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The two-load method is commonly applied to determine the transmission loss for a muffler especially if an impedance tube rig is used. Although one procedure and algorithm is detailed in ASTM E2611, the quality of the transmission loss curve is dependent on several factors that are not discussed in detail in the standard. In this paper, several practical concerns are investigated including (1) the number of channels used in the measurement, (2) the selection of the reference channel, and (3) the choice of data processing algorithm (transfer or scattering matrix). Results are compared for a simple expansion chamber first, then for mufflers of other types. Recommendations are made for obtaining smoother transmission loss curves for various measurement methods.

The insertion loss of louvered terminations positioned at the end of a rectangular duct is determined using acoustic finite element analysis. Insertion loss was determined by taking the difference between the sound power with and without the louvers at the termination. Analyses were conducted in the plane wave regime and the acoustic source was anechoic eliminating any reflections from the source. The effect of different louver configurations on insertion loss was examined. Parameters investigated included louver length, angle, and spacing between louvers. Based on the analyses, equations were developed for the insertion loss of unlined louvers.

The most common approach for measuring the transmission loss of a muffler is to determine the incident power by decomposition theory and the transmitted power by the plane wave approximation assuming an anechoic termination. Unfortunately, it is difficult to construct a fully anechoic termination. Thus, two alternative measurement approaches are considered, which do not require an anechoic termination: the two load method and the two-source method. Both methods are demonstrated on two muffler types: (1) a simple expansion chamber and (2) a double expansion chamber with an internal connecting tube. For both cases, the measured transmission losses were compared to those obtained from the boundary element method. The measured transmission losses compared well for both cases demonstrating that transmission losses can be determined reliably without an anechoic termination. It should be noted that the two-load method is the easier to employ for measuring transmission loss.

It was demonstrated that a bypass duct similar to a Herschel- Quincke tube could be used to increase the transmission loss of mufflers at selected frequencies. In many cases, the duct can be short and thought of as a leak. It was shown that the optimal length and cross-sectional area could be determined by using a simple optimization technique known as the Vincent Circle. Most importantly, it was demonstrated that the attenuation at low frequencies could be improved by as much as 15 dB. To prove the concept, a muffler was designed and optimized using transfer matrix theory. Then, the optimized muffler was constructed and the transmission loss measured using the two-load method. The measured results compared well with prediction from transfer matrix theory. Boundary element simulation was then used to further study the attenuation mechanism.

As one considers the interrelationship between supplier logistics and performance of JIT assembly plants, the question arises concerning how many hours of parts inventory are appropriate. Low inventories reduce holding costs, throughput times, and (by eliminating storage) material handling costs internal to the plant. Moreover, under the lean philosophy, low inventories enable improved control over part quality and supplier performance and they maintain a healthy stress on the system necessary to motivate improvements. The dollar magnitude of these later savings is intangible but generally considered significant. On the other hand, low inventories also increase the frequency of milkrun routes and the number of suppliers on these routes, hence, increasing transportation costs.

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, it has been reported that the 3D printed parts exhibit anisotropic behaviors in microstructure and mechanical properties, that is, depending on the positions (infill orientations) that the parts are placed on a printer platform, the properties of resultant parts can vary greatly. So far, studies on anisotropic behaviors of 3D printed parts have been mostly limited to the static properties (modulus of elasticity, failure strength, etc.); there is a lack on the understanding of mechanical responses of 3D printed parts under dynamic conditions. In the present study, the anisotropic behaviors of 3D printed parts are investigated from the dynamic aspect.

Traditional manufacturing processes such as injection or compression molding are often enclosed and pressurized systems that produce homogenous products. In contrast, 3D printing is exposed to the environment at ambient (or reduced) temperature and atmospheric pressure. Furthermore, the printing process itself is mostly “layered manufacturing”, i.e., it forms a three-dimensional part by laying down successive layers of materials. Those characteristics inevitably lead to an inconsistent microstructure of 3D printed products and thus cause anisotropic mechanical properties. In this paper, the anisotropic behaviors of 3D printed parts were investigated by using both laboratory coupon specimens (bending specimens) and complex engineering structures (A-pillar). Results show that the orientation of the infills of 3D printed parts can significantly influence their mechanical properties.

The design and development of new biologically inspired technologies based on intelligent materials that are capable of sensing the levels of target biomolecules and, if needed, trigger appropriate countermeasures to regulate biological processes and rhythms of the astronauts is being undertaken in our laboratories. This is accomplished by coupling biologically inspired sensors that monitor the levels of the target biomolecules with intelligent polymeric materials that can regulate the release of a countermeasure. The technology developed here integrates sensors and artificial muscle material into a self-regulating device that can perform with minimal crew intervention. Further, it takes advantage of microfabrication technology to construct lightweight and robust responsive delivery systems. These “intelligent” devices address the need for the control and regulation of biological processes and rhythms under spaceflight conditions.

Transfer path analysis is commonly used to determine input forces indirectly utilizing measured responses and transfer functions. Though it is recommended that the source should be detached from the vibrating structure when measuring transfer functions, engineers and technicians frequently have a difficult time in doing so in practice. Recently, a substitute for inverse force determination via transfer path analysis has been suggested. The indirectly determined forces are termed blocked forces and are usable so long as the source and machine are not detached from one another. Blocked forces have the added advantage of being valid even if the machine structure is modified. In this research, a typical automotive engine cover is considered as a receiver structure and is bolted to a plastic source plate excited by an electromagnetic shaker.

Finite element modeling has been used extensively nowadays for predicting the noise and vibration performance of whole engines or subsystems. However, the elastomeric components on the engines or subsystems are often omitted in the FE models due to some known difficulties. One of these is the lack of the material properties at higher frequencies. The elastomer is known to have frequency-dependent viscoelasticity, i.e., the dynamic modulus increases monotonically with frequency and the damping exhibits a peak. These properties can be easily measured using conventional dynamic mechanical experiments but only in the lower range of frequencies. The present paper describes a method for characterizing the viscoelastic properties at higher frequencies using fractional calculus. The viscoelastic constitutive equations based on fractional derivatives are discussed. The method is then used to predict the high frequency properties of an elastomer.

Over the past decade crash rates of younger drivers have been increasing despite a decrease in their driver population. This study examined specific driving maneuvers whose unsuccessful undertaking results in specific types of crashes involving these drivers. Four types of crashes were identified including crashes at intersections, rear end, crashes resulting from passing maneuvers and single vehicle crashes. For all crashes a general trend of decreasing involvement with increasing age was observed indicating that inexperience is the largest single contributor to the increased crash rates of younger drivers. Increasing the level of awareness among young drivers about these issues and their likely crash involvement seems to be the only viable approach.

Helmholtz resonators are often used in the design of vehicle mufflers to target tonal noise at a few specific low frequencies generated by the engine. Due to the uncertainty of temperature variations and different engine speeds, multiple resonators may have to be built in series to cover a narrow band of frequencies. Double-tuned Helmholtz resonators (DTHR) normally consist of two chambers connected in series. Openings or necks are created by punching small slots into a thin-walled tube which provide a natural neck passage to the enclosing volume of the Helmholtz resonator. In this paper, numerical analyses using both the boundary element (BEM) and the finite element (FEM) methods are performed and simulation results are compared against one another. A typical real-world muffler configuration commonly used in passenger vehicles is used in a case study.

Prior research on assessing multiple inlet and outlet mufflers is limited, and only recently have researchers begun to consider suitable metrics for multiple inlet and outlet mufflers. In this paper, transmission loss and insertion loss are defined for multiple inlet and outlet mufflers using a superposition method that can be extended to any m-inlet n-outlet muffler. Transmission loss is determined assuming that the sources and terminations are anechoic. On the other hand, insertion loss considers reflections. For both metrics, the amplitude and phase relationship between the sources should be known a priori. This paper explains both metrics, and measurement of transmission and insertion loss are demonstrated for a 2-inlet 2-outlet muffler with good agreement.

Transmissibility is the most common metric used for isolator characterization. However, engineers are becoming increasingly concerned about energy transmission through an isolator at high frequencies and how the compliance of the machine and foundation factor into the performance. In this paper, the transfer matrix approach for isolator characterization is first reviewed. Two methods are detailed for determining the transfer matrix of an isolator using finite element simulation. This is accomplished by determining either the mobility or impedance matrix for the isolator and then converting to a transfer matrix. It is shown that results are similar using either approach. In both cases, the isolator is first pre-loaded before the transfer matrix is determined. The approach to find isolator insertion loss is demonstrated for an isolator between two plates, and the effect of making changes to the structural impedance on the machine side of the isolator by adding ribs is examined.

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.

Additive manufacturing is slowly changing how components are developed and manufactured. As the technology develops over time, it is anticipated that industry will 3D print sound absorbers in production. Configurations may be considered that would be difficult to manufacture in another way. For exploratory purposes, several designs were 3D printed and positioned in an impedance tube for testing. Though the absorbers developed are based on well-established strategies, the absorbers considered are either difficult to manufacture by another means or take advantage of the unique features of 3D printed parts. The samples measured include long perforations, lightweight panels, and Helmholtz resonators with spiral wound necks. Selected results are compared with acoustic finite element analysis.

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.

The two-load method is used to measure the transmission loss of thin panels in two different sized impedance tubes (3.49 cm and 10.16 cm). Samples were initially tested with a clamped boundary condition. This was followed by tests with an elastomer inserted between the tube and tested sample to adjust the boundary condition at the periphery. In all tests performed, the influence of the sample holding method could not be removed from the test. The measured transmission loss was compared to finite element simulation with good agreement for both impedance tubes. Additionally, the effect of a compliant boundary condition along the periphery of the sample was also validated via simulation.

The two-source method was used to measure the bulk properties (complex characteristic impedance and complex wavenumber) of sound-absorbing materials, and results were compared to those obtained with the more commonly used two-cavity method. The results indicated that the two-source method is superior to the two-cavity method for materials having low absorption. Several applications using bulk properties are then presented. These include: (1) predicting the absorptive properties of an arbitrary thickness absorbing material or (2) layered material and (3) using bulk properties for a multi-domain boundary element analysis.

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.