The wave-crests get more frequent in front of the wave-source and they get fewer behind it.

We notice a higher number of wave-crests in a second when the wave-source is approaching us, so the observed frequency of the wave increases compared to that of the source. When the wave-source is getting farther we notice wave-crests less frequently, so the observed frequency will be lower.

The exact formula is: f’=fc/(c± c_k) where the „-” sign refers to the wave-source getting closer, the „+” sign refers to the one getting farther. In the formula c stands for the speed of the wave and c_k for the speed of the wave-source.