One dimensional (1D) and three dimensional (3D) simulations are widely used in technical acoustics to predict the behavior of duct system elements including fluid machines. In particular, referring to internal combustion engines, the numerical approaches can be used to estimate the Transmission Loss (TL) of mufflers, air boxes, catalytic converters, etc. TL is a parameter commonly used in almost any kind of acoustical filters, in order to assess the passive effects related to their sound attenuation.In this paper, a previous 1D-3D acoustical analysis of a commercial muffler, has been improved and experimentally validated. Features related to the manufacturing process, like the coupling of adjacent surfaces and the actual shape of components, have been noticed to heavily affect the muffler behavior. Hence, although numerical analyses are usually performed on ideal geometries (perfectly matched and shaped), schematizations utilized for acoustic simulations of real mufflers are being suggested to do not neglect these important aspects. On the other hand, for a given initial muffler design, the manufacturing process is assessed to be a critical aspect also for its remarkable effects on the acoustics.In this work, results have been carried out under different muffler operating conditions related to different mean flow velocities and presence or not of internal insulating material. 1D analyses have been performed by implementing a commercial software, solving the nonlinear flow equations which characterize the wave propagation phenomena. 1D approach has also been utilized to evaluate the fluid dynamic behavior of the studied muffler in terms of pressure drop when a mean flow is imposed.3D results are obtained in absence of mean flow by using a commercial software based on Boundary Element approach and solving the three dimensional Helmholtz's equation.Finally, during the experimental tests, the muffler has been treated as an acoustic two-port element.