The impact echo method is a nondestructive testing method that is based on the use of elastic waves. It has been widely used in the nondestructive evaluation of concrete. With the identification of the peak frequency observed in the frequency spectrum, the depth of a reflector can be estimated when the longitudinal wave velocity is known. However, in most applications, an empirical correction factor (ŽÂ²) is used because the frequency observed in the spectrum is not in good agreement with the frequency calculated by the given equation in the impact echo. This paper presents determination of the empirical shape factor, ŽÂ², in impact echo for different rectangular shapes. Two-dimensional dynamic boundary element analysis was conducted as a frequency sweep to simulate the impact test. The deformation modes of the first-mode resonance were examined for each shape by applying boundary element method analysis.