The objective of this paper is to introduce an impact device made by embedding a piezoelectric sensor into a steel sphere and to develop a calibration method for measuring the impact force. The force-time function of impact can be used to normalize the spectrum obtained from an impact-echo test. Furthermore, an approach was proposed to make the normalized spectrum dimensionless. A new technique based on the dimensionless normalized spectrum is developed to quantitatively evaluate the difference in acoustic impedance at the interface between two dissimilar materials. The impact-echo test results show that the dimensionless normalized spectrum is capable of identifying the degree of the difference in acoustic impedance at the interface between two dissimilar materials.