Automotive exhaust systems give a major contribution to the sound quality of a vehicle and must be properly designed in order to produce acceptable acoustic performances. Obviously, noise attenuation is strictly related to the used materials and to its internal geometry. This last influences the wave propagation and the gas-dynamic field.The purpose of this paper is to describe advantages and disadvantages of different numerical approaches in evaluating the acoustic performance in terms of attenuation versus frequency (Transmission Loss) of a commercial two perforated tube muffler under different conditions.At first, a one-dimensional analysis is performed through the 1D GTPower® code, solving the nonlinear flow equations which characterize the wave propagation phenomena. The muffler is characterized as a network of properly connected pipes and volumes starting from 3D CAD information.Then, two different 3D analyses are performed within the commercial STS VNOISE® code. The first one assumes rigid wall hypothesis. In the second, a preliminary FEM modal analysis of the structure is realized. The computed structure natural frequencies are then imported in the BEM model in order to carry out the complex fluid-structure interaction. The effects of this last condition on TL calculation are compared and discussed. In addition, both flow and temperature effects have been included in the modeling and discussed with the previous analyses. Agreement among these numerical evaluations and related limitations are put into evidence for different configurations.