Noise Control Treatments Research Articles

An innovative approach for enclosure design of power generators to meet noise regulation targets

The use of diesel-powered engine generators as a power source is very common for various industrial and residential applications. There are strict regulatory requirements and legislations to limit the noise levels from these generators below statutory limits. The purpose of the paper is to present an approach using statistical energy analysis (SEA) augmented by experimental inputs for the prediction of noise levels for the generators and propose strategies for optimized enclosure design that effectively mitigate noise to achieve regulatory requirements. The study begins by developing an SEA model that represents the acoustic behavior of the generator and enclosure. The airborne sources such as engine, alternator, radiator fan and exhaust are modelled explicitly using experimental noise source characterization. Based on the established SEA model and excitation sources, noise levels are predicted through the calculation of energy flow between subsystems. The predicted sound levels for an existing Diesel Generator with enclosure were validated with its test data to ensure correctness of SEA model. Parametric studies were carried out using simulations on key variables, including critical noise paths, panel thickness, and noise control treatments. These investigations helped identify optimal process parameters that reduce noise levels emitted by genset in adherence to noise regulations.

LCV Vehicle in-cab noise prediction and reduction using SEA based simulation approach

With increase in consumer demand vehicle OEMs are developing quieter vehicles at a design stage using acoustic simulation tools to reduce development cycle times avoiding costly proto builds and test iterations. Present work describes application of statistical energy analysis (SEA) simulation approach for in cab noise prediction and noise reduction through sound package optimization for LCV vehicle. In first step complete vehicle in-cab SEA model is prepared with baseline sound package concept. It consists of comprehensive definition of structural panels, interior and exterior cavities. Noise control treatment is modelled as multi-layer poro-elastic materials. Airborne noise excitations for sources engine, transmission, intake and exhaust muffler are applied. Source strengths for above sources were derived from dyno-coupled powertrain noise tests in the anechoic chamber. Load case of rated speed was considered neglecting tire and wind noise during simulation. Contribution analysis was carried out to find out sensitivity of different panels on interior noise at various frequencies. It was observed that bottom middle floor panel and rear glass panel were major noise contributors. Noise mitigation was carried out by optimizing floor panel sound package and intake air filter within given space constraints. These modifications resulted in overall 2 dB in-cab noise reduction

Azimuthally-distributed wavy inner wall treatment for high subsonic jet noise control

The noise generated by subsonic jet nozzles, commonly encountered in civilian aircraft, is rather significant and propagates in both the upstream and downstream directions due to large-scale and fine-scale turbulence structures. In this paper, a distinctive inner wall treatment strategy, denoted as the Azimuthally-distributed Wavy Inner Wall (AWIW), is proposed, which is aimed at mitigating jet noise. Within this strategy, a circumferentially dispersed treatment wall characterized by a minute wavy pattern is substituted for the smooth inner wall in proximity to the nozzle outlet. To assess the effectiveness of the AWIW treatment, we conducted numerical simulations. The unsteady flow field and far-field noise were predicted by employing Large Eddy Simulations (LES) coupled with the Ffowcs Williams and Hawkings (FW-H) integration method. To gain a comprehensive understanding of the mechanism underlying the noise reduction facilitated by the AWIW treatment, it examined physical parameters such as the Lighthill source acoustic source term, the turbulent kinetic energy acoustic source term, and the shear layer instability. The results reveal that the AWIW treatment expedites the instability within the shear layer of the jet, leading to an early disruption of the jet shear layer, and consequently turbulent structures in varying sizes are generated downstream. This process effectively regulates the generation and emission of jet noise. By controlling the minor scale turbulence through the AWIW treatment, the mid- and high-frequency noise within the distant field can be significantly reduced. In the context of the flow field, the introduction of AWIW also leads to a decrease in drag on the inner wall surface of the jet, thereby improving the overall aerodynamic performance of the nozzle. Considering these attributes, the AWIW strategy emerges as a viable technique for the reduction of jet noise.

Comparison of modelling methodologies for large porous absorbers hanging in a volume

Porous material simulation methodologies are well established. However, most methods, like the Transfer-Matrix Method (TMM) make the assumption that there is a source side and a receiver side. In practice, this works for the majority of industrial noise control treatments. For architectural applications, this is not always the case. Hanging absorbers and curtains are common materials that add absorption to a space, but do not have a boundary condition at a wall or a well-defined source and receiver side. To evaluate the validity of existing simulation methods for this condition, a reverb room test was simulated with a large cube of melamine foam inside the volume. The effective absorption of the cube in the space was then calculated using various simulation methodologies, and compared to results from a measured reverb room absorption test.

Numerical prediction of the sound transmission loss of double panel configurations with acoustic structured and poroelastic materials

The sound transmission loss of multilayer system assemblies made of Noise Control Treatments comprising metamaterials and poroelastic materials, sandwiched between two structural panels is of utmost importance in aerospace, automotive, building and several other engineering applications. The integration of acoustic metamaterials in these systems introduces a number of challenges at the level of numerical simulation in order to predict their acoustic and structural behavior. This paper presents a methodology for the numerical simulation of the sound transmission loss of structural configurations comprising acoustic metamaterials embedded in a glass wools layer, sandwiched between two elastic panels. The metamaterial is designed as a parallel assembly of four sub-metamaterials. Each of these sub-metamaterials is also a series assembly of a periodic unit cell having a neck + cavity + neck type configuration. The four sub-metamaterials are designed so that their first resonance frequencies are grouped together in order to improve the sound transmission loss at the first two natural frequencies of the double panel. The COMSOL Multiphysics finite element method solver is used. The normal incidence and diffuse field sound transmission loss numerical results are presented and discussed in comparison with results from literature for validation.

Low-frequency noise inside metro: contribution analysis and noise control treatment

Low-frequency noise inside metro: contribution analysis and noise control treatment

Effects of familiarity and likability on music masking of aircraft flyover noise for young people

As typical and unique traffic noise, aircraft flyover noise is difficult to be effectively solved by traditional noise control treatments. For unsteady aircraft noise, music masking is considered to be a feasible method for subjective feeling improvement. This study aims to explore the effect of music likability and familiarity on the masking effect and analyze the relationships for young people. 15 music clips from six genres were firstly screened as masking sounds in terms of familiarity and likability difference. Two kinds of subjective scoring methods were adopted for total 60 participants to evaluate the annoyance reduction level. The correlation was analyzed and the masking effect prediction model was obtained from likability and familiarity. The results showed that the likability and familiarity had significant effects of masking effect. According to the annoyance reduction percentage, the masking effect had a significant positive correlation to likability. As for familiarity, there was a critical value for the best masking effect.

A hybrid method of predicting the acoustical properties of multi-layered materials

Many numerical and measuring approaches are widely used in predicting and analysing the acoustic performances of laminated materials for noise control applications. However, numerical methods generally require a set of non-acoustic parameters, and the measuring methods are not available for exceedingly thick materials. The main aim of this article is to address a hybrid approach of evaluating the normal incidence sound transmission loss and absorption coefficients of multi-layered materials. This method is performed with some special parameters that contain the sound transmission and reflection coefficients of each sub-layer of a multi-layer specimen and can be measured by a standing wave tube system. The accuracy and feasibility of the present method are validated by the experimental and numerical comparisons between different methods and samples. Moreover, this present approach can be applied as a numerical tool of estimating the acoustical behaviours of multi-layered structures in noise control treatments, such as automotive, building and aerospace industries.

Ferry M/V Kramer noise mitigation

The Motor Vessel (M/V) Edward V. Kramer is an aluminum vessel that operates as a small passenger ferry, which is owned and operated by the Department of Homeland Security (DHS) and used to transport DHS personnel and materials to Plum Island, NY. It was placed in service in 2018 and right from the start the sound levels inside the Main Deck compartment were found to be excessive. The original vessel specification included a noise limit of 75 dBA in the Main Deck Passenger Lounge and measured levels were as high as 87 dBA. A ship survey of sound and vibration was performed. Noise predictions to determine the controlling sound paths was also performed based on engine sound and vibration source levels. Recommendations for mitigation were presented and carried out by another shipyard. Mitigation included vibration isolation of the main engines and sound attenuation improvements to the Main Deck Passenger Lounge. After completion of the modifications, another survey was performed in 2021 and results show a reduction by as much as 11 dB in the Main Deck Passenger lounge. Noise estimation methods and details on the noise control treatments are given in the paper.

Airflow resistivity measurement and sound absorption performance analysis of sound-absorb cotton

Porous materials are widely used for noise control treatments. Sound-absorb cotton is commonly used in the transportation industry such as automobiles and airplanes because of its excellent sound absorption performance. In this paper, two typical acoustical models were used to evaluate the sound absorption coefficient of sound-absorb cotton. By comparing the measurement results with those of acoustical models, suitable model for sound-absorb cotton can be found. Physical parameters are measured and calculated by experimentally and inverse methods. Porosity and density are directly measured, while airflow resistivity is measured by changing boundary conditions between sample and container. Tortuosity and characteristic length are identified by using material acoustic parameters identification software, when samples considered as limp and rigid skeleton-type respectively. When all parameters of the samples are obtained, then the sound absorption coefficient of the sample can be simulated by material acoustic simulation software. This paper, also investigated the effect on sound absorption coefficient by airflow resistivity due to different boundary conditions. The conclusion of the theoretical model applicable to sound-absorb cotton can be drawn from the results obtained, also proved the best method of airflow resistivity measure.

Subjective Assessment Research of Middle Pressure Regulator Boxes in Urban Gas Industry

In urban gas industry there are many middle pressure regulator boxes which locate in noise sensitive areas like Class 1 and Class 2 acoustic environment functional areas in where exist strict noise emission limit requirement. Compared with high pressure regulation stations these middle pressure ones generate lower level noise but they are still complained because of the boundary noise excessive emission caused by direct exposure outdoors and lack of noise control treatment like acoustic enclosures, sound barriers and walls’ baffle. Most of the existing research focused on the parameters like noise sound pressure level, sound power level and so on of these regulator boxes, and there is little achievement about the subjective assessment of the noise generated by them. But the values of noise subjective assessment are the major reasons and influential index of the surrounding environmental complaints. In this paper, some objective values of subjective assessment base on several typical middle pressure regulator boxes noise emission during gas supply peak are measured, calculated and analyzed, the results offer the scientific evidence for the noise control and subjective feeling improvement of these boxes.

Experimental study of noise mitigation measures on a slab track

Slab track is generally noisier than conventional ballasted track, so noise control measures that can be applied to a slab track are of great importance. Several noise control treatments have been studied under controlled conditions through experiments on a 1:5 scale model slab track with and without the presence of a train body. These treatments consist of absorptive rubber mats applied to the surface of the slab and a low noise barrier introduced close to the track. The noise reduction is evaluated experimentally by using a reciprocal method and compared with the results of numerical simulations. The insertion loss spectra of these treatments have been combined with predicted train pass-by spectra to determine the potential overall noise reductions. It has been found that the absorptive treatment alone has only a small effect on the radiated noise from the track. This is increased by the presence of the train body leading to a reduction of up to 2–3 dB at a standard receiver height of 1.2 m, which suggests that the absorptive layer controls reflections between the car body and the track. All the treatments considered have a greater effect on the noise radiated by the lateral vibration of the rail than on that from the vertical vibration. Their effectiveness mostly increases, by between roughly 0.5 and 1.5 dB, as the train speed is increased from 80 to 300 km/h. As expected, the noise barrier is more effective at lower receiver positions than at higher ones but its effectiveness is reduced by 1–2 dB by the presence of the train body. However, in combination with the absorptive treatment, its effectiveness increases when the train body is present. This shows the importance of including of the presence of the train body in evaluating the effects of acoustic treatments in the track.

Optimizing the bandwidth of plate-type acoustic metamaterials

Plate-type acoustic metamaterials (PAM) consist of a thin film with multiple periodically attached masses. Although these metamaterials can be very lightweight and thin, the resulting sound transmission loss at low frequencies can be much larger than the corresponding mass-law. This is a result of anti-resonances at which the sound transmission through the PAM is strongly reduced. One general challenge, however, is that the anti-resonances are only very narrowband. This makes the application of PAM to noise control problems with broadband noise sources or changing tonal sources difficult. In this contribution, different design strategies to improve the bandwidth of PAM for low-frequency noise control applications (multiple masses per unit cell or stacking multiple PAM layers) are evaluated using optimizations. An efficient modal based model is employed to represent the PAM using their eigenfrequencies and modal masses. The model is validated using simulations and experimental measurements. The optimization results show that it is possible to significantly improve the bandwidth of PAM using the investigated design strategies. In fact, it is shown that the same bandwidths can be achieved either using multiple masses or multiple PAM layers. This allows for some flexibility in the design of suitable noise control treatments with PAM.

Comments on the validity of transfer matrix based models for the prediction of the effect of curved sound packages

In this paper, the validity of using the Transfer Matrix Method (TMM) to account for the effect of practical curved sound packages (i.e. spring-mass treatment) in vibroacoustic systems is assessed. For this purpose, a sub-structuring method that employs a Patch Transfer Functions (PTF) approach is used to efficiently couple standard Finite Element Method (FEM) of the structure and the acoustic cavity with the sound package model. Three models of the noise control treatment are compared, namely (i) a locally reacting model, (ii) a non-locally reacting (NLR) model based on the TMM, and (iii) a FEM. The FEM model considers the actual curved geometry of the SP in order to estimate its dynamic behavior while the simplified TMM based models assume the SP to be flat (i.e. unwrapped if curvature is present), homogeneous and of infinite lateral extent. In addition, these results are systematically compared to a full Finite Element/Boundary Element Methodology. The accuracy and the limitations of adopting the unwrapped approximation in TMM based models are shown through two parametric studies. The first considers the radius of curvature of the system as a parameter while the SP thickness is constant. The second considers the thickness of the SP as a parameter while the radius of curvature of the system is held fixed. For the considered configurations, the first parametric study allows to conclude that the curvature of the SP does not affect the dynamic behavior of the system due to its large dissipation and softness and it can be practically assumed flat if the NLR model is used to model the SP. However, the second parametric study shows that the thickness of the curved SP may affect the efficiency of the NLR model. Moreover, the limitations of locally reacting models are confirmed.

The design of noise attenuation for gas turbine GTS 22 engine

Nowadays, awareness of global society to the green concept where life should be in harmony with nature has been increasing. One of the ideas in the green concept is to reduce excessive sound exposures or noise. Within this context, gas turbine GTS 22 engine is one of mechanical system that generates a very high sound pressure level. Hence, it is instructive to develop noise controls for the gas turbine in order to have acceptable sound pressure level. This research is focused on the design of muffler as noise control treatment after carefully analyzing the dominant noise spectrum of the gas turbine. In this paper, numerical approach is employed to design the muffler. The results are then validated by experimental ones. Moreover, noise reduction (NR) is used to assess the muffler performances. The design of muffler in this research contains all of combination treatment: area cross section expansion chamber, perforated system (micro perforated panel), porous material, partition chamber and concentric tube. The chosen final design of muffler is combination treatment design where NR 38.77 dB is present. This NR leads to sound pressure level of 80.82 dB or 68.95 dBA exist after the muffler applied on the exhaust of gas turbine. The deviation of experimental results compared to numerical ones is 7.14% in average. Moreover, the pressure drop (ΔP) of the muffler is only 0.4042% which is acceptable to keep the mechanical performance.

On the numerical modeling of poroelastic layers in spring-mass systems

The proper modeling of noise control treatments is a key factor in the robustness of the numerical prediction of the sound transmission under airborne excitation. Typical acoustic treatments applied in the industry include one or more layers of poroelastic materials. These are particularly challenging to model since they exhibit a complex, high-dissipative behavior that is not only dependent on the material properties, but also on the interactions with other components. In this article, three different material formulations derived from the poroelasticity theory are employed to describe the poroelastic middle layer of a spring-mass system. The calculated results are contrasted with data measured in a window test bench. It is observed that for the description of softer materials the full poroelastic formulation provides the most accurate results. Conversely, a description as an elastic solid yields the best results for stiffer poroelastic layers. Moreover, the utilization of frequency-dependent elastic properties improves the match between numerical and experimental results, especially in the high-frequency range.

Does acoustically simulated predation risk affect settlement and reproduction of a migratory passerine?

AbstractNest predation is one of the most important drivers of avian life history evolution and population dynamics. Increasing evidence suggests that birds are able to assess nest predation risk and avoid settling in high‐risk areas to increase their reproductive performance. However, the cues used for settlement decisions are poorly known in most species. Population sizes of the migratory wood warblerPhylloscopus sibilatrixare characterized by strong annual fluctuations, which are negatively correlated with the number of forest rodents. Wood warblers might avoid rodent‐rich areas to reduce predation risk arising either from rodents, from rodent‐hunting predators attracted to such areas or from predators not linked to rodents. To evaluate these hypotheses, we conducted a large‐scale field experiment to test whether wood warblers avoided settling in plots with high predation risk simulated by broadcasting vocalizations of rodents or predators. Moreover, we tested whether reproductive performance varied in relation to simulated predation risk. Settlement patterns did not differ between plots with rodent, predator and noise control treatments. Likewise, measures of reproductive performance did not seem to differ across treatments. Thus, the broadcasted vocalizations of rodents and predators did not seem to be perceived as threat by wood warblers. Alternatively, the species might use other cues than those presented here, either other acoustic cues, visual and/or olfactory cues or a combination of cue types during settlement. Further experimental investigations to pin point cues and senses relevant for settlement decisions in wood warblers and birds in general are needed to better understand their life history and population dynamics.

A Statistical Energy Analysis (SEA) model of a fuselage section for the prediction of the internal Sound Pressure Level (SPL) at cruise flight conditions

Comfort plays an increasingly important role in the interior design of airplanes. In general, comfort is defined as ‘freedom from pain, well-being’; in scientific literature, indeed, it is defined as a pleasant state of physiological, psychological and physical harmony between a human being and the environment or a sense of subjective well-being. Cabin noise in passenger aircraft is one of the comfort parameter, which creates straightaway discomfort when exceeding personal thresholds. In general the cabin noise varies by the seat position and changes with flight condition. It is driven by several source types, which are transmitted through different transfer paths into the cabin. In the forward area the noise is mainly dominated by the turbulent boundary layer described by pressure vortexes traveling along the fuselage surface.In this paper evaluation of the Sound Pressure level, for the medium-high frequency range, of an aircraft fuselage section at different stations and locations inside the cabin has been performed numerically by using Statistical Energy Analysis (SEA) method. Different configurations have been considered for the analysis: from the “naked” cabin (only primary structure) up to “fully furnished” (primary structure with interiors and noise control treatments) one. These results are essential to understand which are the main parameters affecting the noise insulation. Furthermore the Power Inputs evaluation has been determined to see the contribution of each considered aeronautic component on the acoustic insulation. Finally, the effect of a viscoelastic damping layer embedded in the glass window has been evaluated.

A Pass-By Noise Prediction Method Based on Source-Path-Receiver Approach Combining Simulation and Test Data

<div class="section abstract"><div class="htmlview paragraph">Optimizing noise control treatments in the early design phase is crucial to meet new strict regulations for exterior vehicle noise. Contribution analysis of the different sources to the exterior acoustic performance plays an important role in prioritizing design changes. A method to predict Pass-by noise performance of a car, based on source-path-receiver approach, combining data coming from simulation and experimental campaigns, is presented along with its validation. With this method the effect of trim and sound package on exterior noise can be predicted and optimized.</div></div>

The identification of minimum-weight sound packages

Generally, heavier noise control treatments are favored over lighter treatments since heavier acoustical materials generally insulate (block) noise sources more effectively than do lighter materials. In automotive applications, however, heavier materials cannot always be adopted because of concerns about the total weight of the vehicle. Thus, it would be useful to identify lightweight acoustical treatments that can mitigate vehicle interior noise. Automotive sound packages have both absorption and transmission characteristics, and there is inevitably a trade-off between these two. Therefore, it is important to study the exchange between the absorption and transmission of acoustical materials particularly as it pertains to weight. Here, we describe a procedure, based on plane wave analysis, that can be used to identify weight reduction opportunities by adjusting the acoustical properties of a generic sound package, consisting of a fibrous layer and a flexible microperforated panel surface treatment, so that it meets a target sound pressure level in a downstream interior space. It has been found for the configuration studied here that there are lightweight sound package configurations that can maintain acoustical performance equivalent to that of heavier noise treatments, and further, it has been found that the lightest treatments tend to favor barrier performance rather than absorption. Further, the impact of acoustical leaks is considered, and it has been found that even very small leaks can result in a very substantial weight penalty if a specified level of acoustical performance is to be ensured.