The relationship between the curve of the spectral density and an increase of the degree of unevenness Φh(Ωo) or undulations ω is shown in the figure.
Road unevenness of wavelengths L between 0.15 m to 80 m lead to excitation frequencies Ω from 0.06 Hz up to 267 Hz (see eq. 1.2-3) for driving speeds v between 5 m/s ( 20 km/h) and 40 m/s ( 160 km/h).