In order to clarify the characteristics of aeroacoustic noise generated by high speed trains, wind tunnel tests are repeated and directional microphone systems, such as an acoustic mirror system, are generally used to localize the sources of aeroacoustic noise. This paper explains the theoretical background of the principle of acoustic mirror and introduces an ellipsoidal shape chosen as the optimized shape of the mirror used for the Maibara large scale low noise wind tunnel. The characteristics of the acoustic mirror are verified experimentally and proved to be much better than those of old one which has a paraboloidal shape. Finally, the method to estimate the contributions of the localized noise sources to the wayside noise level is proposed on the basis of the wind tunnel test data measured with directional microphone systems.