A study on a plate-cavity coupling system induced boom noise and countermeasures
Creators: name: Egab, Laith
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401701
subject: Acoustics and noise control (excl. architectural acoustics)
classificationCode: 401702
subject: Dynamics, vibration and vibration control
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation

description: Reducing the very annoying boom noise which is heard inside the cabin of a moving vehicle is the motivation and the subject of this research thesis. The attraction of a vehicle to a potential consumer is significantly influenced by its interior noise characteristics. Thus the prediction of vehicle interior noise is a significant challenge for vehicle manufacturers at the early concept design phase.<br><br>Vehicular interior noise mainly stems from structural vibration such as the power train, driveline vibration or wheel vibration which is induced by road roughness. Among numerous vehicular noise problems, structural-borne noise such as boom noise is considered to have the most annoying acoustic features. The boom noise influences the acoustic quality, and thus the customer’s perception of the car for the value and quality.<br><br>The boom noise generated from structural vibration is mainly due to the strong modal coupling between the flexible structure and the acoustic cavity. Therefore, studying the vibro-acoustic coupling characteristics – the subject of this research – is important for car manufacturers.<br><br>In this research, the first step was to perform experimental modal analysis to identify the plate-cavity resonance problem. Following this, the Finite Element Method (ANSYS) was applied to simulate the plate and cavity system and the simulation results were compared with experimental results; these new results have formed the basis for later analytical formulations and assumptions. Secondly, the deterministic method, based on an Impedance Mobility Compact Matrix, was implemented in the analytical solution to calculate the sound pressure level inside the acoustic cavity and the flexible plate vibration velocity. The analytical results were verified by the results of the VAOne vibro-acoustic simulation model and experimental results.<br><br>A statistical energy analysis (SEA) was then applied to simulate the plate and cavity system. All SEA parameters were calibrated with analytical formulae and the coupling quotient calculated. The coupling quotient indicated the coupling strength of the plate-cavity system in the mid-high frequency ranges.<br><br>Then the two methods were combined. The use of the combined method produced results which explained how the air cavity and the flexible structure were coupled in the low-mid frequency ranges. The acoustic pressure calculated by the Impedance Mobility method was used to predict the mean energies of the flexible plate and the cavity. The predicted mean energy curves were verified by the SEA simulation results using the AutoSEA software and experimental data from speaker-microphone experiments. The results from the combined methods have revealed that the coupling between the structure and the acoustic cavity depends on the plate thickness and the cavity size. This study provides a better insight into the coupling strength of the plate structure and the cavity. It is this insight which can be used to understand and solve the boom noise problem.<br><br>The effect of interior trim materials on the sound quality of the plate-cavity system was also investigated. The sound pressure results calculated by the impedance mobility method were used to calculate the psychoacoustic metrics using the psychoacoustic analysis method. Experiments were carried out on a simply supported aluminium plate which covered an open ended square box, and the box was excited by a shaker. A good agreement between the analytical and experimental results was obtained, and the proposed model was validated and confirmed for feasibility. The results indicated that the interior trim has an influence on the distribution of the sound loudness and sharpness inside the cavity in the middle and high frequency ranges.<br><br>Finally, the application of the hybrid deterministic-SEA method to predict the sound pressure level and mean energy level responses of the stiffened plate-cavity system was demonstrated. The mean square sound pressure inside the acoustic cavity for nine observation points was averaged and used as an input to calculate the cavity mean energy. A good agreement between analytical and experimental results was obtained, and the proposed method was validated and confirmed for feasibility. The results of the proposed method were also validated by practical engineering applications. It was found that there was a strong coupling between the stiffened bare plate and cavity. This can be demonstrated by the first resonant frequency peak overlap of the stiffened plate and the acoustic cavity. The strength of coupling was found to be reduced by adding the stiffener onto the plate, or increasing the thickness of the plate. A strong coupling of the plate and cavity would induce a high level of boom noise.<br><br>Overall, the results of this research have shed new insights onto the boom noise and its solution which helps to eliminate this most annoying noise phenomenon. Much more research, however, still needs to be undertaken in order to fully resolve this noise issue.