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 internal gas-dynamic field that, on the other hand, needs to be optimised also in terms of pressure losses. In this work, the noise attenuation characteristics of a typical perforated muffler for automotive applications are investigated. Acoustic performances are quantified by the Transmission Loss (TL) parameter, which only depends on the geometrical characteristics of the device. Different numerical analyses are employed. At first, a one-dimensional (1D) simulation code (GT Power™) is used to predict the TL profile in a low frequency range. 1D simulation, in fact, may be only applied under the hypothesis of a planar wave propagation. A more complex 3D FEM/BEM approach is also realised using the VNOISE™ code, , specifically designed for acoustic applications. Obviously, such analysis allows to obtain more accurate results at high frequency, depending on the mesh size. Different flow velocities and gas temperatures are investigated in both 1D and 3D models. The predicted TL profiles are compared and discussed in order to assess the potentiality and limitations of the employed numerical approaches.