Impact of cavity modes on the acoustic environment of a bus cabin: Investigation to find an appropriate way to predict the modes of a bus cabin by three different methods: sound pressure measurements, frequency response measurements and FEM simulation
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Examensarbete för masterexamen
Modellbyggare
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The minimization of noise exposure for passengers and drivers inside a bus is important
to improve their comfort as well as to enhance the product image. The acoustic
modal response of the bus cavity has a major effect on the noise levels perceived by
users. Three different methods were performed in this thesis to find the main modes
in the bus cavity: FEM simulation of a simplified model, Sound Pressure Level
measurements under operation and FRF measurements. The resonant frequencies
obtained with each method were compared in order to identify possible noise sources
amplifying these modes. The frequency range that is thoroughly analyzed in this
report is between 0 Hz and 100 Hz as it includes the lowest modes, which are more
likely to affect the perception of the sound environment in the cabin. The results
show a clear correlation in frequency between the different methods, making it possible
to separate modes excited by the cavity, by the structure of the bus or by other
noise sources, especially at low frequencies. However, the results from the different
methods do not correlate as good above 70 Hz. Possible reasons for this could be
the increased modal density or the damping in the cabin that is not simulated in
the model. On this basis, it can be concluded that the use of both simulation and
experimental measurements to obtain FRFs in the cavity of a bus cabin up to 70 Hz
is beneficial to understand the acoustic behavior of the bus.
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Ämne/nyckelord
acoustical excitation, bus, FEM, frequency response function, FRF, modal analysis, resonance