Frequency analysis of the V_R wave for different models. From left to right: we show Models 1hf, 1hr, 1hd, and 1hf with 2 perturbations. The colour indicates the amplitude of the Fourier transform as a function of time when the L_Z is transformed to . The diagonal lines in all models show the increase in frequency with time, which is faster for the decreasing circular velocity curve mode (1hd). In the ideal model with two perturbations, two lines of the frequency corresponding to the two different impacts are detected at all times.