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Design tools quiet automotive noise: Part 2 - an electric motor application
Test results (and surprises)
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By
Ravi Beniwal, SenSound and Chris DeFilippo, National Instruments
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Page 2 of 3
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Automotive DesignLine
(08/17/2007 5:00 PM EDT)
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The first configuration tested was with the door fully dressed and with a known good motor. The reconstructed pressure spectrum showed a peak at 73 Hz that corresponded to the fundamental order of motor speed. The corresponding reconstructed normal component of the acoustic intensity distribution displayed a hot spot near the arm rest, as seen below.
An acoustic intensity map at 73 Hz shows a hot spot near the armrest.
On physical inspection, it was found that a retainer bolt used to fasten the door panel was missing. The figure below indicates the location where the retainer bolt was missing.
After replacing the retainer bolt, the test was done again. The image below (left) depicts the intensity map after replacing the retainer bolt. While the original hot spot was subsided, a new hot spot emerged. Such a phenomenon is not uncommon as the acoustic energy is redirected by a structural modification and flows out the structure from a different location. The figure below (right) indicates high amplitude vibrations (normal surface velocity) corresponding to the intensity hot spot.
 
A visual inspection of the trim panel showed that a spacer acted like a bridge, allowing vibration transmission from the door module to the trim panel. In this case, structural vibrations were transmitted from the door model to the trim panel and were further transformed into acoustic energy that radiated out as sound.
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