A two-dimensional X-shaped microphone array system can measure horizontal and vertical distributions of exterior noise sources on moving vehicles such as highspeed trains and automobiles, which is essential for reducing noise levels.The system is positioned close to the vehicle track like the letter "X".Numerical simulations show that the spherical wave consideration and the time average of varying directivity are necessary for the system, and that an X-shaped configuration has better resolution of noise sources than a cross-shaped one.Results also show that a microphone interval larger than half-wavelength is allowed by arithmetic decibel averaging over the array directivity for different source positions.