In the interval of a second, due to his own motion, an observer getting closer to the wave-source will notice more wave-crests and when getting farther he will notice fewer wave-crests compared to standing in one place.

When getting closer the measured frequency of the wave increases, when getting farther it decreases compared to the frequency of the source.

The exact formula is: f’=f(c±c_m)/c, where the “+” sign refers to the observer getting closer, the “-“ sign refers to the one getting farther. In the formula c stands for the speed of the wave and c_m for the speed of the observation point.