Top left: Amplitude dispersion of H_n from its mean expressed in percent starting at H₂ and fitted Gaussian distributions extracted from a constant speed measurement of 9.3 s (corresponding to 937 time frames). Blue crosses indicate the standard deviation of the corresponding amplitude dispersion. Top right: Mean and 95% confidence interval of the power spectral densities of all amplitude dispersions estimated with autoregressive models of order 20. Bottom left: Frequency dispersion of H_n from its expected value deduced from engine speed expressed in percent starting at H₂ and fitted Gaussian distributions extracted from a constant speed measurement of 9.3 s (corresponding to 937 time frames). Blue crosses indicate the standard deviation of the corresponding frequency dispersion. Bottom right: Mean and 95% confidence interval of the power spectral densities of all frequency dispersions estimated with autoregressive models of order 20. Data are from a constant speed measurement in a compact urban vehicle.