Evaluation of ground effect is important for acoustic measurement in static-firing tests on rocket motors. The effectiveness of existing acoustic impedance models is examined by comparing with some experimental results. Through the comparison and evaluation of effect of meteorological condition, it is confirmed that the existing impedance models are unsatisfactory for the evaluation of long distance propagation over a hard surface, which corresponds to the far field condition in the present static-firing tests of rocket motors. In this study, a new acoustic impedance model is proposed. From the comparison with the existing acoustic measurement data, it is shown that the new model is effective for both of near and far field propagation. The proposed model is applied to acoustic data measured in the static-firing tests of solid rocket motors, assuming distributed acoustic sources along the exhaust jet axis.