under exhaust conditions, the free-flow exhaust type resulted in an average noise level 23.13% higher than the OEM type. During acceleration, the free-flow exhaust type generated an average noise level 14.54% higher than the OEM type. Employing the HPLPM exhaust type under normal vehicle conditions yielded an average noise level 25.9% higher than the OEM type. Under exhaust conditions, the HPLPM exhaust type led to an average noise level 21.2% higher than the OEM type. Finally, during acceleration, the OEM exhaust type produced an average noise level 0.03% higher than the HPLPM type. Based on this data, HPLPM can reduce noise levels better than free flow muffler. At the higher RPM HPLPM can produce almost the same noise as the OEM type.

The astronauts need to re pressurize the sealed cabin after returning to the sealed cabin. The noise generated in the process of re pressurization is too large, so a silencer shall be set to reduce the noise in the cabin. Through theoretical analysis, this paper selects the small hole injection muffler, makes theoretical calculation, and determines the specific structural size of the muffler. The test shows that when the cabin pressure rises to 85kpa, the use of the small hole injection silencer can reduce the cabin noise from 85.7db to 66.3db, which meets the medical requirements of astronauts.

Air Pollution produced from automobiles has become one of the most critical problems of environmental concern and likely to increase as vehicle population is assumed to grow approximately by 1300 million by the year 2030. In India, automobiles will have to meet the BS-VI emission standards to reduce vehicular pollution that impacts on environment and human health. These norms impose various restrictions on emissions of exhaust gases like NOx, HCs, particulate matter. Many recent exhaust control devices are being developed to meet the required needs. Therefore, the present study aims to reduce CO2 emissions generated in SI engine automobiles using activated carbon/calcite in a muffler. The backpressure parameter of the muffler affects the efficiency of an internal combustion engine. The porosity and diameter of the inner tube of the muffler has a prominent effect on backpressure. Therefore, the muffler with 48, and 60 holes, respectively and 1mm and 2mm diameter, respectively on a perforated tube have been investigated. The result shows that the concentration of CO2 and back pressure is greatly reduced by increasing porosity and diameter, respectively of the tube. Thus, an environmentally friendly and efficient muffler is modeled and simulated.

In this study, the impacts of the inclusion of two semi-circular baffles and their orientations on the acoustic performance of the side outlet muffler have been investigated. The side outlet muffler has a circular simple expansion chamber with an axial inlet and a side outlet and two semi-circular baffles that have been placed inside the expansion chamber at different orientations. The axis of the outlet is at the right angle to the axis of the inlet. The acoustical investigation of the side outlet muffler with two semi-circular baffles is done using the plane wave analysis, the finite element method (FEM), and the two-load technique. Based on the orientations of the two semi-circular baffles, three different models of side outlet muffler with semi-circular baffles have been investigated. The plane wave analysis, FEM, and two-load method are applied to all models and it is found that analytical, computational, and experimental transmission loss (TL) are in good agreement. The analytical modelling successfully predicts the presence of semi-circular baffles in the form of peaks and troughs in the TL of side outlet muffler with semi-circular baffles before the cut-off frequency and thus proves its effectiveness. Among all the models, model_2 gives 42 % higher TL than model_1 and model_3 shows 16.20 % higher TL than model_2. Hence, model_3 proves to be the best design for the side outlet muffler with semi-circular baffles in the attenuation of noise. The model_3 is effective for 1030 Hz–1480 Hz, 1500 Hz–1570 Hz, and 1640 Hz–2400 Hz frequency sound waves. The TL curve, sound pressure contours for model_3, and the band power variation in the 1/3 octave band for all the models have also been presented.

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.

This study proposes a composite acoustic porous duct metamaterial muffler composed of a perforated tortuous channel and an externally wrapped porous layer, integrating both structural resonance and material damping effects. Theoretical models for the perforated plate, tortuous channel, and porous material were established, and analytical formulas for the total acoustic impedance and transmission loss of the composite structure were derived. Finite element simulations verified the accuracy of the models. A systematic parametric study was then performed on the effects of porous material type, thickness, and width on acoustic performance, showing that polyester fiber achieves the best results at a thickness of 30 mm and a width of 5 mm. Further analysis of periodic distribution modes revealed that axial periodic arrangement significantly enhances the peak noise attenuation, radial periodic arrangement broadens the effective bandwidth, and multi-frequency parallel configurations further expand the operating range. Considering practical duct conditions, a single-layer multi-cell array was constructed, and its modal excitation mechanism was clarified. By employing the Particle Swarm Optimization (PSO) algorithm for multi-parameter optimization, the average transmission loss was improved from 26.493 dB to 29.686 dB, corresponding to an increase of approximately 12.05%. Finally, physical samples were fabricated via 3D printing, and four-sensor impedance tube experiments confirmed good agreement among theoretical, numerical, and experimental results. The composite structure exhibited an average experimental transmission loss of 24.599 dB, outperforming the configuration without porous material. Overall, this work highlights substantial scientific and practical advances in sound energy dissipation mechanisms, structural optimization design, and engineering applicability, providing an effective approach for broadband and high-efficiency duct noise reduction.