A modification method for vibration control of structures

Vibration and acoustics requirements are becoming increasingly important in the design of mechanical structures, but they are not usually the primary concern in the design process. So the need of varying the structural behaviour to solve noise and vibration problems often occurs at the prototype stage, giving rise to the so-called structural modification problem. In this paper the problem of determining the best structural modifications is cast in the framework of mathematical programming, by defining a suitable optimisation problem. The method starts from a raw set of experimentally determined frequency response functions (FRFs), and avoids any identification process aimed at the creation of either a modal or a physical model of the structure. Several dynamic requirements can be imposed, depending on the specifications to be satisfied and on the information available about the excitation forces. Typical quantities to control are: FRF modulus, response modulus, response power spectral density and response mean square value. Structural modifications that can be accounted for are lumped masses, springs, viscous dampers, dynamic absorbers and stiffening rods. Some examples are developed to give a straightforward implementation of the method. Results are also presented which involve real-life structures, such as an engine block.