The acoustic performance of mufflers with single-inlet and single-outlet are well described using Insertion Loss (IL) and Transmission Loss (TL). These parameters represent the acoustic damping on the engine emission and on the incident pressure wave respectively. However, for mufflers with multi-inlet these parameters depend also on the sources characteristics, as consequence their use is quite difficult.In the present work the acoustic performance of a double-inlet and single-outlet muffler are experimentally evaluated in terms of reflection and transmission coefficients of each port of the muffler itself. These coefficients are used to evaluate the Insertion Loss of the manifold muffler taking into account specific sources on the inlets.The characteristic coefficients are also used to predict the acoustic emission of the manifold muffler using a known engine source on the two inlets. The resulting acoustic emission is compared with the measured one on the corresponding experimental test.Therefore, in the present work, the experimental procedure and test rig adopted to estimate the acoustic characteristic coefficients are shown.