Adaptation of the large-spanned RC slabs, whose area corresponds to an apartment unit, is increasing in Japan. The authors measured heavy-weight impact sound and impedance of such large slabs at over 30 sites. The structural methods of slabs are: unbond, half-precast concrete, void, and the conventional framework method. There were no distinctive differences among structural methods except in case for void slabs. The impact sound level was determined by the thickness of the slab rather than the area of the slab, even if the area of the slab was over 70 m[sup 2]. In the case of small slabs, where the first eigenfrequency of the slab vibration is around the 63-Hz octave band, there is a tendency of a high-impact sound level. On the other hand, the lower first eigenfrequency may be a benefit for the impact sound level. The stiffness level of the beam seem to depend on frequency.