variance is E [Σjaj
2]). The resulting function is known as
the wave-variance spectrum S(f)*. Typical spectra of
wave systems have a form as shown in Figure 1.15
where the squared amplitudes for each component are
plotted against their corresponding frequencies. The
figure shows the spectrum from a measured wave record,
along with a sample of the data from which it was calculated†.
On the horizontal axis, the wave components are
represented by their frequencies (i.e. 0.1 Hertz (Hz)
corresponds to a period of ten seconds).