Results of the FFT measurements of the H I velocity dispersion maps. The histograms show the number of rings as a function of amplitude (top row) and normalised phase angle (bottom row) of the FFT harmonics of the THINGS sample (883 rings, in blue), the interpolated distribution with 20 rings per galaxy for THINGS (280 rings, in red, normalised to the maximum values of the blue histograms), and the WHISP sample (144 rings, in green). Phase angles are normalised to the period T_k of each order.