Acoustic Mufflers Research Articles

Using empirical data to validate the role of computational fluid dynamics in various stages of aero-acoustic simulations

The purpose of utilizing higher level of understanding techniques is to improve the overall outcome of any process. As a full-service provider of complex engineering solutions to environmental noise problems, there is a need to house specialized knowledge to design and deliver bespoke solutions that are compatible with various constraints that implicate numerous subjects (acoustics, aerodynamics, structural, materials/chemical compatibility). The physics associated with seemingly simple products, such as an industrial acoustic silencer, is often complex. More specifically, its study should be described as aero-vibro-acoustical—whereby (1) airflow causes vibrations in the structure of the silencer, (2) the vibrations generate airborne and structureborne noise, and (3) components of the silencer (i.e., baffles) attenuate noise propagating through the duct. Motivated to expand our understanding of our products’ performances, we are using Siemens software to circumvent exhaustive laboratory testing that is cost-prohibitive, and which is, generally, limited to common geometries and parameters. A systematic approach is necessary to validate correlations between simulated results with empirical data. This is accomplished by, first, correlating the aerodynamic performance of products using computational fluid dynamics (CFD) to predict pressure drop values and the distribution of forces on the structure, to then leverage additional solvers to assess the vibro-acoustical stage of the analysis.

Applying Lean Construction principles to noise mitigation design

The construction industry has fallen behind the times. Since the 1960s, construction labour productivity has decreased while all other non-farming labour efficiencies have doubled or more. 70% of projects are delivered late and over budget. Furthermore, one in five private sector work-related fatalities are from construction accidents. Lean Construction is a program developed by the Associated General Contractors of America (AGC) to challenge the status quo on construction projects and help guide the industry to a better future. Utilizing Lean principles during the design phase of noise mitigation projects can improve project deliverables across all industries. All stakeholders involved in the construction process have incentive to complete projects faster and at a lower cost. The opportunities to apply Lean principles are limitless. This paper focuses on the importance of collaboration, planning, pre-construction, and supply chain management as they pertain to the engineered design process for noise mitigation. Building silencer prototypes before mass-production ensures the end user has an opportunity to provide feedback. Working backwards from milestones and leaving detailed scheduling to the people conducting the work yields more useful/accurate plans. Pre-assembling acoustic barriers and/or delivering modular acoustic enclosures expedite site installations. Scheduling just-in-time acoustic silencer deliveries minimize site congestion.

Photoacoustic optical absorption measurement of aerosols

Atmospheric optical absorption, including the contribution from aerosols, is important in modeling propagation of laser beams and for climate research. Photoacoustic methods have been used in the past for this purpose, as they directly measure absorption, rather than the sum of absorption and scattering. Acoustic operating frequency determines the sensed aerosol size range. The design of a windowless double-Helmholtz photoacoustic aerosol sensing system, operating at an acoustic frequency near 150 Hz, with the capability to measure an optical absorption with a 1/e length of 1 Mm, will be presented. In this design, environmental noise limits the instrument sensitivity. To improve immunity to environmental noise with an open resonator, an aluminum acoustic enclosure was built around the system, with welded vacuum flanges for inputs and outputs. Inlet and outlet acoustic mufflers were designed to limit the in-band noise entering the measurement system. A modulated 10 W 1064 nm fiber laser was used as the excitation source, and a high-sensitivity microphone placed within the open-cell cavity was used to measure the internal photoacoustic pressure fluctuations. The system was modeled using various computational tools, including MATLAB, Simulink, and ANSYS, and then was constructed and tested. Lab measurement results will be presented.

The study for the influence of acoustic silencer foam on the output of sound based on the software of room EQ wizard

Stimulated by market demand and technological progress, the audio industry is booming. Many high-fidelity earphones are no longer satisfactory with simply providing customers with products but tend to provide customers with more opportunities to adjust their own tuning, one of which is to add the earphones with different acoustic silencer foam. This study begins by introducing the different types of earphones and their development history. After that, the specific research content is raised by questioning the prevailing perception of acoustic silencer foam in earphones. By comparing and analyzing the differences reflected by the frequency response curves of headphones equipped with different types of acoustic silencer foam, it is concluded that the smaller the cells of acoustic silencer foam and the presence of membrane will increase its ability to reduce the loudness at high frequency. This study reveals two significant characteristics which affect the performance of acoustic silencer foam and provides a reliable theoretical basis for the tuning technology of foam.

Design of a metamaterial-based muffler for a target frequency range

This work proposes an acoustic metamaterial-based muffler that effectively blocks a transmission noise for a target frequency range. Since the acoustic metamaterial-based muffler consists of arrayed unit cells, its noise attenuation performance is strongly affected by the internal layout of the unit cell. The wave transmission characteristics of an acoustic metamaterial is explained by the effective bulk modulus and dispersion curve of an unit cell. Therefore, the internal layout of the unit cell should be optimally designed so that its band gap should include the target frequency range of a muffler. To the end, an acoustical size optimization problem is formulated to design a unit cell of the muffler and is solved for a given design requirement. The noise blocking frequency range of the unit cell is characterized by the bandgap in its dispersion curve during the optimization process. The wave transmission characteristics of the metamaterial muffler is validated experimentally.

Development of acoustic mufflers for cabin noise reduction in Orion spacecraft

Controlling cabin acoustic noise levels in the Crew Module (CM) of the Orion spacecraft is critical for adequate speech intelligibility, avoid fatigue, and prevent any possibility of temporary and permanent hearing loss to the crew. The primary source of cabin noise for the on-orbit phase of the mission is from the Environmental Control and Life Support System (ECLSS) which recycles and conditions breathing air and maintains cabin pressurization through its ducting network and components. Unfortunately, as a side effect, noise from the ECLSS fans propagates through theses ducts and emanate into the cabin habitable volume via the ECLSS inlet and outlets. To mitigate excessive duct-borne noise, two ECLSS mufflers have been designed to provide significant acoustic transmission loss (TL) so that the cabin noise requirements can be met. Each muffler is meant to be installed in the ducting of the ECLSS air inlet and outlet sides, respectively. Packaging constraints and tight volume requirements necessitated the mufflers to be of complex geometry and compatible with the bends of the ECLSS duct layout. To design and characterize the acoustic performance of the inlet and outlet mufflers, computational acoustic models were developed using the Finite Element Method (FEM) with software. Characterization of the acoustic material and perforations in the mufflers were addressed with poro-elastic theory. Once the mufflers were designed on paper and its TL predicted, prototypes of these mufflers were created using additive manufacturing. The muffler prototypes were subsequently tested for acoustic TL in the laboratory with various configurations of acoustic materials. Comparing the analytical predictions to the test performance yielded excellent correlation for acoustic TL and demonstrated significant broadband noise attenuation. The ECLSS mufflers are currently scheduled to be installed on the Artemis II CM of the Orion spacecraft and will provide significant cabin comfort to crew during the mission.

Design of acoustic meta-material silencer based on coiled up space

Design of acoustic meta-material silencer based on coiled up space

Broadband acoustic silencer with ventilation based on slit-type Helmholtz resonators

Recently, sound attenuation with ventilation is highly needed in many practical applications. In this study, we report on a subwavelength acoustic silencer, named double-layer acoustic silencer (DAS), based on compactly assembled slit-type Helmholtz resonators (SHRs) for low-frequency broadband sound insulation while preserving ventilation. A simple yet insightful theoretical model is first established to characterize the sound insulation performance in terms of transmission loss (TL) and used for microstructure designs of the DAS. The fluctuating TL of the DAS, inevitably produced by the SHR resonances, is then mitigated and optimized via the introduction of viscosity and proper SHR frequency detuning. The overall TL is numerically investigated and experimentally observed to reach beyond 30 dB over the target working band of 0.48–0.95 kHz, with a maximum exceeding 50 dB. In addition, the proposed design also provides perfect ventilation when deployed in a duct environment, due to the straight and conserved airflow cross section. We believe that the proposed acoustic silencer design and its associated theoretical model pave the way for designing and optimizing highly efficient low-frequency subwavelength acoustic liners and silencers.

A new approach to the theory of acoustic multi-port networks with multimode wave and its application to muffler analysis

The paper presents a new approach to the theory of acoustic multi-port networks assuming propagation of multimode wave through the system and applying the scattering matrix formalism. A procedure is proposed which can be applied to improve accuracy of results obtained when analysing acoustic mufflers of complex geometry or other systems composed of several sub-systems joined by duct-like elements. In what follows, the multi-port order is defined by the number of ducts and a selected cross-section of a duct transmitting multimode wave is called the multimode port. The main feature of the presented method is that it allows to analyse each sub-system as a separate multi-port. The scattering matrix representing two selected consecutive sub-systems having a common multimode port could be transformed according to the derived formulae which results in reducing the number of multimode ports by two. The source term can be present in each sub-system. By executing this procedure step by step, the scattering matrix of the whole system can be determined. To ensure good accuracy of final results, the scattering matrix of a discontinuity constituting a degenerated multi-port is calculated with the near field effects taken into account adjusting the number of cut-off modes included in the analysis to the specified continuity conditions at junctions. The next step consists in reduction of the dimension of the “near-field scattering matrix” to the matrix accounting only for cut-on modes propagating in the adjoined ducts. The main advantage and the novelty of the presented procedure is that it can be executed step by step and thus applied to mufflers of complex geometry never leading to the scattering matrix of troublesomely large dimensions and at the same time, ensuring significant accuracy of the obtained results. The proposed method was validated in two ways: by comparing its results with those published in the literature and applying it to some other example mufflers of complex geometry.

RESEARCH OF THE RESONANCE PROPERTIES OF HELMHOLTZ RESONATORS

The object of research. Process of oscillation of the Helmholtz resonators. Investigated problem. Differences between some formulas for the calculation of the resonant frequency of the Helmholtz resonator and the most accurate of them were established. The effect of acoustic design on the Helmholtz resonator frequency value and influence of the attached air mass between the neck of the Helmholtz resonator and free field were investigated. The main scientific results. As a result of a numerical experiment, analytical ratios were obtained that allow obtaining the most accurate results. One of them is the most optimal for calculating the resonator resonant frequency in a free field, and in this case, less than 1 % inaccuracy level can be achieved, given that r/R<0.25. Other one allows to achieve the same low inaccuracy level for a resonator located in an acoustically rigid shield, given that r/R<0.25. Research has shown that the location of the resonator in an acoustically rigid shield leads to significant changes in natural resonance frequency value. The area of practical use of the research results. The detailed research of previously unexplored properties of resonators will make it possible to improve the algorithms for the development of metamaterials, to discover additional parameters with which it is possible to control the characteristics of the metamaterial. Scope of the innovative technological product. Such resonators are used as the basic elements in metamaterials, as structural elements of the sound-absorbing panels, in acoustic mufflers.

Shape Optimization of Acoustic Mufflers Using Genetic Algorithm

Shape Optimization of Acoustic Mufflers Using Genetic Algorithm

Topology optimization of reactive acoustic mufflers using a bi-directional evolutionary optimization method

This article proposes an acoustic muffler design procedure based on finite element models and a Bi-directional Evolutionary Acoustic Topology Optimization. The main goal is to find the best configuration of barriers inside acoustic mufflers used in the automotive industry that reduces sound pressure level in the outlet of the muffler. The acoustic medium is governed by Helmholtz equation and rigid wall boundary conditions are introduced to represent acoustic barriers. The continuum problem is written in the frequency domain and it is discretized using the finite element method. The adopted objective function is Transmission Loss (TL). Increasing TL guarantees that the sound pressure level ratio between outlet and inlet of the muffler is reduced. To find the configuration of acoustic barriers that increases the Transmission Loss function of the muffler an adaptation of the Bi-directional Evolutionary Structural Optimization (BESO) method is used. Applying the proposed design procedure topologies in 2D models are reached, which raises the Transmission Loss function for one or multiple frequencies. Three examples are presented to show the efficiency of the proposed procedure.

22.03: Acoustic ceiling designed for trains passing at 250km/h: Holmestrand – The first station in the world inside a mountain entrance hall

ABSTRACTThe Holmestrand station will be located inside a rock cavern, with two separate tracks for platforms and two tracks for through trains in the same hall. Hence the entrance hall will be designed to allow high speed trains to pass at 250 km/hour, creating challenges concerning pressure, fatigue, wind speed and noise levels within the station hall.The steel structure comprises of a suspended ceiling inside the station of more than 12,000 m2. The ceiling shall prevent drip from the rock and act as an acoustic silencer. In addition, great emphasis has been placed on the architectural configuration. The primary steel is designed for a life time of 100 year.The ceiling is divided into 3 parts, one above each platform and one above the tracks.Main beams and cross beams form a grid that is suspended from the mountain by individual hangers of adjustable lengths. On top of this grid a corrugated aluminium plate prevents drip from the rock, and below this grid acoustic panel is fitted to reduce noise problems. All steel between the rock surface and the corrugated plate on top of the ceiling consists of duplex steel. Below the corrugated steel plate the main and cross beams are hot dip galvanized (HDG) to minimum 200 μm and the acoustic panels are sandwich elements with perforated steel surface. All joint connections are bolted and most of them with preloaded stainless steel bolts. There are many friction grip connections that lead to testing prior to assembly of the preloaded stainless steel bolts and the friction surfaces for the different material combinations.In addition to duplex steel, HDG steel and aluminium the ceiling above the platforms is also supported by weathering steel for architectural reasons. Thus the detailing required special attention in order to secure the required service life without accelerated corrosion from either galvanic effects as combination of different materials are used or the detailing of e.g. weathering steel.The new 14 km double track through the town of Holmestrand will greatly improve the rail services on the Vestfold line. Rambøll is responsible for designing 7 km of this line. Total Cost: NOK 6.6 billion.The work for Rambøll started in 2010 and the station opened on November 27, 2016

Three-dimensional printed acoustic mufflers and aeroacoustic resonators

We explore and present the use of 3D printing technology to design, construct, and test acoustic elements that could be used as a low-frequency Helmholtz-resonator style muffler in a ventilation duct system. Acoustic elements such as these could be quickly prototyped, printed, and tested for any noisy duct environment. These acoustic elements are tested with and without mean flow to characterize their sound absorption (and sound generation) properties. It is found that at particular ranges of air flow speeds the simply designed acoustic muffler acts as a site for aeroacoustic sound generation. Measurement data and 3D model files with Python-scripting will be presented for several muffler designs. This work is supported by the Arkansas Space Grant Consortium in collaboration with NASA's Acoustics Office at the Johnson Space Center.

An innovative acoustic muffler to reduce acoustic leakage from recessed lights, intake vents, exhaust vents, etc., while improving the acoustic environment within a room

In 2011, Bonnie Schnitta of SoundSense received a patent for an Acoustic Muffler. This Muffler addresses the issue of significant sound transmission, or acoustic leakage, through openings in structures, such as a ceiling or wall, where airflow or heat dissipation is required. Examples of applications will be presented, such as openings created by recessed lights, speakers, or intake and exhaust vents. These openings in the structure significantly reduce the STC and/or IIC of the structure. The SoundSense Acoustic Muffler reduces acoustic leakage through an opening in a ceiling or wall that would potentially cause significant degradation in acoustic efficacy of the structure without resulting in any substantial pressure drop, while simultaneously allowing for the required air flow or heat dissipation. The original application of the recessed light and additional similar applications as well as secondary purposes for this muffler will be detailed. One such secondary purpose is when the hole created by the recessed lights in the ceiling contributes to the room sounding better. The patented muffler not only allows this benefit to remain, but serves to inhibit the frequency(s) of concern from disturbing adjacent rooms. Cell tower equipment room noise reduction requiring airflow will also be detailed.

Effect of balconies and upper–lower vents on ventilation and indoor air quality in a wind-induced, naturally ventilated building

Balconies are green features commonly used in residential buildings to improve natural ventilation and air quality. Small vents, if mounted with an acoustic silencer, can reduce noise penetration while still allowing natural ventilation to occur. In this study, the computational fluid dynamics method is used to investigate numerically the effect of balconies with small upper and lower vents on the ventilation and air quality of the 4th, 5th and 6th floors of a 10-storey building. The results show that balconies can significantly increase the natural ventilation on these floors and generally have a more positive effect on the improvement of natural ventilation and the reduction in pollutant concentration on the floor on which they are located rather than on the levels above or below. Practical application: This study will help designers and engineers understand more about the effect of balconies and lower and upper vents on natural ventilation and indoor air quality in buildings. This study will also help them to incorporate with confidence the design of balconies with lower and upper vents.

Multiobjective muffler shape optimization with hybrid acoustics modeling

This paper considers the combined use of a hybrid numerical method for the modeling of acoustic mufflers and a genetic algorithm for multiobjective optimization. The hybrid numerical method provides accurate modeling of sound propagation in uniform waveguides with non-uniform obstructions. It is based on coupling a wave based modal solution in the uniform sections of the waveguide to a finite element solution in the non-uniform component. Finite element method provides flexible modeling of complicated geometries, varying material parameters, and boundary conditions, while the wave based solution leads to accurate treatment of non-reflecting boundaries and straightforward computation of the transmission loss (TL) of the muffler. The goal of optimization is to maximize TL at multiple frequency ranges simultaneously by adjusting chosen shape parameters of the muffler. This task is formulated as a multiobjective optimization problem with the objectives depending on the solution of the simulation model. NSGA-II genetic algorithm is used for solving the multiobjective optimization problem. Genetic algorithms can be easily combined with different simulation methods, and they are not sensitive to the smoothness properties of the objective functions. Numerical experiments demonstrate the accuracy and feasibility of the model-based optimization method in muffler design.

Modeling perforates in mufflers using two-ports

One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers. A very common problem in the modeling of automotive mufflers is that of two flow ducts coupled through a perforate. A new segmentation approach is developed here based on two-port analysis techniques, in order to model perforated pipes using general two-port codes, which are widely available. Examples are given for simple muffler configurations and the convergence of the technique is investigated based on the number of segments used. The results are compared with closed form solutions form the literature. Finally, an analysis of a complicated multichamber perforated muffler system is presented. The two-port simulation results show good agreement with both the measurements, and the simulations using the classical four-port elements.

Development of an educational impedance tube: Experimental vibroacoustic characterization.

One of the great difficulties encountered by designers who work with room acoustics is to know the absorption coefficient of the material to be employed in the project. The tube wave, also known as impedance tube, is a device which determines this coefficient and also determines the acoustic impedance of materials. These values are essential for choosing products that best fit the needs of an acoustic treatment project, either indoors, machinery, pipes, or mufflers. This paper presents the first studies to construction and characterization of an educational impedance tube (Kundt’s tube) called TIGaD (demonstrative impedance tube of the Faculty at Gama). In this first phase we will present a dimensional study of the tube, its construction details, an experimental analysis of its vibroacoustic characteristics, and, finally, its limitations in frequency range. This project belongs to the educational area since this experimental apparatus is being developed to equip the laboratory of acoustics and vibration (NVH Gama). The aim is to obtain low‐cost equipment which makes possible the study of the various acoustic phenomena such as absorption, reflection, impedance, and further demonstration of plane waves and the determination of the speed of sound.

A cochlear analog bio-mimetic muffler.

A noise control device, the structural acoustic silencer, is designed by adopting the tailored structural acoustic filtering exhibited by the mammalian cochlea. In the cochlea, a flexible plate of gradually varying width and thickness separates the upper and lower fluid-filled ducts. Tailoring of the flexible plate properties enables frequency-specific localization of traveling waves, which are slowed down significantly at and near the site of resonance. While such slowing of the traveling wave leads to efficient energy coupling to the flexible plate thereby reducing transmitted acoustic fluctuations, the gradually varying impedance reduces reflections and allows the frequency-component to travel to its resonance site. The design of the bio-mimetic muffler employing non-biological materials is performed using three dimensional finite element analysis and validated against experimental data. The relation between coupled dispersion and transmission loss in the noise control device is explored. The coupled wave propagation in the engineered device is compared with the wave propagation in a passive cochlea.