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

Abstract

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.