In a present paper, floor impact sounds of void slabs are numerically studied using FEM with coupling of the in-plane and bending displacements developed before by author. This method has a feature that a three dimensional model can be treated as a two dimensional one. Eigenvalue analysis on the void slabs was carried out due to this program and the changes in natural frequencies were examined. When the total amount of the void is constant, the magnitudes of flexural rigidity and dynamical response of the void slabs were not varied by the patterns of void layout, and the increase in dynamical response is not so much in case of the ratio of the void in the slab becomes over 60 %. In addition, the calculation method of the equivalent thickness on void slabs is newly proposed based on the area of the void. Through these studies, it has been revealed that the void slabs are considerably advantageous construction.
