If one measures the power density spectra Φh(Ω) of different roads and represents them on a double-logarithmic scale, then similar characteristic curves result for all types of roads as shown in the figure.
Here the power density spectra can be approximated by straight lines, which then can be described by the following equation:
(1.2-19)

Here Φh(Ωo) represents the power spectral density dependent on Ωo (reference angular path frequency), which is usually selected as Ω0 = 10-2cm-1 = 1 m-1 . This corresponds to a reference wavelength of Lo = 2π/ Ω0 = 6.28m. Φh(Ωo) is also called the degree of unevenness of the road.
W defines the gradient of the straight line and is also called the Index of Unevenness (undulation) (in German: "Welligkeit"). The index of unevenness (undulation) of the road varies depending on road design between 1.7 and 3.3 (standardized road: w = 2).
The degree of unevenness (and undulation) is considered as an assessment criterion for the condition of a road. An increase of Φh(Ωo) corresponds to a larger unevenness of the road, while an increasing value of 'W' implies a higher proportion of long waves in the spectrum.