The contaminating effect of external damping sources to the overall measured damping of mechanical structures has always been an issue. Although these sources are qualitatively known, they are often not considered if damping properties are experimentally determined, yielding erroneous results. The aim of this paper is to quantify some of these undesired effects on the overall measured damping value. Free vibrations of steel plate specimens are used to list up several causes of external damping sources. As small modifications to the test setup may lead to totally different results, appropriate actions are offered to design a more accurate test setup. Known observations, such as the damping value’s dependence on the specimen size and the excitation level, are confirmed. Finally, it is shown that the damping capacity of one type of steel alloy can usually not be generalised to other steel alloys.