In recent years, the motorcycle muffler design is moving to dissipative silencer architectures. Due to the increased of restrictions on noise emissions, both dissipative and coupled reactive-dissipative mufflers have substituted the most widely used reactive silencers. This led to higher noise efficiency of the muffler and size reduction.A dissipative muffler is composed by a perforated pipe that crosses a cavity volume filled by a fibrous porous material. The acoustic performance of this kind of muffler are strictly dependent on the porosity of the perforated pipe and the flow resistivity characteristic of the porous material. However, while the acoustic performance of a reactive muffler is almost independent from the presence of a mean flow for typical Mach numbers of exhaust gases, in a dissipative muffler the acoustic behaviour is strictly linked to the mass flow rate intensity.In this paper, an experimental analysis on a commercial dissipative muffler for motorcycles is presented. More in details, the acoustic performance of the silencer is determined by using a test rig that is based on the multi-microphone technique and the two source method. The acoustic test rig used allows carrying out the measurements in presence of mean flow in order to evaluate the influence of the mean flow on the muffler acoustic performance. The possibility to study the muffler behaviour in presence of mean flow makes the experimental conditions closer to the real operating ones. The aim of the present work is to determine how the acoustic response of a dissipative muffler is influenced by the fluid-dynamic interaction between the mean flow and the flow resistivity chamber.