In structure-borne sound problems in buildings, knowledge of the contact forces between vibrating equipment and a receiving element is often required. When multiplying these forces with the simultaneously measured velocities at the contact points of the equipment, the structure-borne sound power can be determined, which is the quantity governing the sound energy transfer from a source space to a receiving space. Measuring contact forces directly is however not straightforward. Therefore, a structural-acoustical method is used to determine the contact forces inversely. This technique is already used in the automotive industry, but applications in buildings are rather rare. The installation used is a vibrating platform, which induces tonal, low-frequency vibrations into the building. Therefore, the used sound source has to perform well for low frequencies. The resulting forces are used for structure-borne sound power calculation and verified by comparing the sound pressure, caused by this injected power, with measured sound pressure.