New European directives force manufacturers of internal combustion engines to develop clean, fuel- effective and silent engines. Exhaust system manufacturers have to develop new concepts of exhaust systems which comply the noise emission standards without affecting the engine efficiency adversely. An original active exhaust silencer has been developed, based on an electrically controllable valve combined with a buffer volume. The system is able to attenuate the exhaust noise without passive attenuation prior to the active silencer. There have been developed electrical equivalent models to dimension the silencer wherein the engine acoustical characteristics, such as the acoustic impedance and source strength at the exhaust, have been included. A cold engine test rig is developed which generates realistic exhaust noise using compressed air with similar characteristics as an operational engine. The test rig permits to develop new concepts of exhaust systems economically, using low cost materials and standard measurement equipment. The prototype active silencer has been validated on the cold engine test rig.