Source location technology has been broadly used for structure monitoring. Source location (damage detection) of the structure is a challenging task due to the complex underground conditions. The basic assumption in many currently available techniques is that the velocity of wave, which is generated by structural damage and propagates in the structure, is considered in the constant value to estimate the source location. If the wave velocity is assumed to be a fixed velocity in the target structure, significant error can occur for detecting the damage location. In this study, laboratory tests were performed to analyze the accuracy of the source location according to the wave velocities. This study proposes a velocity model with the receiver distance, with which we verify the accuracy of the source location. Our results from the proposed velocity model produce an erorr rate of only 4%, which is significantly lower than the error rate of the original source location with a varied velocity. This velocity model achieves a high degree of reliability in damage monitoring.