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Abstract: Identifying an appropriate method for modelling automotive dissipative silencers normally requires one to choose between analytic and numerical methods. It is common in the literature to justify the choice of an analytic method based on the assumption that equivalent numerical techniques are more computationally expensive. The validity of this assumption is investigated here, and the relative speed and accuracy of two analytic methods are compared to two numerical methods for a uniform dissipative silencer that contains a bulk reacting porous material separated from a mean gas flow by a perforated pipe. The numerical methods are developed here with a view to speeding up transmission loss computation, and are based on a mode matching scheme and a hybrid finite element method. The results presented demonstrate excellent agreement between the analytic and numerical models provided a sufficient number of propagating acoustic modes are retained. However, the numerical mode matching method is shown to be the fastest method, significantly outperforming an equivalent analytic technique. Moreover, the hybrid finite element method is demonstrated to be as fast as the analytic technique. Accordingly, both numerical techniques deliver fast and accurate predictions and are capable of outperforming equivalent analytic methods for automotive dissipative silencers. [Copyright &y& Elsevier]

Abstract: A numerical matching technique known as point collocation is used to model mathematically large dissipative splitter silencers of a type commonly found in HVAC ducts. Transmission loss predictions obtained using point collocation are compared with exact analytic mode matching predictions in the absence of mean flow. Over the frequency range in which analytic mode matching predictions are available, excellent agreement with point collocation transmission loss predictions is observed for a range of large splitter silencers. The validity of using point collocation to tackle large dissipative silencers is established, as is the computational efficiency of the method and its suitability for tackling dissipative silencers of arbitrary, but axially uniform, cross-sections. [Copyright &y& Elsevier]

The acoustic behavior of perforated dissipative circular mufflers with empty extended inlet/outlet is investigated in detail by means of a two-dimensional (2D) axisymmetric analytical approach that matches the acoustic pressure and velocity across the geometrical discontinuities, and the finite element method (FEM). The complex characteristic impedance, wavenumber, and perforation impedance are taken into account to evaluate the axial wavenumber in the fibrous material and the central perforated pipe. Two different analytical procedures are presented that allow the computation of the modal wave coefficients for the muffler sections. Benchmarking against FE calculations exhibited an excellent agreement. Both approaches are also compared with experimental work for further validation. Several effects are examined, including the extended inlet/outlet ducts, absorbent resistivity, the porosity of the perforations, and the muffler dimensions, and compared with muffler configurations of earlier studies, such as reactive extended inlet/outlet expansion chambers, dissipative mufflers without extensions, and mufflers with absorbent-filled extensions. It is shown that the use of empty extensions leads to quarter-wave resonances providing an improved acoustic performance of the muffler at low to mid frequencies. Also, the presence of sound-absorbing material partially retains the desirable behavior of the dissipative mufflers at higher frequencies. [Copyright &y& Elsevier]

Identification of regions on a vibrating structure which radiate energy to the far field is critical in many areas of engineering. Non-negative intensity is a means to visualize contributions of local surface regions to sound power from vibrating structures. Whilst the non-negative intensity has been used for structures under deterministic excitation due to structural forces or harmonic incident acoustic pressure excitation, it has not been considered for analyzing a structure under stochastic excitation. This work analytically formulates non-negative intensity in the wavenumber domain to investigate the surface areas on a vibrating planar structure that are contributing to the radiated sound power in the far field. The non-negative intensity is derived in terms of the cross spectrum density function of the stochastic field and the sensitivity functions of either the acoustic pressure or normal fluid particle velocity. The proposed formulation can be used for both infinite planar structure and finite plate in an infinite baffle. To demonstrate the technique, a simply supported baffled panel excited by a turbulent boundary layer as well as an acoustic diffuse field is considered and those regions contributing to the radiated sound power are identified. It is demonstrated that the non-negative intensity distribution is dependent on the stochastic excitation. It is also found that for a panel under stochastic excitation the more the non-negative intensity distribution is concentrated within the panel surface, the more efficient the panel radiates sound to the far field. [ABSTRACT FROM AUTHOR]