The impact-echo technique is a nondestructive evaluation for defects in concrete structures by analyzing elastic waves due to a mechanical impact. In order to improve the technique, a new procedure to evaluate defects in concrete is investigated, applying a scanning procedure. Thus, stack imaging of spectral amplitudes based on impact-echo (SIBIE) is developed. In order to detect artificial voids, SIBIE is applied to plain and reinforced concrete slabs. It is confirmed that the location and the presence of voids in concrete slab can be visually identified by SIBIE. In the case, however, that spacing of bars becomes narrow, it is found that detection of the void in intensely reinforced region might not be easy.