Internal damping of hollow structures, particularly retrospectively, is often difficult and expensive. The nature of the damping material and how it will be used will also be relevant if weight penalties are of significance. It would seem advantageous to establish how effective a damping treatment will be before deciding whether or not to use it. This paper presents an investigation into different methods of applying passive damping treatments to such structures. The merits of three treatments, extensional, constrained-layer, and visco-sphere in-fill damping are discussed. Using the dynamic properties of the viscoelastic damping material, the effect of the individual damping treatments are modelled using MATLAB, for the optimised treatments are applied to the box beam. This allowed for thickness optimisation of the layers of viscoelastic material, particularly for the constrained layer damping. The effectiveness of these treatments in damping the lowest flexural modes present in a steel box beam is investigated using hammer and electromagnetic shaker excitation. These results are then compared with predicted values and each other.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.