Abstract Renewed interest in reducing interior noise in transportation vehicles has motivated research in low frequency, structural-acoustic analysis. The internal sound field in the enclosed cavity is significantly affected by the acoustic modal characteristics of the cavity, by the dynamic behavior of the surrounding structure, and by the nature of the coupling of these two dynamic systems. The present work is intended to cast more light on the acoustic-structure interaction between the car compartment structure and the enclosed cavity. The system is studied using ANSYS finite element (FE) code. The modeling involved shell finite elements for the structure and three-dimensional (3D) acoustic elements for the cavity. The 3D FE modal analysis produced results visualizing the complex picture of acoustic-structure coupling. It was found that strong coupling between the thin-walled structure and the acoustic enclosure exists in the vicinity of any acoustic resonance. Also it was found that "combined" acoustic-structure modes of vibration exist in the vicinity of an acoustic resonance, which means that the coupled system manifests a new type of energy exchange.69599

Key Words: Acoustic-structure interaction  vibration  finite elements

1 INTRODUCTION

The automotive industry is involved in a continuous endeavor to improve the noise and vibration characteristics of passenger vehicles. Under road conditions, the noise spectrum inside a passenger vehicle in the low frequency range <400Hz is found to be mainly structure-borne noise (see e.g. [1] ). The vibration energy generated from various sources is transmitted into the compartment cavity through structural connections. Thus vibration characteristics of the cavity and its boundary are very important factors which dominate acoustic response in a vehicle passenger compartment. Unpredictable noise problems can occur when the natural dynamic properties of the car body and compartment system coupled with the enclosed cavity are not well predicted. Among the various vehicle noise problems, structure-borne noise such as booming has been a subject of detailed investigations (see e.g. [2] and [3]).

Many studies have been carried out with an ultimate aim to understand the problem and reduce the boom noise (e.g. [4]).For various reasons, recent research has emphasized the low frequency noise in the range from 20 to 200Hz. It has been found that the internal sound field in the enclosed cavity is significantly affected by the acoustic modal characteristics of the cavity, by the dynamic behavior of the surrounding structure, and by the nature of the coupling of these dynamic systems. In addition, depending upon the relative values of the panel and cavity resonant frequencies, sound transmitted to the interior may be amplified rather than reduced.

More recently, it was proved that the well-designed trim-air gap system can play an important role in improving the acoustic response characteristics of the compartment [5]. Analysis and experiment show that the compartment resonance can be controlled by changing the gap thickness or trim mass. On the other hand, Song et al.[6] presented an active vibration control system for structural acoustic coupling of a 3D vehicle cabin model. The structural-acoustic coupling system is analyzed by combining the structural data from modal testing with the acoustic data from the finite element method.

The present study is intended to cast additional light on the acoustic-structure coupling in the gas-structure system discussed by using three-dimensional (3D) finite element (FE)modeling. The thin-walled passenger compartment interacting with the acoustic cavity is modeled employing simplifying assumptions. The external structure is assumed to be elastic, and therefore it is modeled using thin elastic shell elements. The fluid domain is discretized utilizing 3D pressure-formulated, acoustic elements. With respect to the modal analysis of the coupled problem, a special emphasis is placed not only on the changes in the frequency spectra, but also on the respective mode shapes which have rarely been treated in the literature. The present research highlights the very special and unique free-vibration behavior of systems comprising a thin-walled structure and an acoustic cavity, which is direct result of the acoustic-shell coupling.