Complex and irregular waveforms

The shape of an ac wave can get exceedingly complicated, but as long as it has a definite period, and as long as the polarity keeps switching back and forth between positive and negative, it is ac.
Fig. 1 shows an example of a complex ac wave. You can see that there is a period, and therefore a definable frequency. The period is the time between two points on succeeding wave repetitions.
With some waves, it can be difficult or almost impossible to tell the period. This is because the wave has two or more components that are nearly the same magnitude. When this happens, the frequency spectrum of the wave will be multifaceted. The energy is split up among two or more frequencies.

Fig. 1: An irregular waveform.