Signals in the time and frequency domain - Signals, Processes, and Systems - page 47

Illustration 39: Stochastic noise
The upper picture shows stochastic noise in the time domain (for 1s) and below this the amplitude
spectrum of the above noise. As the time domain develops randomly regularity of the frequency spectrum
within the period of time under consideration is not to be expected (otherwise the signal would not be
stochastic). In spite of many "irregular lines" it is not a typical line spectrum for otherwise the time
domain would have to be periodic!
Pure randomness: stochastic noise
Noise is a typical and extremely important example of a non-periodic oscillation. It has a
highly interesting cause, namely a rapid sequence of unpredictable individual events.
In the roar of a waterfall billions of droplets hit the surface of the water in a completely
irregular sequence. Every droplet goes "ping" but the overall effect is one of noise. The
applause of a huge audience may also sound like noise, unless they clap rhythmically to
demand an encore (which simply represents a certain order, regularity or periodicity!)