Abstract
When two ducts of different circular cross-sectional area are joined together to form an acoustical transmission system, an additional impedance is introduced. This particular problem is found commonly in air conditioning systems among others. Simple models are used to solve the sound wave equation, and they assume that the plane wave mode is responsible for the most part of the transmission/insertion loss calculation. From previous works, it appears that, although higher-order modes can propagate at certain frequencies, their transmission efficiency is low and they do not cause a critical deviation from the assumption of one-dimensional propagation. The models are based on numerical/semi-analytical and asymptotic approximations to transform the sound pressure wave equation into an acoustic impedance equation with proper boundary conditions. The final aim is to calculate the transfer impedance of the duct section which can be used to calculate the sound field radiated. The model should be extended to ducts with incompressible mean flow for low flow Mach numbers, and further work is currently under progress on this subject. However, for low mean flow, the effect on the transmission loss has been found to be only marginal for some particular geometries. [Work supported by Fondecyt, Grant No. 1020196.]
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