The problem of the effect of structural modifications on sound radiation from a structure is very important with many applications in the design of a variety of products. In practise, for the design of acoustically optimum structures simplified theoretical models are used which can lead to unexpected behaviour of the real structures. Alternatively, measurements or numerical simulation can be used. The disadvantage of these two methods is that the study of alternative structures (structures with different properties) is not easy since for every modification a new structure must be manufactured or a new numerical model must be created. Moreover, these two methods do not easily give an insight into the physical mechanisms of sound radiation from
modified structures. In the first part of this thesis the problem of vibration and sound radiation from a plate with an attached beam stiffener is studied theoretically.
This extends the current theories of vibroacoustic behaviour of infinite plates with an infinitely long beam discontinuity and infinite plates with an infinite number of equidistant beam discontinuities forming a periodic structure.
Firstly, the propagation of flexural waves and the subsequent sound radiation from an infinitely long plate strip is considered. The scattering of plate flexural waves by a finite beam across its width is considered. Changes to the mean square velocity, sound power and radiation efficiency of the plate
strip due to the introduction of the beam stiffener are identified. Simplified approximate analytical expressions for the low-frequency range, well below
the critical frequency, are also presented. This model is extended for the case of a finite rectangular plate by incorporating wave reflection from the two
additional boundaries of the plate. Expressions for the radiation efficiency and the mean squared velocity of the stiffened plate are derived. The results
are compared with results derived using well established numerical methods.
For structures with more complicated modifications the derivation of analytical expressions is not easy. For this reason numerical optimisation is
often used. In this thesis numerical optimisation is used to optimise the modes of a structure in order to radiate acoustic energy weakly into the
acoustic medium. The effectiveness of point mass and line stiffener modification to create acoustically optimum modeshapes in a flat simply supported plate is firstly studied. The results show significant reduction in the sound
power radiated by the optimised structural modes. These results are also verified experimentally.
This optimisation method has certain advantages when it is used on automotive panels. The main advantage is that the panel under consideration can
be isolated from the rest of the automotive structure and it can be optimised alone. This drastically reduces the time required and makes the optimisation practically applicable. A simplified car model is used and the proposed
optimisation is applied to one of its floor panels. Firstly, point masses and
line stiffeners are used as structural modifications to create weakly radiating
modeshapes. Then more commonly used geometrical modifications are used on the floor panel. Two such modifications are studied; swages and domes. The results show a significant reduction in the radiation effciency and sound
power radiated by the optimised panel.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.