For the high speed boom, the main sound path to the passenger cabin goes through the right dash area. Unbalanced 2nd order forces of the inline, four cylinder engine drive the rigid vertical acceleration of the powerplant. The top and bottom surface velocities of the powerplant increase with engine speed, therefore, acoustic excitation of the engine compartment also rises. Along its path from the engine compartment to the occupant's ear positions, the excitation is amplified due to three closely spaced resonances. First, the 174 Hz (5200 RPM) cabin diagonal acoustic mode amplifies excitation levels. Next, with increasing engine speed this influence is reduced but now the 180 Hz (5400 RPM) dash panel structural mode amplifies sound transmission into the cabin. Finally, at an engine speed of 6000 RPM, the 200 Hz engine compartment vertical acoustic mode becomes the dominant factor. The proximity of the three modes leads to a very broad amplification of cabin sound pressure levels above 4500 RPM. With complicated acoustic-structural interaction, the benefit of investigating all relevant phenomena together becomes obvious. Vibro-Acoustic analysis allows the extraction of coupled acoustic-structural resonances showing all transfer path relevant details. A benefit of acoustic excitation is the fact that only modes contributing to the realized phenomena are well excited, while nonparticipating modes are suppressed. Engineers can get an impression of the system characteristics relevant to their work, thus optimal system modifications during prototype development can be identified and implemented. In comparison to structural or closed cavity investigations, data acquisition in external open cavities may require an acoustically suitable environment in which the test is conducted except for the fact that excitation is now done with acoustic sources, data acquisition requires no special hard- or software. Data analysis tools are also very similar to the ones used for structural analysis, but some important differences, such as scaling, must be observed.
Vibro-acoustic modal analysis of automobile body cavity noise
Sound and Vibration ; 34 , 6 ; 20-23
2000
4 Seiten, 9 Bilder, 2 Tabellen
Article (Journal)
English
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