The heavy soft impact source is a rubber ball with a mass of 2.5 kg which is used as a standard device to excite floor constructions. This device is dropped from a height of 1 m in the source room, and the maximum sound pressure level is measured inside the receiving room either in one-third octave bands from 50 Hz to 630 Hz or in octave bands between 63 Hz and 500 Hz. The A-weighted sum of these values together with some corrections for reverberation time and room volume is finally used to assess the acoustic insulation against impact noise.From a metrological point of view, the rubber ball is an absolute standard. Its properties are defined mainly by the force exerted on a hard receiver in octave bands between 31.5 Hz and 500 Hz. At PTB, a new facility for testing rubber balls was set up to measure this force. In the contribution, the setup is described, the calibration of the force sensor by a calibrated impact hammer is explained and a first estimate of the measurement uncertainty is derived.