The Doppler effect applies to any periodic phenomena - anything that happens at regular intervals. The Doppler effect is a result of two things:
Here, the two detectors receive an incoming periodic signal from the emitter, the source of signals. The signals are emitted both to the right and left. The white dots represent the ticking of wristwatch time along each history. As the emitter increases its speed, you can see the effect of rel-time dilation in the increasing spaces between the ticks (as projected onto the ct-axis, to be precise)
As you can see, the reception of signals: