In recent years the importance of noise vibration and harshness (NVH) has risen greatly in part due to automotive legislative law related to drive-by noise tests. Total noise levels are not the only concern for the automotive industry: many companies are now marketing their vehicles by quoting its level of quietness. Indeed, a customer's perceived quality of a vehicle is based upon the quality of sound the vehicle produces whether during cruising on a road or just shutting a door.NVH has become more of an issue for vehicle manufacturers since the development of plastic under-hood components, such as air intake manifolds. This is especially true now as legislative law has pushed sound levels lower, and customer demands have increased. Several manufacturers have found air flow noise through plastic manifolds can cause NVH problems. In the past, tackling air-flow noise problems have proved difficult.