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An automotive OEM was faced with quickly having to reduce the noise, vibration, and harshness (NVH) levels of a new model that were above those of comparable vehicles. The problematic noise levels were about 6 dB higher on its vehicle than for the quietest competitor.
Company design engineers called in LMS International engineering consultants who used source ranking and benchmarking analysis, investigation of critical noise paths, and countermeasure evaluation by frequency response function (FRF) testing techniques to determine the root causes. They found primary NVH contributors were airborne noise and noise transferred through the engine mounts. A new bracket was developed to reduce the engine mount contribution, and trim materials were added to the floor, firewall, and hood. The result was that noise levels were reduced by 8 dB.
Here's how modern analysis tools allowed such a reduction.
A severe noise problem
Just before the subject vehicle was scheduled to go into production, the OEM identified a severe engine noise during full-throttle acceleration. The company turned to LMS to solve the problem while also providing other important information. The latter included an analysis of the best competitive vehicle to set targets for interior noise, especially during acceleration (see below).
The acoustic insulation of the subject vehicle was compared with best-in-class competitors to provide a design target.
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The OEM wanted LMS engineers to also identify reasons for the different noise levels and propose design changes to improve the car. Final requirement was for LMS to deliver a modified prototype that equaled the NVH performance of the competitive vehicle.
A sensor suite was used to measure interior noise.
For the project, LMS used a combination of advanced technologies and vehicle experience. The former included "fast" technologies, such as fast transfer path analysis (TPA), to quickly identify the general areas causing problems in the vehicle, and detailed technologies, such as TPA and acoustic source quantification (ASQ), to understand the noise mechanisms and determine the root causes of the difficulties. While overcoming engineering challenges, LMS engineers simultaneously transferred what it learned to the customer, making it possible to optimize the vehicle and subsystem development process.
Previous methods
The traditional approach to addressing interior noise problems uses physical tests that attempt to pinpoint noise sources. For example, a tube may be placed in the air intake in order to remove the nozzle noise from interior noise measurements. Or the intake manifold might be shielded to eliminate shell noise radiated from its housing.
The problem with these types of tests is that they provide only approximate indications of the source of the problem. Without an in-depth analysis of the cause of the problem, design engineers typically face a long and expensive trial-and-error process that usually requires making costly modifications whose impact is far from guaranteed.
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