Error due to the regression of modal coefficients C_n and s_n (a), and its consequence on the discrepancy between modal decomposition and definition (9) on z_in (b). (a) Mean relative error ε between fitted modal coefficient and its actual value X = {s_n, C_n}. Regression is based on N_v = 25 values of v_RMS. (b) L₂-norm of the error over z_in for every frequencies, between definition given by equation (9) and modal decomposition approximation defined by equation (2). This error is plotted with respect to the degree of the fitting polynomial of C_n and s_n.