Design and construction of modem passenger automobiles involve a great deal of research and development (R&D) for noise, vibration, and harshness (NVH) control and reduction. Target levels are not only set by government offices for noise emitted to the environment, but also by carmakers themselves for noise radiated and vibrations transmitted to a vehicle's cabin where drivers and other occupants ride along. A variety of noise and vibrations sources in a vehicle are always on the minds of designers and NVH engineers for special attention. Examples of those sources are internal combustion (IC) engines, rolling tires, gearboxes, exhaust pipes, suspension mechanisms, airflow, etc. Ideally, the best application of countermeasures for noise control and noise reduction is at the sources of those noises by eliminating their causes. When those root causes cannot be mitigated satisfactorily, absorbing and blocking technologies for reduction of noise and vibrations are deployed throughout the vehicle. This paper deals with characterizing and evaluating the effectiveness of a patched dash panel in blocking the vibrations and noise transmitted primarily from the engine compartment to the cabin of a passenger vehicle. In this work, comparisons were made for the vibro-acoustic performances of two steel dash panels: one known as monolithic panel or untreated panel, and a second one identified as patched panel. Evaluations and interrogations of the two panels were conducted in a sound transmission loss (STL) chamber and by employing a novel laser-assisted reconstruction technology known as HELS (Helmholtz equation least squares) method. It is worth mentioning that the panels were excited by non-contact, uniformly diffused sound fields generated by power loudspeakers stationed in the reverberation chamber of the STL facilities. The inputs to the HELS formulations were a) the normal surface velocities taken by a laser beam from a finite number of points on the surface of panels, and b) the acoustic pressures taken from a few points by an array of microphones located in the nearfield from the surface of the panels. With those limited input data, the HELS formulations reconstructed the normal surface velocities over the entire surface of the panels under interrogation. This reconstruction capability of the HELS method gives NVH engineers the flexibility to take limited input normal velocity data from the surface of the structure and acoustic pressures in the nearfield and reconstruct the normal velocities over the entire surface of the target structure. This capacity for data reconstruction of the HELS formulations is especially practical when some areas of the structures are not accessible for data acquisition. The panels were characterized in terms of their structural damping ratios, STL, time-average acoustic intensity, time-average acoustic power, radiated sound pressure level (SPL), normal surface velocities, operational deflection shapes (ODS), etc.
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