The instantaneous frequency (IF) of a pure tone emitted by a transiting acoustical source varies due to the acoustical Doppler effect, which is usually used to estimate the velocity of the source. Hence, the common practice is first to estimate the IFs by the time-frequency analysis (TFA) of the signals and then to derive the velocity from the IFs. In this paper, a TFA method based on the chirplet transform is proposed where the IFs of the chirplets are formulated by the nonlinear kinetic function with respect to the source velocity. By iteratively fitting the chirplets to the signals it directly renders the velocity of the source. The efficiency of the proposed method is validated by the microphone recorded noise data due to a flying helicopter during an experiment carried out in May, 2018, northeast to the coast of Sanya, Hainan province, China.