Acoustic Attenuation Performance Research Articles

Low-frequency sound attenuation by coiled-up meta-liner with nonuniform cross sections under grazing flow

We report a kind of coiled-up meta-liners with nonuniform cross sections (CMNC), which can efficiently attenuate low-frequency sound waves under grazing flow with a deep subwavelength thickness (e.g., ∼λ/17 at 500 Hz). At a grazing flow Mach number of 0.26, the average transmission loss of the meta-liner is 12.6 dB at 500–1000 Hz, which is twice as much as that of a double-degree-of-freedom acoustic liner of the same size. Physically, the nonuniform cross-sectional distribution and significant cross-sectional area ratio enhances vortex shedding, thus resulting in severe acoustic energy dissipation. The excellent low-frequency acoustic attenuation performance of CMNC is investigated thoroughly with experimental, theoretical, and numerical methods. This work provides an avenue for low-frequency noise reduction in grazing flow scenarios (e.g., in a high bypass ratio turbofan engine).

Nonlinear Acoustic Behavior Prediction and Analysis of Straight Through Perforated Tube Silencer With Flow

Straight through perforated tube silencers are widely used in intake and exhaust noise control, and their acoustic behaviors are influenced by the gas flow and high amplitude sound remarkably. To investigate the acoustic attenuation performance of straight through perforated tube silencers in the presence of flow and high amplitude sound, an approach based on the three-dimensional (3D) time-domain computational fluid dynamics (CFD) simulation is employed and validated by comparing predictions and measurements. Transmission loss of the straight through perforated tube silencer under pure tone and multi-tone sound excitations is predicted. Results show that, under the pure tone sound excitations, the acoustic attenuations are affected remarkably by the high amplitude sound, and the nonlinear effects turn to weaken as flow velocity increases. With the sound pressure level (SPL) decreasing along the axis of the silencer, the acoustic nonlinearity in the orifice weakens gradually. The shedding vortices are restricted to the region near perforations because of the transport of flow. When the multi-tone sound excitation is enforced on the inlet, transmission loss of the silencer is different from the pure tone excitation at the same SPL of every compositional frequency.

Noise reduction through a waveguide structure consisting of expansion chambers with a geometrical defect

Noise control helps to make working environments safer and keep operations in line with health and safety standards. Exhaust noise is the main component of noise pollution in urban environments. In this paper, we focus on noise control by improving acoustic attenuation performance using a one-dimensional waveguide structure composed of simple periodic expansion chambers with a geometric defect. This defect is located at the center of the periodic structure and results from a modification in the length of the central chamber of the system. The objective is to study the properties of acoustic transmission and transmission loss and to examine the effect of defects in a periodic acoustic structure. The system’s spatial periodicity enables us to design wide band gaps where sound waves cannot propagate. This characteristic is very important for reducing noise in our environment. The effect of the cross-sectional ratio on the band gap behavior was also examined in this work. In addition, we have shown that the presence of a defect in a regular structure leads to a perturbation of the structure’s spatial periodicity. This leads to the creation of defect modes or resonance modes in the band gaps. We also controlled the number and amplitude of defect peaks within the band gap by varying the length of the defect. The results of this work are of interest for various applications, such as the creation of wide acoustic bands, low-frequency noise reduction, and acoustic wave filtering.

Acoustic attenuation prediction and analysis of perforated hybrid mufflers with non-uniform flow based on frequency domain linearized Navier-Stokes equations

The three-dimensional frequency-domain linearized Navier-Stokes equations (LNSEs) with consideration of eddy viscosity are developed to evaluate the acoustic attenuation performance of perforated hybrid mufflers in the presence of non-uniform flow. The computations are performed in two steps: time averaged flow variables are acquired by using steady-state computational fluid dynamics (CFD) method and then mapped into the acoustic mesh, and the acoustic perturbation variables are obtained by solving frequency-domain LNSEs, where the sound-absorbing material is treated as an equivalent fluid with complex sound speed and density. The predictions of transmission losses of the two-pass perforated hybrid mufflers in the presence of non-uniform flow are in good consistencies with the measurements, which verifies the correctness of LNSEs. The effect of Mach numbers on acoustic attenuation performance of the mufflers with different filling densities and perforated components is investigated in detail. The transmission loss of the mufflers with various filling densities are increased by complex airflow in the lower frequency range. For the mufflers where not all components have been perforated, the flow lowers low-frequency resonance peak. The influence of complex airflow on acoustic attenuation performance of all configurations is weakened at higher frequencies.

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 of an instrumentation system for assessing the acoustic attenuation performance of ducted Helmholtz resonators in the presence of grazing flow

The Helmholtz resonator (HR) has been widely used as a duct silencer to reduce low-frequency noise. However, the presence of grazing flow could influence the noise attenuation performance of the HR in ductwork systems. This study aimed to design an instrumentation system for studying the acoustic attenuation performance of ducted HRs in the presence of grazing flow. The instrumentation system includes a centrifugal fan, diffusion section, anechoic termination, flow development section, test section, and sound source. In this study, a HR as a ducted silencer was installed in the test section of the instrumentation system. Transmission loss (TL) due to the HR insertion was measured in the presence of forward and reverse grazing flows. The results showed that the instrumentation system succeeded in assessing the noise attenuation performance of the HR under a high sound pressure level and different flow rates and directions.

Analysis of Helmholtz resonator wall elasticity effects on the performance of periodic acoustic metamaterial

In this paper, a metamaterial made of a porous layer with embedded periodic Helmholtz resonators is studied and the effects of the resonator wall elasticity on the acoustic attenuation performance of the metamaterial are investigated numerically. The analytical transmission loss results using the parallel transfer matrix method show good agreement with finite element results for the rigid wall case. Elastic materials with different thicknesses are considered for the wall of the neck and the top, lateral and bottom walls of the resonator cavity. It is shown that the transmission loss of the metamaterial degrades when the bottom wall of the resonator cavity is elastic. The elasticity of the bottom wall has a significant impact on the performance of the metamaterial. This analysis will help for better design of acoustic metamaterials with periodic Helmholtz resonators.

Numerical investigation of multi-designed mufflers in internal combustion engines for noise reduction

Some of the most harmful sources of noise pollution to humans are the exhaust gases of internal combustion engines and factory equipment such as compressors and turbines. An environment contaminated by these sounds can also cause brain cancer. These noise issues can be addressed by installing expansion chambers in the exhaust path. Although numerous studies have been conducted on mufflers, three-dimensional mufflers with non-coaxial inlet and outlet dimensions have not been emphasized in previous studies. To improve the acoustic attenuation performance of the reactive muffler, the effects of different parameters on transmission loss need to be investigated, which is performed in the present paper. The geometry of the muffler was considered one of the noise control parameters. The muffler was modeled using the Helmholtz equation and simulated in COMSOL for analysis. The results demonstrate that the noise level drops with an increase in length, width, and height. The optimal values for the length, width, and height of the muffler are calculated to be 700, 320, and 170 mm, respectively. In addition, the optimum size of the non-coaxial muffler is found 60 mm. Compared to existing mufflers, the proposed muffler reduces noise by 13% to 34%. These research findings will assist the automotive or other related industries to reduce the noise produced by vehicles and other industrial devices.

Acoustic attenuation analysis of expansion chamber mufflers with non-uniform cold and hot flow

In the presence of flow, the acoustic field and the flow field inside mufflers will interact with each other. In order to capture this phenomenon, the frequency-domain linearized Navier–Stokes equations (LNSEs) are employed to investigate the vortex-sound interaction inside expansion chamber mufflers. The calculations are performed in three steps: (1) to obtain the background mean flow variables by using the steady-state CFD simulation, (2) to map CFD results to acoustic meshes, and (3) to calculate the acoustic perturbation by using the finite element method based on LNSEs. Primarily, the transmission loss of single expansion chamber with inlet/outlet extensions is predicted in the presence of cold flow and the numerical predictions of LNSEs showed consistent agreements with measurements published in the literature. With the increase of inlet flow velocity, transmission loss in the low frequency range is obviously influenced by the vortex-sound interaction, which is attributed to the energy transfer between the fluidic and acoustic scales inside the muffler. Further, the acoustic attenuation performance of dual-chamber mufflers with flow is investigated numerically and experimentally, and the LNSEs predictions agree well with measurements. In the presence of cold flow, the transmission loss curves of the mufflers in the low frequency range is disturbed by the acoustic dissipation and amplification, while the effect of hot flow on the acoustic attenuation behavior of the mufflers is almost the same as that of cold flow, except that the non-coaxial mufflers with side-inlet is remarkably influenced by temperature. The studies demonstrated that the background mean velocity gradient is the most important impact factor for acoustic attenuation performance of the dual-chamber mufflers in low frequency range, whether in the presence of cold or hot flow.

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.

A Virtual Sensing Active Noise Control System Based on a Functional Link Neural Network for an Aircraft Seat Headrest

PurposeThe goal of this work is to propose a local Active Noise Control system for an aircraft seat’s headrest. This system should be able to extend the quiet zone beyond the physical microphones that can be placed behind the passenger’s head due to space constraints. Furthermore, the control algorithm should perform efficiently when non-linear phenomena exist during the sound propagation.MethodsA functional link neural network and a computationally efficient Multiple Input Multiple Output approach are used for the system’s implementation. The quiet zone is extended by linearly estimating the acoustic pressure in front of the headrest surface. The main novelty of this paper is the combination of these methods in an attempt to improve acoustic pressure attenuation performance, while keeping computational complexity low.ResultsThe proposed control algorithm has been evaluated through numerical simulations, including Finite Element Method and experimental tests at an aircraft cabin mock-up. The results show that for a real-world acoustic disturbance, a 10 dB reduction in sound pressure level was achieved 10 cm away from the headrest surface. In addition, the attenuation of some harmonics can reach 20 dB and in most cases is bigger than the linear FxLMS algorithm.ConclusionsTo summarize, it has been demonstrated that a multichannel functional link neural network using a simple virtual sensing technique can efficiently attenuate synthesized and real world acoustic disturbances captured in a tilt-rotor aircraft’s cabin. Finally, it can create an adequate quiet zone for gentle head movement while maintaining its stability.

Numerical Study of Acoustic-Structure Interaction of Selected Helicoidal Resonator with Flexible Helicoidal Profile

The paper presents the results of numerical studies of acoustic-structure interaction of selected helicoidal resonator with helicoidal profile made of an elastic material. Considered a well-recognized acoustic system for one representative type of acoustic helicoidal resonator with two resonant frequencies that correspond to previous studies of the author. Due to the large range of flexible materials to study, this work focuses on the change of material density, Poisson’s ratio and Young’s modulus as the basic parameters describing the properties of elastic materials. The results indicate a significant interaction between the acoustic attenuation performance of helicoidal resonator and elasticity of the helicoidal profile. These interactions are most evident in the frequency range in which the helicoidal resonator is revealed to be effective acoustic damper.

Acoustic Attenuation Performance Analysis and Optimisation of Expansion Chamber Coupled Micro-perforated Cylindrical Panel Using Response Surface Method

This paper describes boundary element method (BEM), experimental and optimization studies conducted to understand the potential of expansion tube coupled micro-perforated cylindrical panel (MPCP) to enhance the acoustic attenuation for in-duct noise control issues. Due to complex structure of the MPCP and for the correct prediction of acoustic attenuation, BEM is adopted on the basis of PLM Simcenter 3D software to compute the sound transmission loss (TL). As the MPCP is cylindrical in-shape with numbers of sub-milimeter holes, additive manufacturing based 3D printing was utilized for the model prototyping to reduce current design limitation and enabled fast fabrication. The TL measurement based two-load method is adopted for modal validation. Subsequently, a parametric studies of the MPCP concerning the perforation hole diameter, perforation ratio and depth of air space are carried out to investigate the acoustical performance. Optimization via response surface method (RSM) is used as it allows evaluating the effects of multiple parameters as required in this study. The model validation result shows that the error between the BEM and and measured values is relatively small and show a good agreement. The R-square value is 0.89. The finding from parametric study shows that a widen peak attenuation can be achieve by reducing the perforation hole diameter and one way to increase the transmission loss amplitude is by increasing the air cavity depth. Finally, the optimized MPCP model was adopted to the commercial vacuum cleaner for the verification. The sound pressure level (SPL) of the vacuum cleaner is significantly attenuated within the objective frequency of 1.7 kHz and its A-weighted SPL is reduced by 1.8 dB.

Finite Element Approach for Three-Dimensional Linearized Potential Equation with Application to Muffler Acoustic Attenuation Prediction

In this paper, a three-dimensional finite element method for solving the linearized potential equation (LPE) is implemented to predict the acoustic attenuation performance of mufflers in the presence of nonuniform flow. For accurate physical representations, the porous material is modeled as equivalent fluid with complex speed of sound and density while the background mean flow in fluid domain is captured by solving Reynolds-averaged Navier–Stokes equations. The treatment details for boundary integrals at different boundary conditions and expressions for transmission loss are presented. Good agreements of predicted and measured transmission loss of two reactive and a hybrid mufflers validated the present approach. Finally, the convective effect of nonuniform flow on the acoustic attenuation behavior of the hybrid muffler is examined numerically in detail.

Topology-optimized ventilation barrier for mid-to-high frequency ultrabroadband sound insulation

An ultrabroadband ventilated acoustic barrier, composed of periodic subwavelength units (with a thickness of 50 mm), is put forward here by a density-based topology optimization method. We propose a unit with a cross channel as the initial unit for the topology optimization process. Compared with the initial one, the optimized barrier reduces the unit volume filling ratio by nearly 50 %. The efficient sound attenuation band is shifted to a lower frequency by nearly 665 Hz, and the broadband is broadened by nearly 870 Hz. Then, a simplification of the optimized unit is carried out, and the influence of the structure parameters on the acoustic attenuation performance is discussed. Numerical simulations and experiments are conducted to verify the sound-blocking performance. The optimized and simplified structures can block 90 % of the incident energy in [1420 Hz, 5140 Hz] and [1571 Hz, 5150 Hz], respectively, and both show excellent omnidirectional sound insulation performance. In addition, the ventilative capabilities are further evaluated by simulating the airflow across the two structures. The proposed barrier may provide new insight into the acoustic applications calling for soundproofing in free surroundings.

Case study: Numerical study of the noise reduction characteristics of corrugated perforated pipe mufflers

Resistant mufflers are commonly used in the wide band noise control of the exhaust of tractor internal combustion engines due to their simple structure, broadband frequency performance and long service life. In this paper, a corrugated perforated pipe muffler was proposed based on an improved design of a straight-through perforated pipe muffler to reduce the exhaust noise of internal combustion engines. The acoustic attenuation performance of the corrugated perforated tube muffler under the action of nonuniform flow and a temperature gradient was predicted by using the one-way flow-acoustic coupling method, which combines computational fluid dynamics and the acoustic finite element method. The pressure loss and self-noise of the corrugated perforated tube muffler were compared with those of the straight-through perforated tube muffler. The influence of the structural parameters of the corrugated perforated tube mufflers on the transmission loss was analyzed. The significance level of the perforation diameter, peak height, distance between adjacent peaks, and peak width on the transmission loss of the corrugated perforated tube muffler was studied by multiple linear regression analysis.

Acoustic Attenuation Performance of Two-Pass Perforated Hybrid Mufflers with Mean Flow

The one-dimensional (1-D) analytical approach is developed to predict and analyze the acoustic attenuation performance of two-pass perforated hybrid mufflers in the presence of mean flow. The expressions of complex acoustic wavenumber and impedance of long fiber glass wool are presented by using the impedance tube measurement and curve fitting. The 1-D approaches as well as the fitting expressions are validated by comparing the predicted and measured transmission loss of two-pass perforated hybrid mufflers with a different Mach number. Results illustrate that the 1-D predictions agree well with measurements below the plane wave cut-off frequency and deviate in higher frequency range. The influences of geometric parameters including the lengths of extended tubes, perforations on the right bulkhead and the outlet tube, and mean flow on the acoustic attenuation behavior of the hybrid mufflers are examined.

A three-dimensional finite element analysis of tuning the selected acoustic helicoidal resonator by the change of its mandrel diameter

This work presents the possibility of tuning the acoustic helicoidal resonator by the change of it’s central mandrel diameter. Set of numerically computed systems with different mandrel diameters were executed. For numerical investigations, by the use of a finite element method, the acoustic helicoidal resonator was placed inside an infinite long cylindrical duct. Also the transmission loss is achieved as an acoustic attenuation performance parameter. The results show that the mandrel is a very important part of the helicoidal resonator and it provides a proper physical functioning of this solution, as an acoustic resonator. Also this paper presents the normalized range of mandrel diameters, which provides the best performance.

Metamaterial-based muffler with broadband tunability in a limited space: optimal design, theoretical investigation and experiment

A versatile unit cell for metamaterial-based mufflers that can be mounted in a limited space is proposed and optimized without increasing their size to reduce duct noise in the low frequency band as well as middle and high frequency bands. The acoustical behavior of the proposed unit cell is explained in terms of its dispersion curve. The acoustic metamaterial filled with the proposed unit cells allows inflowing fluid to pass freely but blocks incoming noise within the target frequency range. The wave transmission characteristics of the metamaterial-based muffler were identified by using the dispersion curve of the unit cell during the optimization process. The dimensions of the internal structure of the proposed unit cell were systematically determined using an optimization method for the target frequency range with its outer dimensions fixed. Especially, the length of the optimized unit cell designed for the low frequency band is less than one-fifteenth of the working wavelength. The structural features of the optimized unit cell are discussed from a theoretical viewpoint. Two types of optimized unit cells were combined in one metamaterial-based muffler to reduce noise over a wider frequency band. The acoustic attenuation performance of the optimized metamaterial-based muffler was validated experimentally.

Numerical Investigation of Back Pressure and Acoustic Attenuation Performance of Two and Three Chamber Exhaust Muffler

Two and Three chamber expansion muffler are commonly used in light category vehicle for meeting the noise level legislature requirement. The Helmholtz resonator and expansion chamber are used in exhaust muffler to attenuate the noise in low and middle frequency range. In this work, the effect of expansion chamber size on back pressure is determined using CFD and the same geometry were used to study the acoustic attenuation performance using FEA in the frequency range of 0 to 3000 Hz. Regarding the size of individual expansion chamber there is about 17% increase and decrease in size from base model geometry were considered for performance comparison in both two and three chamber type of muffler. It is found that minimum back pressure of about 241 Pa is observed for 2 chamber base model (2C-B) with slightly higher turbulent kinetic energy value of 77.53 J/Kg. The sound pressure level (SPL) at outlet pipe is less for 3 chamber mufflers and the obtained value is less than 120 dB for most of the frequency range of 0 Hz to 3000 Hz. Similarly transmission loss (TL) is higher for 3 chamber mufflers and the value remains same for 0 to 1600 Hz frequency range and peak fluctuation occurs at higher frequency. The obtained results shows that 3C-A model is having higher noise attenuation performance with back pressure value within acceptable limit.