Consider a seaway with irregular waves of which the energy distribution over the wave frequencies (the wave-spectrum) is known. These waves are input to a system that possesses linear characteristics. These frequency characteristics are known, for instance via model experiments or computations. The output of the system is the motion of the boating structure. This motion has an irregular behavior, just as the seaway that causes the motion. The block diagram of this principle is given in figure 3.6.