Structure-acoustic analysis; finite element modelling and reduction methods

Abstract: This thesis investigates structure-acoustic systems by use of finite element analysis. The systems studied here are limited to those that consist of an enclosed acoustic fluid cavity, which is coupled to a flexible structure and/or a porous sound absorbing material domain. This type of analysis is applicable to a wide range of engineering problems, for example, studying the interior noise in a vehicle or the sound transmission loss in a wall between two dwellings. The geometrical properties of the studied system and the frequency limit of interest for the analysis determine the size of the system of equations to be solved. This size often becomes very large and the solution time becomes long. To decrease the size of the system, and thereby speeding up the analysis, methods are investigated and developed, using substructuring and modal reduction, for the analysis of structure-acoustic problems. The geometric problem domain is divided into a number of subdomains and a reduced set of basis vectors is derived for each of these subdomains. The set of basis vectors for each domain is derived to include information about both the internal behaviour of the subdomain and the coupling to the other subdomains. The reduced description enables efficient solution of the total system. An important feature is to include the description of porous sound absorbing materials in the reduction process of the structure-acoustic problems. The derived procedures are employed in engineering applications; particularly in the study of the sound transmission loss of lightweight double-leaf walls in the low-frequency range. The objective is to include a detailed geometric description of the problem enabling a structured evaluation of the influence of various geometrical and material properties of the studied wall on the predicted sound transmission loss.