SUMMARYThe issues of observability and stabilization are analysed for a vibrating string equipped, respectively, either with a point sensor measuring its displacement or a vertical tie-down viscous damper. It is well known that the aforementioned properties hold for the stationary devices of these types only when they are placed at the 'irrational points' of the string. The latter creates obvious di culties in applications. In this article we discuss how mobile point sensors and dampers, bouncing in a rather simple fashion between any two points on the string, can be employed instead in a more 'feasible and robust' way.