The present quiet and comfortable automobiles are the result of years of research carried out by NVH engineers across the world. Extensive studies helped engineers to attenuate the noise generated by major sources such as engine, transmission, driveline and road excitations to a considerable extent, which made other noise sources such as intake, exhaust and tire perceivable inside. Many active and passive methods are available to reduce the effect of said noise sources, but enough care needs to be taken at the design level itself to eliminate the effect of cavity resonances. Experimental investigation of cavity resonances of real systems is necessary besides the FEA model based calculations.
Acoustic cavity resonance of vehicle sub systems show their presence in the interior noise through structure borne and air borne excitations. Cavity resonances for some systems e.g. intake can only be suppressed through resonators. The exact location and nature of acoustic cavity resonance needs to be found as accurately as possible to bring out the best from a resonator.
Different approaches are used to excite and identify the cavity resonances depending on the operational differences of various systems. Cabin cavity modes are analyzed by arranging an array of microphones inside the cabin and exciting the cavity with help of a low frequency volume acceleration source. The cavity modes of inflated tire are analyzed by placing accelerometers circumferentially and exciting the cavity by intermittent deflation. The cavity resonances of intake system are investigated using microphones by providing excitation through a low frequency volume acceleration source. Acoustic modal analysis is carried out to identify and understand the mode shapes.