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In this paper procedures for estimating the sound absorption coefficient when the specimen has inherently low absorption are discussed. Examples of this include the measurement of the absorption coefficient of pavements, closed cell foams and other barrier materials whose absorption coefficient is nevertheless required, and the measurement of sound absorption of muffler components such as perforates. The focus of the paper is on (1) obtaining an accurate phase correction and (2) proper correction for tube attenuation when using impedance tube methods. For the latter it is shown that the equations for tube attenuation correction in the standards underestimate the actual tube attenuation, leading to an overestimate of the measured absorption coefficient. This error could be critical, for example, when one is attempting to qualify a facility for the measurement of pass-by noise.

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.

The Boundary Element Method (BEM) is a computational method for solving the acoustic wave equation when the acoustic domain has an irregular or arbitrary shape. The BEM is distinguished from other numerical methods such as the finite element method in that with the BEM only the surface or the boundary of the acoustic domain needs to be discretized. In this paper some examples are presented concerning problems in automotive industry involving the radiation of sound from engines and other vibrating structures, the acoustical response of passenger compartments of vehicles and the attenuation of mufflers and other exhaust or intake system components.

In this paper the application of the Boundary Element Method (BEM) to problems in acoustics and noise control will be reviewed. The BEM is a computational method for solving the acoustic wave equation when the acoustic domain has an irregular or arbitrary shape. Examples of such problems in the automotive industry include the radiation of sound from engines and other vibrating structures, the scattering (diffraction) of sound from irregular surfaces and obstacles, the acoustical response of passenger compartments of vehicles and the attenuation of mufflers and other exhaust or intake system components. The BEM is distinguished from other numerical methods such as the finite element method in that with the BEM only the surface or boundary of the acoustic domain must be discretized. This is an important feature in solving radiation problems, where the domain is infinite or semi-infinite, but is also beneficial for cavity and muffler problems as well.