Square root of the noise on the squared velocity dispersion amplitude for orders k = 1 to 4 of the FFT as a function of σ₀ for two distinct values of dσ and various number of points N in the FFT. The curves correspond to Eq. A.5 with dσ = 1 km s⁻¹ (dashed) and dσ = 4 km s⁻¹ (dotted), each corresponding to a different value of N (from top to bottom: 1000, 2000, 4000 and 8000). Coloured symbols correspond to different orders measured on toy models. The similar behaviour of orders leads to superposed points.