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Transmission loss (TL) is a good performance measure of mufflers since it represents the muffler's inherent capability of sound attenuation. There are several existing numerical methods, which have been widely used to calculate the TL from numerical simulation results, such as the four-pole and three-point methods. In this paper, a new approach is proposed to evaluate the transmission loss based on the reciprocal work identity. The proposed method does not assume plane wave propagation in the inlet and outlet ducts, and more importantly, does not explicitly apply the anechoic termination impedance at the outlet. As a result, it has the potential of extending TL computation above the plane wave cut-off frequency.

A process for predicting the transmission and insertion losses of multi-component exhaust systems is detailed in this paper. A two-tiered process incorporating boundary element analysis to evaluate multi-component systems is implemented. At the component level, the boundary element method is used to predict the transfer matrix for larger components where plane wave behavior is not expected within the component. The transfer matrix approach is then used to predict insertion loss for built-up systems with interconnecting duct or pipe work. This approach assumes plane wave behavior at the inlet and outlet of each component so it is limited to the low frequency regime. Results are compared with experimental results for HVAC systems.

In many industries, muffler and silencer design is primarily accomplished via trial and error. Prototypes are developed and tested, or numerical simulation (finite or boundary element analysis) is used to assess the performance. While these approaches reliably determine the transmission loss, designers often do not understand why their changes improve or degrade the muffler performance. Analyses are time consuming and models cannot be changed without some effort. The intent of the current work is to demonstrate how plane wave muffler models can be used in industry. It is first demonstrated that plane wave models can reliably determine the transmission loss for complicated mufflers below the cutoff frequency. Some tips for developing dependable plane wave models are summarized. Moreover, it is shown that plane wave models used correctly help designers develop intuition and a better understanding of the effect of their design changes.

A recently developed superposition approach for determining the insertion loss of a two-inlet muffler is reviewed. To validate the approach, calculated and measured insertion losses are compared for a small engine muffler with two inlets and one outlet. After which, the phasing between the two inputs is varied and the insertion loss is evaluated. Results show that the insertion loss is strongly affected by the phasing between sources at low frequencies while phasing between sources has a lesser impact at high frequencies. At the conclusion of the paper, the theory for applying the superposition approach to transmission loss is reviewed.

Over 30 years ago, A. H. Vincent of Westland Helicopters demonstrated that if a structure is excited harmonically, the response at another position (at a fixed frequency) will trace a circle in the complex plane as a result of a dynamic stiffness modification between two points. As either the real or imaginary part of an introduced dynamic stiffness is varied from minus infinity to plus infinity, the structural or acoustic response on any position will map a circle in the complex plane. This paper reviews the basis for this little known principle for vibro-acoustics problems and illustrates the viability for a cantilevered plate example. The applicability of the method is then considered for strictly acoustic systems like intake and exhaust systems. Specifically, it is shown that the response traces a circle in the complex plane if either the real or imaginary parts of the source or termination impedance are varied from minus to plus infinity.

The measurement of sound absorption coefficient (SAC) of porous materials is covered by both American and international standards. However, by using the standards alone it is difficult to achieve consistently repeatable results given the large number of variables such as sample cutting and preparation, sample fit and position in the tube, and sample material variability. This paper will review the standards briefly and examine what is available in the literature to guide users in making consistently repeatable SAC measurements. The paper will also show some of the authors' results and interpret these results in light of the standards and technical literature on the subject.

This paper examines the feasibility of using the boundary element method (BEM) and the Rayleigh integral to assess the sound radiation from engine components such as oil pans. Two oil pans, one cast aluminum and the other stamped steel, are used in the study. All numerical results are compared to running engine data obtained for each of these oil pans on a Cummins engine. Measured running-engine surface velocity data are used as input to the BEM calculations. The BEM models of the oil pains are baffled in various ways to determine the feasibility of analyzing the sound radiated from the oil pan in isolation of the engine. Two baffling conditions are considered: an infinite baffle in which the edge of the oil pan are attached to an infinite, flat surface; and a closed baffle in which the edge of the oil pan is sealed with a rigid structure. It is shown that either of these methods gives satisfactory results when compared to experiment.

The two-load method is commonly used to determine the transmission loss of a muffler or silencer. Several practical measurement considerations are examined in this paper. First of all, conical adapters are sometimes used to transition between impedance tubes and the muffler. It is demonstrated that the effect of adding the adapter can be quite significant at low frequencies especially if the adapter is short in length. The effect of changing the length of the adapter was examined via measurement and plane wave theory. Secondly, the effect of selecting the reference microphone was examined experimentally. It was found that measurements are improved by selecting a downstream reference. Finally, the effect of using different frequency response function estimation algorithms (H1, H2 and Hv) was compared sans flow. This had little effect on the measurement.

A mathematical model is presented to help understand sheet metal deformation during forming. The particular purpose of this model is to predict the forming limit diagram (FLD). The present model is an extension of a previous analysis by Jones and Gillis (JG) in which the deformation is idealized into three phases: (I) homogeneous deformation up to maximum load; (II) deformation localization under constant load; (III) local necking with a precipitous drop in load. In phase III the neck geometry is described by a Bridgman type neck. The present model extends the JG theory which was applied to the right hand side of the FLD only. The main difference in treating the two different sides of the FLD lies in the assumptions regarding the width direction deformations. For biaxial stretching, the right hand side, the minor strain rate is assumed to be homogeneous throughout the process.

This paper documents a finite element approach to predict the attenuation of muffler and silencer systems that incorporate diesel particulate filters (DPF). Two finite element models were developed. The first is a micro FEM model, where a subset of channels is modeled and transmission matrices are determined in a manner consistent with prior published work by Allam and Åbom. Flow effects are considered at the inlet and outlet to the DPF as well as viscous effects in the channels themselves. The results are then used in a macro FEM model of the exhaust system where the transmission relationship from the micro-model is used to simulate the DPF. The modeling approach was validated experimentally on an example in which the plane wave cutoff frequency was exceeded in the chambers upstream and downstream to the DPF.

Helmholtz resonators are normally an afterthought in the design of mufflers to target a very specific low frequency, usually the fundamental firing frequency of the engine. Due to space limitations in a complex muffler design, a resonator may have to be built by punching a few small holes on a thin-walled tube to create a neck passage into a small, enclosed volume outside the tube. The short neck passage created by punching a few small holes on a thin-walled tube can pose a great challenge in numerical modeling, especially when the boundary element method (BEM) is used. In this paper, a few different BEM modeling approaches are compared to one another and to the finite element method (FEM). These include the multi-domain BEM implemented in a substructure BEM framework, modeling both sides of the thin-walled tube and the details of each small hole using the Helmholtz integral equation and the hypersingular integral equation, and modeling just the mid surface of the thin-walled tube.

A simplified method to model perforated tubes in mufflers is the equivalent transfer impedance approach. Various empirical formulas that consider the porosity, hole diameter, wall thickness, and flow type have been proposed to date. They normally work very well under the conditions that the formulas are intended for. However, there are situations that the empirical formulas may not be able to cover. In this paper, we propose a simple BEM-based numerical procedure to determine the transfer impedance from a small perforate sample, and then send the transfer impedance to the muffler BEM model for analysis purposes. Numerical results are verified in three test cases.

Insertion loss in one-third or octave bands is widely used in industry to assess the performance of large silencers and mufflers. However, there is no standard procedure for determining the transmission loss in one-third or octave bands using measured data or simulation. In this paper, assuming that the source is broadband, three different approaches to convert the narrowband transmission loss data into one-third and octave bands are investigated. Each method is described in detail. To validate the three different approaches, narrowband transmission loss data of a simple expansion chamber and a large bar silencer is converted into one-third and octave bands, and results obtained from the three approaches are demonstrated to agree well with one another.

The source in an intake/exhaust system is commonly modeled as a source strength and impedance combination. Both the strength and impedance are normally measured and measurement accuracy depends on selecting an appropriate acoustic load combination. An incident wave decomposition method is proposed which is based on acoustic wave decomposition concepts instead of an electric circuit analogy providing a more straightforward approach to investigating the effect of acoustic load selection. Based on studying wave reflections in the system, the uncertainty for determining source impedance is estimated.

Diesel engines produce harmful exhaust emissions and high exhaust noise levels. One way of mitigating both exhaust emissions and noise is via the use of after treatment devices such as Catalytic Converters (CC), Selective Catalytic Reducers (SCR), Diesel Oxidation Catalysts (DOC), and Diesel Particulate Filters (DPF). The objective of this investigation is to characterize and simulate the acoustic performance of different types of filters so that maximum benefit can be achieved. A number of after treatment device configurations for trucks were selected and measured. A measurement campaign was conducted to characterize the two-port transfer matrix of these devices. The simulation was performed using the two-port theory where the two-port models are limited to the plane wave range in the filter cavity.

Typical automotive muffler designs contain complex internal components such as extended inlet/outlet tubes, thin baffles with eccentric holes, internal connecting tubes, perforated tubes, perforated baffles, flow plugs and sound-absorbing materials. An accurate performance prediction for highly complicated muffler designs would greatly reduce the effort in fabricating and testing of multiple design iterations for engineers. This paper discusses the use of a component-based computer simulation tool for design and analysis of exhaust mufflers. A comprehensive computer program based on the Direct Mixed-Body Boundary Element Method was developed to predict the transmission loss characteristics of muffler systems. The transmission loss is calculated by an improved four-pole method that does not require solving the boundary element matrix twice at each frequency, and hence, it is a significantly faster approach when compared to the conventional four-pole method.

3D printing is a revolutionary manufacturing method that allows the productions of engineering parts almost directly from modeling software on a computer. With 3D printing technology, future manufacturing could become vastly efficient. However, the procedures used in 3D printing differ substantially among the printers and from those used in conventional manufacturing. The objective of the present work was to comprehensively evaluate the mechanical properties of engineering products fabricated by 3D printing and conventional manufacturing. Three open-source 3D printers, i.e., the Flash Forge Dreamer, the Tevo Tornado, and the Prusa, were used to fabricate the identical parts out of the same material (acrylonitrile butadiene styrene). The parts were printed at various positions on the printer platforms and then tested in bending. Results indicate that there exist substantial differences in mechanical responses among the parts by different 3D printers.

Microperforated panel (MPP) absorbers are rugged, non-combustible, and do not deteriorate over time. That being the case, they are especially suitable for long term use in harsh environments. However, the acoustic performance is modified when contaminated by dust, dirt, or fluids (i.e. oil, water). This paper examines that effect experimentally and correlates the absorption performance with Maa's theory for micro-perforated panels. Transfer impedance and absorption coefficient are measured for different levels of aluminum oxide and carbon dust accumulation. The amount of dust contamination is quantified by measuring the luminance difference between clean and dirty panels with a light meter. The porosity and hole diameter in Maa's equation are modified to account for dust obstruction. The effect of coating the MPP with oil, water, and other appropriate viscous fluids was also measured. This effect was simulated by modifying the viscous factor in Maa's equation.

The high temperature and high neutron flux environment of a thermionic space power reactor presents a challenge in the design of the sheath insulator within a thermionic fuel element. The present alumina insulator design is suspect to degradation due to the neutron flux. The alumina insulator also requires a barrier coating to isolate it from the liquid alkali metal coolant. Although the alumina sheath development is progressing, the alumina insulator remains a potential point of significant performance loss in the thermionic fuel element. The recent successes in depositing polycrystalline diamond film onto cylindrical refractory metal substrates has led to the consideration of diamond as a potentially ideal sheath insulator. Investigations have been conducted into the durability of diamond thin film under exposure to simulated thermionic reactor conditions.

Bench tests are an important step to developing mufflers that perform adequately with acceptable pressure drop. Though the transmission loss of a muffler without flow is relatively simple to obtain using the two-load method, the presence of mean flow modifies the muffler behavior. The development of an insertion loss test rig is detailed. A blower produces the flow, and a silencer quiets the flow. Acoustic excitation is provided by a loudspeaker cluster right before the test muffler. The measurement platform allows for the measurement of flow-induced noise in the muffler. Also, the insertion loss of the muffler can be determined, and this capability was validated by comparison to a one-dimensional plane wave model.