Abstract: Since the inhomogeneity in the atmosphere, an infrasonic wave consists of different normal modes, of different phase and group velocities. When an infrasonic wave is received by an acoustic array, the time delays of the different modes on the array are also different. The main energy of every mode is distributed in separated frequency ranges. In this paper, a method which computes the average, value and azimuth of the phase velocity for every normal mode is proposed and the spectrum of every mode is estimated with velocity filtering. And after Weinerr-Hopf filtering, the waveform of any mode in any position of space can be estimated from the data received in an acoustic array. Finally the computer simulation is given, and it is shown that the estimated waveform is very close to the assumed one.