The influence of the restitution coefficient and multiple impacts on the force exerted by ISO tapping machines

ISO tapping machines serve as absolute standards to describe the sensitivity of buildings and building elements in terms of impact noise. They consist of five hammers with a mass of 500 g each. The hammers strike the test object at a velocity corresponding to a free fall from a height of 4 cm with a time interval of 0.1 s between subsequent impacts. The force exerted by ISO tapping machines depends on the interaction between source and receiver. This interaction is governed by two independent phenomena: the mobility (mis)match and the restitution coefficient, i.e. the ratio of the hammer velocity after and before the impact. It is theoretically between 0 and 1 which introduces an uncertainty range of +/- 3 dB for the level of the interacting force. This contribution extends the existing equation for the blocked force of ISO tapping machines to cover multiple impacts since their occurrence is related to the force and the restitution coefficient. It is then shown experimentally that the restitution coefficient can cover the full theoretical range in practical situations. Fortunately, different types of tapping machines exhibit nearly the same restitution coefficient on the same receiver, and no systematic influence of the restitution coefficient on the hammer velocity before the impact or on the time intervals between impacts is observed. A first attempt to predict the restitution coefficient based on the mobility match between source and receiver did not give satisfactory results. A prediction of the restitution coefficient thus remains an open question.